Dr. U. SatyanarayanaDr. U. Chakrapani

(with Clinical Concepts & Case Studies)

Co-published with

SECTION 1

Dr. U. SatyanarayanaM.Sc., Ph.D., F.I.C., F.A.C.B.

Professor of Biochemistry & Director (Research)Dr. Pinnamaneni Siddhartha Institute of Medical Sciences

(Dr. NTR University of Health Sciences)Chinaoutpalli, Gannavaram (Mdl)

Krishna (Dist), A.P., India

Dr. U. ChakrapaniM.B.B.S., M.S., D.N.B.

(with Clinical Concepts & Case Studies)

Since 1960

Books & Allied Pvt. Ltd.ELSEVIER

A division of Reed Elsevier India Pvt. Ltd.

Co-published with

Cjpdifnjtusz-!5fSatyanarayana and Chakrapani

ELSEVIERA division ofReed Elsevier India Private Limited

Mosby, Saunders, Churchill Livingstone, Butterworth-Heinemann andHanley & Belfus are the Health Science imprints of Elsevier.

© 2013 Dr. U. SatyanarayanaFirst Published: March 1999Revised Reprint: August 2000Second Revised Edition: June 2002Revised Reprint: 2004, 2005Third Revised Edition (multicolour): 2006Revised Reprint: 2007, 2010Fourth Revised Edition: 2013

All rights are reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without the prior permission of the publishers.

ISBN: 978-81-312-3601-7

Medical knowledge is constantly changing. As new information becomes available, changes in treatment, procedures, equipment and the use of drugs become necessary. The author, editors, contributors and the publisher have, as far as it is possible, taken care to ensure that the information given in this text is accurate and up-to-date. However, readers are strongly advised to confirm that the information, especially with regard to drug dose/usage, complies with current legislation and standards of practice. Please consult full prescribing information before issuing prescriptions for any product mentioned in this publication.

This edition of Biochemistry, 4e by Dr. U. Satyanarayana and Dr. U. Chakrapani is co-published by an arrangement with Elsevier, a division of Reed Elsevier India Private Limited and Books and Allied (P) Ltd.

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Cover DesignDepicts the universal energy currency of the living world—ATP, predominantly synthesized by the mitochondria of the cell (the functional unit of life), in comparison with the international currencies—$, £, €, `, ¥.

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This book ‘Biochemistry’ has undoubtedly become one of the most preferred text books (in India andmany other countries) by the students as well as teachers in medical, biological and other allied sciences.It is certainly a book of choice and a true companion to all learning biochemistry, hence appropriatelyregarded by many as ‘Bible of Biochemistry’. This book has undergone three editions, several reprints, andrevised reprints in a span of 13 years.

The advances in biochemistry are evergrowing due to exponential growth of the subject. Further, thecritical comments, frank opinions and constructive suggestions by teachers and students need to beseriously considered. All this necessitates frequent revision of the book.

In this fourth edition, a thorough revision and update of each chapter with latest advances has beendone. The main emphasis of this edition is an improved orientation and treatment of human biochemistryin health and disease. A wide variety of case studies with relevant biochemical profiles (along with diagnosisand discussion) are newly added as an appendix. In addition, several newer aspects of biochemistry arecovered in this edition, some of them are listed below.

l Triacylgylcerol/fatty acid cycle l ω-fatty acidl Metabolic syndrome l Soluble and insoluble fiberl Glucose toxicity l Trans fatty acidsl Estimated average glucose l Nutrigenomicsl Peptide nucleic acids l Detailed information on antivitaminsl Pseudogenes l Dental cariesl Recombinant ribozymes l Amino acids as neurotransmitters

l Epigenetic regulation of gene expression l Disorders of membrane transportl Metagenomics l Diagnostic importance of various body fluids and tissuesl Therapeutic diets l Enzyme patterns in diseasesl Atkins diet l Cystatin Cl Dietary antioxidants l Pleural fluidl High fructose corn syrups l High sensitive CRP

It is a fact that I represent a selected group of individuals authoring books, having some time atdisposal, besides hard work, determination and dedication. I consider myself as an eternal reader and aregular student of biochemistry. However, it is beyond my capability to keep track of the evergrowingadvances in biochemistry due to exponential growth of the subject. And, this makes me nervous wheneverI think of revising the book. I honestly and frankly admit that I have to depend on mature readers forsubsequent editions of this book.

AN INVITATION TO READERS, WELL WISHERS AND SUBJECT EXPERTS

I have to admit that it is not all the time possible for me to meet the readers individually and get theirfeedback. I sincerely invite the readers, my well wishers and experts in biochemistry subject to feel free andwrite to me (Email ID: uppalasatya@yahoo.com) expressing their frank opinions, critical comments andconstructive suggestions. And this will help me to further improve the book in subsequent revisions.

Dr. U. SATYANARAYANA

Preface to the Fourth Edition

Preface to the First EditionPreface to the First Edition

Biochemistry is perhaps the most fascinating subject as it deals with the chemical language of life, be ithuman, animal, plant or microorganism. No other science subject has as much application as biochemistry tothe disciplines of medicine, health, veterinary, agriculture bioengineering and technology. This necessitates atotally different outlook for the books on biochemistry subject.

There are many biochemistry textbooks on the market. Some of them are purely basic while others areapplied, and there are very few books which cover both these aspects together. For this reason, the studentslearning biochemistry in their undergraduate courses have to depend on multiple books to acquire a soundknowledge of the subject.

This book, ‘Biochemistry’ is unique with a simultaneous and equal emphasis on basic and applied aspectsof biochemistry. This textbook primarily is an integration of medical and pure sciences, comprehensively writtento meet the curriculum requirements of undergraduate courses in medical, dental, pharmacy, life-sciences andother categories (agriculture, veterinary, etc.) where students learn biochemistry as one of the subjects.

The tendency among the students (particularly medical) is to regard biochemistry as being mostlyconcerned with unimportant and complicated metabolic (chemical) pathways. This book gives a new orienta-tion to the subject of biochemistry so that the students appreciate the great importance and significance ofthe application of biochemistry to medicine.

This book is designed to develop in students a sustained interest and enthusiasm to learn and develop theconcepts in biochemistry in a logical and stepwise manner. It incorporates a variety of pedagogic aids, besidescolour illustrations to help the students understand the subject quickly and to the maximum. The summaryand biomedical/clinical concepts are intended for a rapid absorption and assimilation of the facts and conceptsin biochemistry. The self-assessment exercises will stimulate the students to think rather than merely learnthe subject. In addition, these exercises (essays, short notes, fill in the blanks, multiple choice questions) setat different difficulty levels, will cater to the needs of all the categories of learners.

It will not be out of place to mention here how-and when-the book was born. The entire book was writtenin the early morning hours (between 2 AM-6 AM; when the world around is fast asleep), during which periodI carry out my intellectual activities. After a sound sleep, a fresh mind packed with creative ideas and innovativethoughts, has largely helped me to write this book. My wife pleaded with me that I should not write topics likediabetes, cancer, AIDS at home. In deference to her sentiment, I made a serious attempt to write those topicsduring my leisure time in the Department. But when I went through them in my serene mood of the earlymorning hours, I had to discard them in disappointment and rewrite them. Truly, each page of this book wasconceived in darkness and born at daybreak !

This textbook is a distillation of my knowledge and teaching experience in biochemistry, acquired duringthe past 25 years. It contains predigested information on biochemistry for good understanding, assimilationand reproducibility. Each page is crafted with a fine eye. The ultimate purpose of this book is to equip thereader with comprehensive knowledge in biochemistry with reference to basic as well as applied aspects.

Although I have made every effort to make the book error free, I am under no illusion. I welcomecomments, criticism and suggestions from the faculty, students and other readers, and this will help me makeimprovements in the next edition.

Dr. U. SATYANARAYANA

[ ii ]

[ iii ]

I owe a deep debt of gratitude to my parents, the late Sri U. Venkata Subbaiah, and Smt. Vajramma, forcultivating in me the habit of early rising. The writing of this book would never have been possible withoutthis healthy habit. I am grateful to Dr. B. S. Narasinga Rao (former Director, National Institute of Nutrition,Hyderabad) for disciplining my professional life, and to my eldest brother Dr. U. Gudaru (former Professor ofPower Systems, Walchand College of Engineering, Sangli) for disciplining my personal life.

My elder son, U. Chakrapani (MBBS) deserves a special place in this book. He made a significantcontribution at every stage of its preparation—writing, verification, proof-reading and what not. I had the rareprivilege of teaching my son as he happened to be a student of our college. And a major part of this book waswritten while he was learning biochemistry. Thus, he was the first person to learn the subject of biochemistryfrom my handwritten manuscript. The student-teacher relation (rather than the father-son) has helped me inreceiving constant feedback from him and restructure the book in a way an undergraduate student wouldexpect a biochemistry textbook to be.

Next, I thank Dr. G. Pitcheswara Rao (former Professor of Anatomy, SMC, Vijayawada) for his constructivecriticism and advice, and Dr. B. Sivakumar (Director, National Institute of Nutrition, Hyderabad) for his helpfulsuggestions on the microfigures.

Last but not least, I thank my wife Krishna Kumari and my younger son, Amrutpani, without whosecooperation and encouragement this book could never have been written. The manuscript was carefullynurtured like a new born baby and the book has now become a full-pledged member of our family.

ACKNOWLEDGEMENTS TO THE FOURTH EDITION

I am grateful to a large number of faculty members, students, friends and pen friends who directly orindirectly helped me to revise and improve the content and quality of the book. I have individually andpersonally thanked all of them (who number a few hundreds!). I once again express my gratitude to them.

I thank Dr (Mrs) U.B. Vijaya Lakshmi, MD, Associate Professor of Biochemistry at our college whoparticipated to comprehensively prepare case studies with biochemical correlations, besides improving thebiomedical/ clinical aspects in some chapters. My special thanks goes to one student, and an ardent fan of mybooks, Mr. Y. Nagendra Sastry (Ph.D), who has been studying my books regularly for over 7-8 years. Hisconstant feedback and suggestions have certainly contributed to improve this book. I express my gratitude toMr. M.S.T. Jagan Mohan (my former colleague), who has helped me with his frequent interactions to revisethe book, and make it more student-friendly.

I express my sincere thanks to Mr Arunabha Sen, Director, Books & Allied (P) Ltd, Kolkata for his wholehearted support and constant encouragement in revising the book, and taking all pains to bring it out to mysatisfaction. I thank Mr. Shyamal Bhattacharya for his excellent page making and graphics-work in the book.I am grateful to Mr. Abhijit Ghosal for his help in the cover design.

I thank my wife, Krishna Kumari, my younger son Amrut Pani and my daughter-in law Oohasri fortheir constant support and encouragement. My special thanks to my grand daughter Maahe (2 years) whoseever smiling face, sweet words and deeds infuse energy into my academic activities. I am grateful to

Uppala Author-Publisher interlinks, Vijayawada for sponsoring and supporting me to bring out this edition.

Dr. U. SATYANARAYANA

Acknowledgements

The term Biochemistry was introduced by Carl Neuberg in 1903. Biochemistry broadly deals with thechemistry of life and living processes. There is no exaggeration in the statement, ‘The scope of biochemistryis as vast as life itself !’ Every aspect of life-birth, growth, reproduction, aging and death, involves biochemistry.For that matter, every movement of life is packed with hundreds of biochemical reactions. Biochemistry is themost rapidly developing and most innovative subject in medicine. This becomes evident from the fact that overthe years, the major share of Nobel Prizes earmarked for Medicine and Physiology has gone to researchersengaged in biochemistry.

The discipline of biochemistry serves as a torch light to trace the intricate complexicities of biology,besides unravelling the chemical mysteries of life. Biochemical research has amply demonstrated that all livingthings are closely related at the molecular level. Thus biochemistry is the subject of unity in the diversifiedliving kingdom.

Advances in biochemistry have tremendous impact on human welfare, and have largely benefited mankindand their living styles. These include the application of biochemistry in the laboratory for the diagnosis ofdiseases, the products (insulin, interferon, growth hormone etc.) obtained from genetic engineering, and thepossible use of gene therapy in the near future.

Organization of the Book

This textbook, comprising 43 chapters, is organized into seven sections in the heirarchical order oflearning biochemistry.

l Section I deals with the chemical constituents of life—carbohydrates, lipids, proteins and amino acids,nucleic acids and enzymes.

l Section II physiological chemistry includes digestion and absorption, plasma proteins, hemoglobin andprophyrins, and biological oxidation.

l Section III incorporates all the metabolisms (carbohydrates, lipids, amino acids, nucleotides, minerals)

l Section IV covers hormones, organ function tests, water, electrolyte and acid-base balance, tissue proteinsand body fluids, and nutrition.

l Section V is exclusively devoted to molecular biology and biotechnology (DNA-replication, recombination,and repair, transcription and translation, regulation of gene expression, recombinant DNA and biotechnology)

l Section VI gives relevant information on current topics such as human genome project, gene therapy,bioinformatics, prostaglandins, diabetes, cancer, AIDS etc.

l Section VII deals with the basic aspects for learning and understanding biochemistry (bioorganicchemistry, biophysical chemistry, tools of biochemistry, genetics, immunology).

Each chapter in this book is carefully crafted with colour illustrations, headings and subheadings tofacilitate quick understanding. The important applications of biochemistry to human health and disease are puttogether as biomedical/clinical concepts. Icons are used at appropriate places to serve as ‘landmarks’.

The origins of biochemical words, confusables in biochemistry, practical biochemistry and clinicalbiochemistry laboratory, case studies with biochemical correlations, given in the appendix are novel features.

The book is so organized as to equip the readers with a comprehensive knowledge of biochemistry.

[ iv ]

Scope of Biochemistry

Contents

S E C T I O N O N E

Chemical Constituents of Life

1 ➤ Biomolecules and the cell 3

2 ➤ Carbohydrates 93 ➤ Lipids 284 ➤ Proteins and amino acids 435 ➤ Nucleic acids and nucleotides 696 ➤ Enzymes 857 ➤ Vitamins 116

S E C T I O N T W OPhysiological Biochemistry

8 ➤ Digestion and absorption 165

9 ➤ Plasma proteins 182

10 ➤ Hemoglobin and porphyrins 196

11 ➤ Biological oxidation 221

S E C T I O N T H R E EMetabolisms

12 ➤ Introduction to metabolism 241

13 ➤ Metabolism of carbohydrates 244

14 ➤ Metabolism of lipids 285

15 ➤ Metabolism of amino acids 330

16 ➤ Integration of metabolism 380

17 ➤ Metabolism of nucleotides 387

18 ➤ Mineral metabolism 403

S E C T I O N F O U RClinical Biochemistry and Nutrition

19 ➤ Hormones 427

20 ➤ Organ function tests 45321 ➤ Water, electrolyte and

acid-base balance 468

22 ➤ Tissue proteins and body fluids 48723 ➤ Nutrition 502

S E C T I O N F I V EMolecular Biology and Biotechnology

24 ➤ DNA-replication, recombination and repair 52325 ➤ Transcription and translation 54226 ➤ Regulation of gene expression 56627 ➤ Recombinant DNA and biotechnology 578

S E C T I O N S I XCurrent Topics

28 ➤ Human genome project 61929 ➤ Gene therapy 62530 ➤ Bioinformatics 63431 ➤ Metabolism of xenobiotics (detoxification) 63832 ➤ Prostaglandins and related compounds 64433 ➤ Biological membranes and transport 65034 ➤ Free radicals and antioxidants 65535 ➤ Environmental biochemistry 66236 ➤ Insulin, glucose homeostasis,

and diabetes mellitus 66937 ➤ Cancer 68538 ➤ Acquired immunodeficiency

syndrome (AIDS) 695

S E C T I O N S E V E NBasics to Learn Biochemistry

39 ➤ Introduction to bioorganic chemistry 70340 ➤ Overview of biophysical chemistry 70841 ➤ Tools of biochemistry 71942 ➤ Immunology 73243 ➤ Genetics 737

A P P E N D I C E S Answers to Self-assessment Exercises 745

I Abbreviations used in this book 751II Origins of important biochemical words 756

III Common confusables in biochemistry 759IV Practical biochemistry—principles 763V Clinical biochemistry laboratory 769

VI Case studies with biochemical correlations 772

INDEX 779

38

37

36

35

34

33

32

■ Biomolecules and the Cell 3

■ Carbohydrates 9

■ Lipids 28

■ Proteins and Amino acids 43

■ Nucleic acids and Nucleotides 69

■ Enzymes 85

■ Vitamins 116

3

2

1

7

6

5

4

CHEMICAL CONSTITUENTS OF LIFECHEMICAL

CHEMICAL

CONSTITUENTS OF LIF

CONSTITUENTS OF LIFE

E

Section

Section

I

I

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3

The cell speaks :

“I am the unit of biological activity;Organized into subcellular organelles;

Assigned to each are specific duties;Thus, I truly represent life!”

TTThe living matter is composed of mainlysix elements—carbon, hydrogen, oxygen,nitrogen, phosphorus and sulfur. These elementstogether constitute about 90% of the dry weightof the human body. Several other functionallyimportant elements are also found in the cells.These include Ca, K, Na, Cl, Mg, Fe, Cu, Co, I,Zn, F, Mo and Se.

Carbon—a unique element of life

Carbon is the most predominant and versatileelement of life. It possesses a unique property toform infinite number of compounds. This isattributed to the ability of carbon to form stablecovalent bonds and C C chains of unlimitedlength. It is estimated that about 90% ofcompounds found in living system invariablycontain carbon.

Chemical molecules of life

Life is composed of lifeless chemicalmolecules. A single cell of the bacterium,Escherichia coli contains about 6,000 different

organic compounds. It is believed that man maycontain about 100,000 different types ofmolecules although only a few of them havebeen characterized.

Complex biomolecules

The organic compounds such as amino acids,nucleotides and monosaccharides serve as themonomeric units or building blocks of complexbiomolecules—proteins, nucleic acids (DNA andRNA) and polysaccharides, respectively. Theimportant biomolecules (macromolecules) withtheir respective building blocks and majorfunctions are given in Table 1.1. As regardslipids, it may be noted that they are notbiopolymers in a strict sense, but majority ofthem contain fatty acids.

Structural heirarchy of an organism

The macromolecules (proteins, lipids, nucleicacids and polysaccharides) form supramolecularassemblies (e.g. membranes) which in turnorganize into organelles, cells, tissues, organsand finally the whole organism.

11Biomolecules and the CellChapter

Section 1 Chemical Constituents of Life

4 BIOCHEMISTRY

Chemical composition of man

The chemical composition of a normal man,weighing 65 kg, is given in Table 1.2. Water isthe solvent of life and contributes to more than60% of the weight. This is followed by protein(mostly in muscle) and lipid (mostly in adiposetissue). The carbohydrate content is rather lowwhich is in the form of glycogen.

THE CELL

The cell is the structural and functional unitof life. It may be also regarded as the basic unitof biological activity.

The concept of cell originated from thecontributions of Schleiden and Schwann (1838).However, it was only after 1940, thecomplexities of cell structure were exposed.

Prokaryotic and eukaryotic cells

The cells of the living kingdom may bedivided into two categories

1. Prokaryotes (Greek : pro – before; karyon –nucleus) lack a well defined nucleus and possessrelatively simple structure. These include thevarious bacteria.

2. Eukaryotes (Greek : eu – true; karyon –nucleus) possess a well defined nucleus and aremore complex in their structure and function.The higher organisms (animals and plants) arecomposed of eukaryotic cells.

A comparison of the characteristics betweenprokaryotes and eukaryotes is listed in Table 1.3.

EUKARYOTIC CELL

The human body is composed of about 1014

cells. There are about 250 types of specializedcells in the human body e.g. erythrocytes,nerve cells, muscle cells, cells of pancreas.An eukaryotic cell is generally 10 to 100 min diameter. A diagrammatic representationof a typical rat liver cell is depicted inFig.1.1.

The plant cell differs from an animal cell bypossessing a rigid cell wall (mostly composed ofcellulose) and chloroplasts. The latter are thesites of photosynthesis.

TABLE 1.1 The major complex biomolecules of cells

Biomolecule Building block Major functions(repeating unit)

1. Protein Amino acids Fundamental basis of structure andfunction of cell (static and dynamic functions).

2. Deoxyribonucleic acid (DNA) Deoxyribonucleotides Repository of hereditary information.

3. Ribonucleic acid (RNA) Ribonucleotides Essentially required for protein biosynthesis.

4. Polysaccharide (glycogen) Monosaccharides (glucose) Storage form of energy to meet short termdemands.

5. Lipid Fatty acids, glycerol Storage form of energy to meet long termdemands; structural components of membranes.

TABLE 1.2 Chemical composition of a normal man

(weight 65 kg)

Constituent Percent (%) Weight (kg)

Water 61.6 40

Protein 17.0 11

Lipid 13.8 9

Carbohydrate 1.5 1

Minerals 6.1 4

Chapter 1 : BIOMOLECULES AND THE CELL 5

TABLE 1.3 Comparison between prokaryotic and eukaryotic cells

Characteristic Prokaryotic cell Eukaryotic cell

1. Size Small (generally 1-10 m) Large (generally 10-100 m)

2. Cell membrane Cell is enveloped by a rigid cell wall Cell is enveloped by a flexible plasma membrane

3. Sub-cellular Absent Distinct organelles are foundorganelles (e.g. mitochondria, nucleus, lysosomes)

4. Nucleus Not well defined; DNA is found Nucleus is well defined, surrounded by aas nucleoid, histones are absent membrane; DNA is associated with histones

5. Energy metabolism Mitochondria absent, enzymes of Enzymes of energy metabolism are locatedenergy metabolism bound to in mitochondriamembrane

6. Cell division Usually fission and no mitosis Mitosis

7. Cytoplasm Organelles and cytoskeleton Contains organelles and cytoskeletonabsent (a network of tubules and filaments)

The cell consists of well defined subcellularorganelles, enveloped by a plasma membrane.By differential centrifugation of tissuehomogenate, it is possible to isolate eachcellular organelle in a relatively pure form(Refer Chapter 41). The distribution of majorenzymes and metabolic pathways in differentcellular organelles is given in the chapteron enzymes (Refer Fig.6.6). The subcellularorganelles are briefly described in the followingpages.

Nucleus

Nucleus is the largest cellular organelle,surrounded by a double membrane nuclearenvelope. The outer membrane is continuouswith the membranes of endoplasmic reticulum.At certain intervals, the two nuclear membraneshave nuclear pores with a diameter of about 90nm. These pores permit the free passage of theproducts synthesized in the nucleus into thesurrounding cytoplasm.

Fig. 1.1 : Diagrammatic representation of a rat liver cell.

Plasma membrane

Coated pits

Mitochondrion

Lysosome

Nucleolus

Nucleus

Peroxisome

Cytosol

Ribosomes

Cytoskeleton

Golgi apparatus

Smooth endoplasmic reticulum

VacuoleRough endoplasmic reticulum

6 BIOCHEMISTRY

Nucleus contains DNA, the repository ofgenetic information. Eukaryotic DNA isassociated with basic protein (histones) in theratio of 1 : 1, to form nucleosomes. An assemblyof nucleosomes constitutes chromatin fibres ofchromosomes (Greek: chroma – colour; soma –body). Thus, a single human chromosome iscomposed of about a million nucleosomes. Thenumber of chromosomes is a characteristicfeature of the species. Humans have 46chromosomes, compactly packed in the nucleus.

The nucleus of the eukaryotic cell contains adense body known as nucleolus. It is rich inRNA, particularly the ribosomal RNA whichenters the cytosol through nuclear pores.

The ground material of the nucleus is oftenreferred to as nucleoplasm. It is rich in enzymessuch as DNA polymerases and RNA polymerases.

Hutchinson-Gilford progeria syndrome(HGPS) is a rare condition of aging beginning atbirth (incidence I in 5 million births). HGPSoccurs as a result of distortion of nuclearenvelope due to accumulation of abnormalprotein namely lamina A.

MitochondriaThe mitochondria (Greek: mitos – thread;

chondros – granule) are the centres for thecellular respiration and energy metabolism. Theyare regarded as the power houses of the cellwith variable size and shape. Mitochondria arerod-like or filamentous bodies, usually withdimensions of 1.0 3 m. About 2,000mitochondria, occupying about 1/5th of the totalcell volume, are present in a typical cell.

The mitochondria are composed of a doublemembrane system (Refer Fig.11.5). The outermembrane is smooth and completely envelopsthe organelle. The inner membrane is folded toform cristae (Latin – crests) which occupy alarger surface area. The internal chamber ofmitochondria is referred to as matrix or mitosol.

The components of electron transport chainand oxidative phosphorylation (flavoprotein,cytochromes b, c1, c, a and a3 and couplingfactors) are buried in the inner mitochondrialmembrane. The matrix contains several enzymesconcerned with the energy metabolism of

carbohydrates, lipids and amino acids (e.g., citricacid cycle, -oxidation). The matrix enzymesalso participate in the synthesis of heme andurea. Mitochondria are the principal producersof ATP in the aerobic cells. ATP, the energycurrency, generated in mitochondria is exportedto all parts of the cell to provide energy for thecellular work.

The mitochondrial matrix contains a circulardouble stranded DNA (mtDNA), RNA andribosomes. Thus, the mitochondria are equippedwith an independent protein synthesizingmachinery. It is estimated that about 10% of themitochondrial proteins are produced in themitochondria.

The structure and functions of mitochondriaclosely resemble prokaryotic cells. It ishypothesized that mitochondria have evolvedfrom aerobic bacteria. Further, it is believed thatduring evolution, the aerobic bacteria developeda symbiotic relationship with primordialanaerobic eukaryotic cells that ultimately led tothe arrival of aerobic eukaryotes.

Endoplasmic reticulum

The network of membrane enclosed spacesthat extends throughout the cytoplasmconstitutes endoplasmic reticulum (ER). Some ofthese thread-like structures extend from thenuclear pores to the plasma membrane.

A large portion of the ER is studded withribosomes to give a granular appearance whichis referred to as rough endoplasmic reticulum.Ribosomes are the factories of proteinbiosynthesis. During the process of cellfractionation, rough ER is disrupted to form smallvesicles known as microsomes. It may be notedthat microsomes as such do not occur in the cell.

The smooth endoplasmic reticulum does notcontain ribosomes. It is involved in the synthesisof lipids (triacylglycerols, phospholipids, sterols)and metabolism of drugs, besides supplying Ca2+

for the cellular functions.

Golgi apparatus

Eukaryotic cells contain a unique cluster ofmembrane vesicles known as dictyosomes

Chapter 1 : BIOMOLECULES AND THE CELL 7

which, in turn, constitute Golgi apparatus (orGolgi complex). The newly synthesized proteinsare handed over to the Golgi apparatus whichcatalyse the addition of carbohydrates, lipids orsulfate moieties to the proteins. These chemicalmodifications are necessary for the transport ofproteins across the plasma membrane.

Certain proteins and enzymes are enclosed inmembrane vesicles of Golgi apparatus andsecreted from the cell after the appropriatesignals. The digestive enzymes of pancreas areproduced in this fashion.

Golgi apparatus are also involved in themembrane synthesis, particularly for theformation of intracellular organelles (e.g.peroxisomes, lysosomes).

Lysosomes

Lysosomes are spherical vesicles envelopedby a single membrane. Lysosomes are regardedas the digestive tract of the cell, since they areactively involved in digestion of cellularsubstances—namely proteins, lipids, carbo-hydrates and nucleic acids. Lysosomal enzymesare categorized as hydrolases. These includethe enzymes (with substrate in brackets)—

-glucosidase (glycogen), cathepsins (proteins),lipases (lipids), ribonucleases (RNA).

The lysosomal enzymes are responsible formaintaining the cellular compounds in a dynamicstate, by their degradation and recycling. Thedegraded products leave the lysosomes, usually

by diffusion, for reutilization by the cell.Sometimes, however, certain residual products,rich in lipids and proteins, collectively known aslipofuscin accumulate in the cell. Lipofuscin isthe age pigment or wear and tear pigment whichhas been implicated in ageing process. As the celldies, the lysosomes rupture and release hydrolyticenzymes that results in post-morteum autolysis.

The digestive enzymes of cellular compoundsare confined to the lysosomes in the best interestof the cell. Escape of these enzymes into cytosolwill destroy the functional macromolecules of thecell and result in many complications. Theoccurrence of several diseases (e.g. arthritis,muscle diseases, allergic disorders) has been partlyattributed to the release of lysosomal enzymes.

Inclusion cell (I-cell) desease is a rarecondition due to the absence of certain hydrolasesin lysosomes. However, these enzyme aresyntherized and found in the circulation. I-celldisease is due to a defect in protein targetting, asthe enzymes cannot reach lysosomes.

Peroxisomes

Peroxisomes, also known as microbodies, aresingle membrane cellular organelles. They arespherical or oval in shape and contain theenzyme catalase. Catalase protects the cell fromthe toxic effects of H2O2 by converting it to H2Oand O2. Peroxisomes are also involved in theoxidation of long chain fatty acids (> C18), andsynthesis of plasmalogens and glycolipids. Plantscontain glyoxysomes, a specialized type of

+ A living cell is a true representative of life with its own organization and specializedfunctions.

+ Accumulation of lipofuscin, a pigment rich in lipids and proteins, in the cell has beenimplicated in ageing process.

+ Leakage of lysosomal enzymes into the cell degrades several functional macromoleculesand this may lead to certain disorders (e.g. arthritis).

+ Zellweger syndrome is a rare disease characterized by the absence of functionalperoxisomes.

8 BIOCHEMISTRY

peroxisomes, which are involved in theglyoxylate pathway.

Peroxisome biogenesis disorders (PBDs), area group of rare diseases involving the enzymeactivities of peroxisomes. The biochemicalabnormalities associated with PBDs includeincreased levels of very long chain fatty acids(C24 and C26) and decreased concentrations ofplasmalogens. The most severe form of PBDs isZellweger syndrome, a condition characterizedby the absence of functional peroxisomes. Thevictims of this disease may die within one yearafter birth.

Cytosol and cytoskeleton

The cellular matrix is collectively referred toas cytosol. Cytosol is basically a compartmentcontaining several enzymes, metabolites andsalts in an aqueous gel like medium. More recentstudies however, indicate that the cytoplasmactually contains a complex network of proteinfilaments, spread throughout, that constitutescytoskeleton. The cytoplasmic filaments are of

three types – microtubules, actin filaments andintermediate filaments. The filaments which arepolymers of proteins are responsible for thestructure, shape and organization of the cell.

INTEGRATION OFCELLULAR FUNCTIONS

The eukaryotic cells perform a wide range ofcomplex reactions/functions to maintain tissues,and for the ultimate well-being of the wholeorganism. For this purpose, the variousintracellular processes and biochemical reactionsare tightly controlled and integrated. Division ofa cell into two daughter cells is good example ofthe orderly occurrence of an integrated series ofcellular reactions.

Apoptosis is the programmed cell death orcell suicide. This occurs when the cell hasfulfilled its biological functions. Apoptosis maybe regarded as a natural cell death and it differsfrom the cell death caused by injury due toradiation, anoxia etc. Programmed cell death isa highly regulated process.

1. Life is composed of lifeless chemical molecules. The complex biomolecules, proteins,nucleic acids (DNA and RNA), polysaccharides and lipids are formed by the monomericunits amino acids, nucleotides, monosaccharides and fatty acids, respectively.

2. The cell is the structural and functional unit of life. The eukaryotic cell consists of welldefined subcellular organelles, enveloped in a plasma membrane.

3. The nucleus contains DNA, the repository of genetic information. DNA, in associationwith proteins (histones), forms nucleosomes which, in turn, make up the chromosomes.

4. The mitochondria are the centres for energy metabolism. They are the principal producersof ATP which is exported to all parts of the cell to provide energy for cellular work.

5. Endoplasmic reticulum (ER) is the network of membrane enclosed spaces that extendsthroughout the cytoplasm. ER studded with ribosomes, the factories of proteinbiosynthesis, is referred to as rough ER. Golgi apparatus are a cluster of membranevesicles to which the newly synthesized proteins are handed over for further processingand export.

6. Lysosomes are the digestive bodies of the cell, actively involved in the degradation ofcellular compounds. Peroxisomes contain the enzyme catalase that protects the cell fromthe toxic effects of H2O2. The cellular ground matrix is referred to as cytosol which, infact, is composed of a network of protein filaments, the cytoskeleton.

7. The eukaryotic cells perform a wide range of complex functions in a well coordinated andintegrated fashion. Apoptosis is the process of programmed cell death or cell suicide.

12CarbohydratesChapter

Section 1 Chemical Constituents of Life

9

The carbohydrates speak :

“We are polyhydroxyaldehydes or ketones;Classified into mono-, oligo- and polysaccharides;

Held together by glycosidic bonds;Supply energy and serve as structural constituents.”

CCCarbohydrates are the most abundant organicmolecules in nature. They are primarilycomposed of the elements carbon, hydrogen andoxygen. The name carbohydrate literally means‘hydrates of carbon’. Some of the carbohydratespossess the empirical formula (C.H2O)n wheren 3, satisfying that these carbohydrates are infact carbon hydrates. However, there are severalnon-carbohydrate compounds (e.g. acetic acid,C2H4O2; lactic acid, C3H6O3) which also appearas hydrates of carbon. Further, some of thegenuine carbohydrates (e.g. rhamnohexose,C6H12O5; deoxyribose, C5H10O4) do not satisfythe general formula. Hence carbohydrates cannotbe always considered as hydrates of carbon.

Carbohydrates may be defined aspolyhydroxyaldehydes or ketones or compoundswhich produce them on hydrolysis. The term‘sugar’ is applied to carbohydrates soluble inwater and sweet to taste.

Functions of carbohydratesCarbohydrates participate in a wide range of

functions

1. They are the most abundant dietary sourceof energy (4 Cal/g) for all organisms.

2. Carbohydrates are precursors for manyorganic compounds (fats, amino acids).

3. Carbohydrates (as glycoproteins and glyco-lipids) participate in the structure of cellmembrane and cellular functions such as cellgrowth, adhesion and fertilization.

4. They are structural components of manyorganisms. These include the fiber (cellulose) ofplants, exoskeleton of some insects and the cellwall of microorganisms.

5. Carbohydrates also serve as the storageform of energy (glycogen) to meet the immediateenergy demands of the body.

CLASSIFICATIONOF CARBOHYDRATES

Carbohydrates are often referred to assaccharides (Greek: sakcharon–sugar). Theyare broadly classified into three major groups—monosaccharides, oligosaccharides and poly-saccharides. This categorization is based on the

BIOCHEMISTRY10

TABLE 2.1 Classification of monosaccharides with selected examples

Monosaccharides (empirical formula) Aldose Ketose

Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone

Tetroses (C4H8O4) Erythrose Erythrulose

Pentoses (C5H10O5) Ribose Ribulose

Hexoses (C6H12O6) Glucose Fructose

Heptoses (C7H14O7) Glucoheptose Sedoheptulose

number of sugar units. Mono- and oligo-saccharides are sweet to taste, crystalline incharacter and soluble in water, hence they arecommonly known as sugars.

MonosaccharidesMonosaccharides (Greek : mono-one) are the

simplest group of carbohydrates and are oftenreferred to as simple sugars. They have thegeneral formula Cn(H2O)n, and they cannot befurther hydrolysed. The monosaccharides aredivided into different categories, based on thefunctional group and the number of carbon atoms

Aldoses : When the functional group in

monosaccharides is an aldehyde C O

H

, they

are known as aldoses e.g. glyceraldehyde,glucose.

Ketoses : When the functional group is a keto

C O group, they are referred to as ketoses

e.g. dihydroxyacetone, fructose.

Based on the number of carbon atoms, themonosaccharides are regarded as trioses (3C),tetroses (4C), pentoses (5C), hexoses (6C) andheptoses (7C). These terms along with functionalgroups are used while naming monosaccharides.For instance, glucose is an aldohexose whilefructose is a ketohexose (Table 2.1).

The common monosaccharides and disaccha-rides of biological importance are given in theTable 2.2.

Oligosaccharides

Oligosaccharides (Greek: oligo-few) contain2-10 monosaccharide molecules which are

liberated on hydrolysis. Based on the number ofmonosaccharide units present, the oligo-saccharides are further subdivided todisaccharides, trisaccharides etc.

Polysaccharides

Polysaccharides (Greek: poly-many) are poly-mers of monosaccharide units with high mole-cular weight (up to a million). They are usuallytasteless (non-sugars) and form colloids withwater. The polysaccharides are of two types –homopolysaccharides and heteropolysaccharides.

MONOSACCHARIDES—STRUCTURAL ASPECTS

Stereoisomerism is an important character ofmonosaccharides. Stereoisomers are thecompounds that have the same structuralformulae but differ in their spatial configuration.

A carbon is said to be asymmetric when it isattached to four different atoms or groups. Thenumber of asymmetric carbon atoms (n)determines the possible isomers of a givencompound which is equal to 2n. Glucosecontains 4 asymmetric carbons, and thus has 16isomers.

Glyceraldehyde—the reference carbohydrate

Glyceraldehyde (triose) is the simplest mono-saccharide with one asymmetric carbon atom. Itexists as two stereoisomers and has been chosenas the reference carbohydrate to represent thestructure of all other carbohydrates.

Chapter 2 : CARBOHYDRATES 11

TABLE 2.2 Monosaccharides and disaccharides of biological importance

Monosaccharides Occurrence Biochemical importance

Trioses

Glyceraldehyde Found in cells as phosphate Glyceraldehyde 3-phosphate is an intermediatein glycolysis

Dihydroxyacetone Found in cells as phosphate Its 1-phosphate is an intermediate in glycolysis

Tetroses

D-Erythrose Widespread Its 4-phosphate is an intermediate incarbohydrate metabolism

Pentoses

D-Ribose Widespread as a constituent of For the structure of RNA and nucleotideRNA and nucleotides coenzymes (ATP, NAD+, NADP+)

D-Deoxyribose As a constituent of DNA For the structure of DNA

D-Ribulose Produced during metabolism It is an important metabolite in hexosemonophosphate shunt

D-Xylose As a constituent of glycoproteins Involved in the function of glycoproteinsand gums

L-Xylulose As an intermediate in uronic acid pathway Excreted in urine in essential pentosuria

D-Lyxose Heart muscle As a constituent of lyxoflavin of heart muscle

Hexoses

D-Glucose As a constituent of polysaccharides The ‘sugar fuel’ of life; excreted in urine in(starch, glycogen, cellulose) and diabetes. Structural unit of cellulose in plantsdisaccharides (maltose, lactose,sucrose). Also found in fruits

D-Galactose As a constituent of lactose Converted to glucose, failure leads to(milk sugar) galactosemia

D-Mannose Found in plant polysaccharides For the structure of polysaccharidesand animal glycoproteins

D-Fructose Fruits and honey, as a constituent Its phosphates are intermediates of glycolysisof sucrose and inulin

Heptoses

D-Sedoheptulose Found in plants Its 7-phosphate is an intermediate in hexosemonophosphate shunt, and in photosynthesis

Disaccharides Occurrence Biochemical importance

Sucrose As a constituent of cane sugar and Most commonly used table sugar supplyingbeet sugar, pineapple calories

Lactose Milk sugar Exclusive carbohydrate source to breast fedinfants. Lactase deficiency (lactose intolerance)leads to diarrhea and flatulence

Maltose Product of starch hydrolysis, An important intermediate in the digestion ofoccurs in germinating seeds starch

BIOCHEMISTRY12

D- and L-isomers

The D and L isomers are mirror images ofeach other. The spatial orientation of H and

OH groups on the carbon atom (C5 forglucose) that is adjacent to the terminal primaryalcohol carbon determines whether the sugar isD- or L-isomer. If the OH group is on the rightside, the sugar is of D-series, and if on the leftside, it belongs to L-series. The structures ofD- and L-glucose based on the reference mono-saccharide, D- and L-glyceraldehyde (glycerose)are depicted in Fig.2.1.

It may be noted that the naturally occurringmonosaccharides in the mammalian tissues aremostly of D-configuration. The enzyme machineryof cells is specific to metabolise D-series ofmonosaccharides.

Optical activity of sugars

Optical activity is a characteristic feature ofcompounds with asymmetric carbon atom.When a beam of polarized light is passedthrough a solution of an optical isomer, it will berotated either to the right or left. The termdextrorotatory (d+) and levorotatory (l–) areused to compounds that respectively rotate theplane of polarized light to the right or to the left.

An optical isomer may be designated asD(+), D(–), L(+) and L(–) based on its structural

relation with glyceraldehyde. It may be notedthat the D- and L-configurations of sugars areprimarily based on the structure ofglyceraldehyde, the optical activities however,may be different.

Racemic mixture : If d- and l-isomers arepresent in equal concentration, it is known asracemic mixture or dl mixture. Racemic mixturedoes not exhibit any optical activity, since thedextro- and levorotatory activities cancel eachother.

In the medical practice, the term dextrose isused for glucose in solution. This is because ofthe dextrorotatory nature of glucose.

Configuration of D-aldoses

The configuration of possible D-aldosesstarting from D-glyceraldehyde is depicted inFig.2.2. This is a representation of Killiani-Fischer synthesis, by increasing the chain lengthof an aldose, by one carbon at a time. Thus,starting with an aldotriose (3C), aldotetroses (4C),aldopentoses (5C) and aldohexoses (6C) areformed. Of the 8 aldohexoses, glucose, mannoseand galactose are the most familiar. Amongthese, D-glucose is the only aldose mono-saccharide that predominantly occurs innature.

Configuration of D-ketoses

Starting from dihydroxyacetone (triose), thereare five keto-sugars which are physiologicallyimportant. Their structures are given in Fig.2.3.

Epimers

If two monosaccharides differ from eachother in their configuration around a singlespecific carbon (other than anomeric) atom, theyare referred to as epimers to each other (Fig.2.4).For instance, glucose and galactose are epimerswith regard to carbon 4 (C4-epimers). That is,they differ in the arrangement of OH group atC4. Glucose and mannose are epimers withregard to carbon 2 (C2-epimers).

The interconversion of epimers (e.g. glucoseto galactose and vice versa) is known as

Fig. 2.1 : D-and-L- forms of glucose compared withD- and L- glyceraldehydes (the reference carbohydrate).

D-Glyceraldehyde

H C O

CH2OH

H C OH

D-Glucose

H C OH

H C OH

HO C H

H C OH

H C O

CH2OH CH2OHL-Glucose

HO C H

HO C H

H C OH

HO C H

H C O

L-Glyceraldehyde

HO C H

H C O

CH2OH

Chapter 2 : CARBOHYDRATES 13

epimerization, and a group of enzymes—namely—epimerases catalyse this reaction.

Enantiomers

Enantiomers are a special type ofstereoisomers that are mirror images ofeach other. The two members are designated asD- and L-sugars. Enantiomers of glucose aredepicted in Fig.2.5.

Majority of the sugars in the higher animals(including man) are of D-type (Fig.2.5).

The term diastereomers is used to representthe stereoisomers that are not mirror images ofone another.

STRUCTURE OF GLUCOSE

For a better understanding of glucosestructure, let us consider the formation ofhemiacetals and hemiketals, respectivelyproduced when an aldehyde or a ketone reactswith alcohol.

HCOH

HOCH

CHO

CH2OHD-Xylose

HCOHHOCH

D-Arabinose

CH2OH

CHO

HCOH

HCOHHCOH

HCOH

HCOH

CHO

CH2OHD-Ribose

HCOH

D-Erythrose

CH2OH

HCOH

CHO

HOCH

CHO

HOCH

HCOH

CH2OHD-Lyxose

D-ThreoseCH2OH

HCOH

HOCH

CHO

D-GlyceraldehydeCH2OH

HCOH

CHO

Aldotriose(3C)

Aldotetroses(4C)

Aldopentoses(5C)

HCOH

HOCH

HCOH

HCOH

CHO

CH2OH

D-Gulose

HCOH

HOCH

D-Idose

CH2OH

CHO

HCOH

HOCHHOCH

HCOH

HCOH

CHO

CH2OH

D-Mannose

HOCHHOCH

HCOH

D-Glucose

CH2OH

CHO

HCOH

HCOH

HCOH

HCOHHCOH

HCOH

D-Allose

CH2OH

HCOH

CHO

HCOH HCOH

CHO

CH2OH

D-Altrose

HOCH HOCH

D-Talose

CH2OH

HOCH

CHO

HOCH

HCOHHCOH

HOCH

CHO

HCOH

HOCH

CH2OH

D-Galactose

Aldo-hexoses

(6C)

Fig. 2.2 : The structural relationship between D-aldoses shown in Fischer projection. (The configuration around C2 (red) distinguishes the members of each pair).

BIOCHEMISTRY14

HemiacetalAlcoholAldehydeOH

R1 C H

OR2+ R2 OH

O

H1R C

The hydroxyl group of monosaccharides canreact with its own aldehyde or keto functionalgroup to form hemiacetal and hemiketal. Thus,the aldehyde group of glucose at C1 reactswith alcohol group at C5 to form two typesof cyclic hemiacetals namely and , as depictedin Fig.2.6. The configuration of glucose isconveniently represented either by Fischerformulae or by Haworth projection formulae.

Pyranose and furanose structures

Haworth projection formulae are depicted bya six-membered ring pyranose (based on pyran)or a five-membered ring furanose (based onfuran). The cyclic forms of glucose are known as

-D-glucopyranose and -D-glucofuranose(Fig.2.7).

Anomers—mutarotation

The and cyclic forms of D-glucose areknown as anomers. They differ from each otherin the configuration only around C1 known asanomeric carbon (hemiacetal carbon). In case of

anomer, the OH group held by anomericcarbon is on the opposite side of the group

CH2OH of sugar ring. The reverse is true for-anomer. The anomers differ in certain physical

and chemical properties.

Mutarotation : The and anomers ofglucose have different optical rotations. Thespecific optical rotation of a freshly preparedglucose ( anomer) solution in water is +112.2°which gradually changes and attains anequilibrium with a constant value of +52.7°. Inthe presence of alkali, the decrease in opticalrotation is rapid. The optical rotation of

-glucose is +18.7°. Mutarotation is defined asthe change in the specific optical rotationrepresenting the interconversion of and

CH2OH

C O

CH2OH

Dihydroxyacetone

C O

CH2OH

HOCH

HCOH

CH2OH

D-Xylulose

CH2OH

HCOH

CH2OH

C O

HCOH

D-Ribulose

CH2OH

HCOH

CH2OH

C O

HOCH

HCOH

D-Fructose D-Sedoheptulose

HCOH

HOCH

C O

CH2OH

HCOH

CH2OH

HCOH

Fig. 2.3 : Structures of ketoses of physiological importance.

CH2OH

H C OH

HO C H

HO C H

H C OH

H C O

D-Galactose D-Glucose

2

H C OH

H C O

H C OH

HO C H

H C OH

CH OH

D-Mannose

HO C H

CH2OH

H C OH

HO C H

H C O

H C OH

22

4 4

Fig. 2.4 : Structures of epimers (glucose and galactoseare C4-epimers while glucose and mannose are

C2-epimers).

D-Glucose

H

C O

H C OH

HO C H

H C OH

H C OH

H C H

HOL-Glucose

H

O C

HO C H

H C OH

HO C H

HO C H

H C H

OH

Fig. 2.5 : Enantiomers (mirror images) of glucose.

Chapter 2 : CARBOHYDRATES 15

forms of D-glucose to an equilibrium mixture.Mutarotation depicted in Fig. 2.6, is summarizedbelow.

-D-Glucose Equilibrium mixture -D-Glucose + 112.2° + 52.7° + 18.7°

(Specific optical rotation [ ]20D )The equilibrium mixture contains 63%

-anomer and 36% -anomer of glucose with

1% open chain form. In aqueous solution, the form is more predominant due to its stableconformation. The and forms of glucose areinterconvertible which occurs through a linearform. The latter, as such, is present in aninsignificant quantity.

Mutarotation of fructose : Fructose alsoexhibits mutarotation. In case of fructose, thepyranose ring (six-membered) is converted tofuranose (five-membered) ring, till an equilibriumis attained. And fructose has a specific opticalrotation of –92° at equilibrium.

The conversion of dextrorotatory (+) sucroseto levorotatory fructose is explained underinversion of sucrose (see later in this chapter).

REACTIONS OF MONOSACCHARIDES

Tautomerization or enolization

The process of shifting a hydrogen atom fromone carbon atom to another to produce enediolsis known as tautomerization. Sugars possessinganomeric carbon atom undergo tautomerizationin alkaline solutions.

When glucose is kept in alkaline solution forseveral hours, it undergoes isomerization to form

Fig. 2.6 : Mutarotation of glucose representing and anomers (A) Fischer projections (B) Haworth projections.

Fig. 2.7 : Structure of glucose-pyranoseand furanose forms.

O

Pyran

-D-Glucopyranose

OCH2OH

H

OH

H

HO

H H

H

OH

OH

O

OH

H

OH

HOH

H

H

CH2OH

H C OH

-D-Glucofuranose

O

Furan

(A)

(B)

D-Glucose(aldehyde form)

CH2OH

H C OH5

H C OH

HO C H

H C OH

H C O1

-D-Glucose

HOC

H C5

CH2OH

H

(+ 18.7 )

1

H C OH

HO C H

H C OH

O

-D-Glucopyranose -D-Glucopyranose

(+ 112.2 )-D-Glucose

H C OH

OHH

CH2OH

H C5

HO C H

C

O

1

H C OH

OCH2OH

H

OH

H

HO

H H

H

OH

OH

OCH2OH

H

OH

H

HO

H

HH

OH

OH

D-Glucose(aldehyde form, acyclic)

OHCH2OH

H

OH

H

HO

H

H

OH

O C H

BIOCHEMISTRY16

D-fructose and D-mannose. This reaction—known as the Lobry de Bruyn-von Ekensteintransformation—results in the formation of acommon intermediate—namely enediol—for allthe three sugars, as depicted in Fig.2.8.

The enediols are highly reactive, hence sugarsin alkaline solution are powerful reducingagents.

Reducing properties

The sugars are classified as reducing or non-reducing. The reducing property is attributed tothe free aldehyde or keto group of anomericcarbon.

In the laboratory, many tests are employed toidentify the reducing action of sugars. Theseinclude Benedict’s test, Fehling’s test, Barfoed’stest etc. The reduction is much more efficientin the alkaline medium than in the acidmedium.

The enediol forms (explained above) or sugarsreduce cupric ions (Cu2+) of copper sulphateto cuprous ions (Cu+), which form a yellowprecipitate of cuprous hydroxide or ared precipitate of cuprous oxide as shownnext.

Sugar

Enediol

Sugar acid

CuSO4

2+Cu +Cu

2H2O + Cu2O 2Cu(OH)

It may be noted that the reducing property ofsugars cannot help for a specific identification ofany one sugar, since it is a general reaction.

Oxidation

Depending on the oxidizing agent used, theterminal aldehyde (or keto) or the terminalalcohol or both the groups may be oxidized. Forinstance, consider glucose :

1. Oxidation of aldehyde group (CHO COOH) results in the formation of gluconic acid.

2. Oxidation of terminal alcohol group(CH2OH COOH) leads to the production ofglucuronic acid.

ReductionWhen treated with reducing agents such as

sodium amalgam, the aldehyde or keto group ofmonosaccharide is reduced to correspondingalcohol, as indicated by the general formula :

2HH C O

R

H C OH

R

H

The important monosaccharides and theircorresponding alcohols are given below.

D-Glucose D-SorbitolD-Galactose D-DulcitolD-Mannose D-MannitolD-Fructose D-Mannitol + D-SorbitolD-Ribose D-Ribitol

Sorbitol and dulcitol when accumulate intissues in large amounts cause strong osmoticeffects leading to swelling of cells, and certainpathological conditions. e.g. cataract, peripheralneuropathy, nephropathy. Mannitol is useful toreduce intracranial tension by forced diuresis.

D-MannoseR

HO C H

HO C H

H C O

R

H C OH

HO C H

H C O

D-Glucose

H

C O

RD-Fructose

H C OH

HO C H

Enediol(common)

H C OH

C OH

HO C H

R

Fig. 2.8 : Formation of a common enediol fromglucose, fructose and mannose

(R corresponds to the end 3 carbon common structure).

Chapter 2 : CARBOHYDRATES 17

Dehydration

When treated with concentrated sulfuric acid,monosaccharides undergo dehydration with anelimination of 3 water molecules. Thus hexosesgive hydroxymethyl furfural while pentoses givefurfural on dehydration (Fig.2.9). These furfuralscan condense with phenolic compounds( -naphthol) to form coloured products. This isthe chemical basis of the popular Molisch test.In case of oligo- and polysaccharides, they arefirst hydrolysed to monosaccharides by acid, andthis is followed by dehydration.

Bial’s test : Pentoses react with strong HCl toform furfural derivatives which in turn react withorcinol to form green coloured complex. Bial’stest is useful for detection of xylose in urine inessential pentosuria.

Mucic acid test : Galactose when treated withnitric acid forms insoluble mucic acid crystals.

Osazone formation

Phenylhydrazine in acetic acid, when boiledwith reducing sugars, forms osazones in areaction summarized in Fig.2.10.

As is evident from the reaction, the first twocarbons (C1 and C2) are involved in osazoneformation. The sugars that differ in their

configuration on these two carbons give thesame type of osazones, since the difference ismasked by binding with phenylhydrazine. Thusglucose, fructose and mannose give the sametype (needle-shaped) osazones.

Reducing disaccharides also give osazones—maltose sunflower-shaped, and lactose powder-puff shaped.

Formation of esters

The alcoholic groups of monosaccharidesmay be esterified by non-enzymatic orenzymatic reactions. Esterification of carbo-hydrate with phosphoric acid is a commonreaction in metabolism. Glucose 6-phosphateand glucose 1-phosphate are good examples.ATP donates the phosphate moiety in esterformation.

GLYCOSIDES

Glycosides are formed when the hemiacetalor hemiketal hydroxyl group (of anomericcarbon) of a carbohydrate reacts with a hydroxylgroup of another carbohydrate or a non-carbohydrate (e.g. methyl alcohol, phenol,glycerol). The bond so formed is known asglycosidic bond and the non-carbohydratemoiety (when present) is referred to as aglycone.

Fig. 2.9 : Dehydration of monosaccharideswith concentrated H2SO4.

Glucose

+

Phenylhydrazine

H2N NH C6H5H C O

H C OH

R

GlucohydrazoneR

N NH C6H5H C

H C OH

RGlucosazone

H2N NH C6H5

N NH C6H5H C

N NH C6H5C

Fig. 2.10 : A summary of osazone formation(R represents C3 to C6 of glucose).

Conc. H2SO4

3H2O

Hydroxymethyl furfuralD-Glucose

H C O

H C OH

HO C H

H C OH

H C OH

CH2OH

H C O

C

H C

H C

C

CH2OH

O

3H2O

Conc. H2SO4

H C O

H C OH

H C OH

CH2OH

H C OH

D-Ribose

H C O

C

H C

H CO

H CFurfural

BIOCHEMISTRY18

The monosaccharides are held together byglycosidic bonds to result in di-, oligo- orpolysaccharides (see later for structures).

Naming of glycosidic bond : Thenomenclature of glycosidic bonds is based onthe linkages between the carbon atoms and thestatus of the anomeric carbon ( or ). Forinstance, lactose—which is formed by a bondbetween C1 of -galactose and C4 of glucose—is named as (1 4) glycosidic bond. The otherglycosidic bonds are described in the structureof di- and polysaccharides.

Physiologically important glycosides

1. Glucovanillin (vanillin-D-glucoside) is anatural substance that imparts vanilla flavour.

2. Cardiac glycosides (steroidal glycosides) :Digoxin and digitoxin contain the aglyconesteroid and they stimulate muscle contraction.

3. Streptomycin, an antibiotic used in thetreatment of tuberculosis is a glycoside.

4. Ouabain inhibits Na+ – K+ ATPase andblocks the active transport of Na+.

5. Phlorhizin produces renal damage inexperimental animals.

DERIVATIVES OF MONOSACCHARIDESThere are several derivatives of monosaccha-

rides, some of which are physiologicallyimportant (Fig.2.11)

1. Sugar acids : Oxidation of aldehyde orprimary alcohol group in monosaccharide resultsin sugar acids. Gluconic acid is produced fromglucose by oxidation of aldehyde (C1 group)whereas glucuronic acid is formed when primaryalcohol group (C6) is oxidized.

2. Sugar alcohols (polyols) : They areproduced by reduction of aldoses or ketoses. Forinstance, sorbitol is formed from glucose andmannitol from mannose.

3. Alditols : The monosaccharides, onreduction, yield polyhydroxy alcohols, known asalditols. Ribitol is a constituent of flavincoenzymes; glycerol and myo-inositol arecomponents of lipids. Xylitol is a sweetener usedin sugarless gums and candies.

4. Amino sugars : When one or morehydroxyl groups of the monosaccharides arereplaced by amino groups, the products

formed are amino sugars e.g. D-glucosamine,D-galactosamine. They are present as consti-tuents of heteropolysaccharides.

N-Acetylneuraminic acid (NANA) is aderivative of N-acetylmannose and pyruvic acid.It is an important constituent of glycoproteinsand glycolipids. The term sialic acid is used toinclude NANA and its other derivatives.

Certain antibiotics contain amino sugarswhich may be involved in the antibiotic activitye.g. erythromycin.

5. Deoxysugars : These are the sugars thatcontain one oxygen less than that present in theparent molecule. The groups CHOH and

CH2OH become CH2 and CH3 due to theabsence of oxygen. D-2-Deoxyribose is the mostimportant deoxysugar since it is a structuralconstituent of DNA (in contrast to D-ribose inRNA). Feulgen staining can specifically detectdeoxyribose, and thus DNA in tissues. Fucose isa deoxy L-galactose found in blood groupantigens, and certain glycoproteins.

6. L-Ascorbic acid (vitamin C) : This is awater-soluble vitamin, the structure of whichclosely resembles that of a monosaccharide.

DISACCHARIDES

Among the oligosaccharides, disaccharidesare the most common (Fig.2.12). As is evidentfrom the name, a disaccharide consists of twomonosaccharide units (similar or dissimilar) heldtogether by a glycosidic bond. They arecrystalline, water-soluble and sweet to taste. Thedisaccharides are of two types

1. Reducing disaccharides with free aldehydeor keto group e.g. maltose, lactose.

2. Non-reducing disaccharides with no freealdehyde or keto group e.g. sucrose, trehalose.

Maltose

Maltose is composed of two -D-glucoseunits held together by (1 4) glycosidic bond.The free aldehyde group present on C1 of secondglucose answers the reducing reactions, besides

Chapter 2 : CARBOHYDRATES 19

the osazone formations (sunflower-shaped).Maltose can be hydrolysed by dilute acid or theenzyme maltase to liberate two molecules of

-D-glucose.

In isomaltose, the glucose units are heldtogether by (1 6) glycosidic linkage.

Cellobiose is another disaccharide, identicalin structure with maltose, except that the formerhas (1 4) glycosidic linkage. Cellobiose isformed during the hydrolysis of cellulose.

Sucrose

Sucrose (cane sugar) is the sugar of commerce,mostly produced by sugar cane and sugar beets.Sucrose is made up of -D-glucose and -D-fructose. The two monosaccharides are heldtogether by a glycosidic bond ( 1 2), betweenC1 of -glucose and C2 of -fructose. Thereducing groups of glucose and fructose areinvolved in glycosidic bond, hence sucrose is anon-reducing sugar, and it cannot form osazones.

Sucrose is an important source of dietarycarbohydrate. It is sweeter than most othercommon sugars (except fructose) namely glucose,lactose and maltose. Sucrose is employed as asweetening agent in food industry. The intestinalenzyme—sucrase—hydrolyses sucrose to glucoseand fructose which are absorbed.

Inversion of sucroseSucrose, as such is dextrorotatory (+66.5°).

But, when hydrolysed, sucrose becomeslevorotatory (–28.2°). The process of changein optical rotation from dextrorotatory (+)to levorotatory (–) is referred to as inversion.The hydrolysed mixture of sucrose, containingglucose and fructose, is known as invert sugar.The process of inversion is explained below.

Hydrolysis of sucrose by the enzyme sucrase(invertase) or dilute acid liberates one moleculeeach of glucose and fructose. It is postulated thatsucrose (dextro) is first split into -D-glucopyranose (+52.5°) and -D-fructofuranose,both being dextrorotatory. However, -D-fructofuranose is less stable and immediately getsconverted to -D-fructopyranose which isstrongly levorotatory (–92°). The overall effect isthat dextro sucrose (+66.5°) on inversion isconverted to levo form (–28.2°).

LactoseLactose is more commonly known as milk

sugar since it is the disaccharide found in milk.Lactose is composed of -D-galactose and -D-glucose held together by (1 4) glycosidicbond. The anomeric carbon of C1 glucose is free,hence lactose exhibits reducing properties andforms osazones (powder-puff or hedgehog shape).

Fig. 2.11 : Structures of monosaccharide derivatives (selected examples).

D-Glucuronic acid

HO C H

H C OH

H C O

H C OH

H C OH

COOHGlycerol

CH2OH

CH2OH

H C OH

N-Acetylneuraminic acid

H

O

HO

H HH OH

H3C C HN

O

COO–

CH2OH

H OHH OH

HD-Glucosamine

OCH2OH

H

OH

H

HO

H H

H OH

NH2D-2-Deoxyribose

OHOCH2

HH

OH H

HH

OH

myo -Inositol

OH

H

OH

H

HO

H

HH

OH

OH

OH

H

BIOCHEMISTRY20

Lactose of milk is the most importantcarbohydrate in the nutrition of young mammals.It is hydrolysed by the intestinal enzyme lactaseto glucose and galactose.

LactuloseLactulose is a synthetic dissccharide containing

galactose and fructose. It is neither digested norabsorbed in the inestine. Lactulose is useful forthe treatment of hepatic encephalopathy, adisorder characterized by elevated plasmaammonium levels. Lactulose converts ammonia(NH3) in the lumen to ammonium ion (NH4

+). Thisresults in a reduction in the plasma NH3, sinceNH4

+ ions are not easily absorbed.

POLYSACCHARIDES

Polysaccharides (or simply glycans) consist ofrepeat units of monosaccharides or theirderivatives, held together by glycosidic bonds.They are primarily concerned with two importantfunctions-structural, and storage of energy.

Polysaccharides are linear as well asbranched polymers. This is in contrast tostructure of proteins and nucleic acids which areonly linear polymers. The occurrence ofbranches in polysaccharides is due to the factthat glycosidic linkages can be formed at anyone of the hydroxyl groups of a monosaccharide.

Polysaccharides are of two types

1. Homopolysaccharides on hydrolysis yieldonly a single type of monosaccharide. Theyare named based on the nature of themonosaccharide. Thus, glucans are polymers ofglucose whereas fructosans are polymers offructose.

2. Heteropolysaccharides on hydrolysis yielda mixture of a few monosaccharides or theirderivatives.

HOMOPOLYSACCHARIDES

StarchStarch is the carbohydrate reserve of plants

which is the most important dietary source forhigher animals, including man. High content ofstarch is found in cereals, roots, tubers, vegetablesetc. Starch is a homopolymer composed ofD-glucose units held by -glycosidic bonds. It isknown as glucosan or glucan.

Starch consists of two polysaccharidecomponents-water soluble amylose (15-20%)and a water insoluble amylopectin (80-85%).Chemically, amylose is a long unbranchedchain with 200–1,000 D-glucose units held by (1 4) glycosidic linkages. Amylopectin, on theother hand, is a branched chain with (1 6)glycosidic bonds at the branching points and (1 4) linkages everywhere else (Fig.2.13).Amylopectin molecule containing a fewthousand glucose units looks like a branchedtree (20–30 glucose units per branch).

Fig. 2.12 : Structures of disaccharides—maltose, sucrose and lactose.

O

CH2OH

H

H

OH

HO

H

OCH2OH

H

OH

H

HO

H H

H

OH

1 2O

HOH2C

Glucose FructoseSucrose

( -D-glucosyl (1 2) -D-fructose)

OCH2OH

H

OH

H

HO

H H

H

OH

1O

OCH2OH

H

OH

H

H H

H

OH

OH4

Glucose GlucoseMaltose

( -D-glucosyl (1 4) -D-glucose)

OCH2OH

H

OH

H

HO

H HH

OH

1 O

OCH2OH

H

OH

H

H

HH

OH

OH4

Galactose GlucoseLactose

( -D-galactosyl (1 4) -D-glucose)

Chapter 2 : CARBOHYDRATES 21

Starches are hydrolysed by amylase(pancreatic or salivary) to liberate dextrins, andfinally maltose and glucose units. Amylase actsspecifically on (1 4) glycosidic bonds.

Dextrins

Dextrins are the breakdown products ofstarch by the enzyme amylase or dilute acids.Starch is sequentially hydrolysed throughdifferent dextrins and, finally, to maltose andglucose. The various intermediates (identified byiodine colouration) are soluble starch (blue),amylodextrin (violet), erythrodextrin (red) andachrodextrin (no colour).

Dextrans

Dextrans are polymers of glucose, producedby microorganisms. They are used as plasmavolume expanders in transfusion, andchromatography (e.g. gel filtration).

InulinInulin is a polymer of fructose i.e., fructosan.

It occurs in dahlia bulbs, garlic, onion etc. It isa low molecular weight (around 5,000) poly-saccharide easily soluble in water. Inulin is notutilized by the body. It is used for assessingkidney function through measurement ofglomerular filtration rate (GFR).

Glycogen

Glycogen is the carbohydrate reserve inanimals, hence often referred to as animal starch.It is present in high concentration in liver,followed by muscle, brain etc. Glycogen is alsofound in plants that do not possess chlorophyll(e.g. yeast, fungi).

The structure of glycogen is similar to that ofamylopectin with more number of branches.Glucose is the repeating unit in glycogen joinedtogether by (1 4) glycosidic bonds, and

Fig. 2.13 : Structure of starch ( -amylose and amylopectin).

OCH2OH

H

OH

H

H H

H

OH

1O

OCH2OH

H

OH

H

H H

H

OH

4

D-Glucose D-Glucose

O

-Amylose

n

OCH2OH

H

OH

H

H H

H

OH

1O

OCH2OH

H

OH

H

H H

H

OH

4O

1

Branch

OCH2OH

H

OH

H

H H

H

OH

O

OCH2

H

OH

H

H H

H

OH

6

O

O

O

O

OH

H

H

OH

H

CH2OH

H

H

O

Amylopectin

(1 6) Branch

Main chain

BIOCHEMISTRY22

(1 6) glycosidic bonds at branching points(Fig.2.14). The molecular weight (up to 1 108)and the number of glucose units (up to 25,000)vary in glycogen depending on the source fromwhich glycogen is obtained.

Cellulose

Cellulose occurs exclusively in plants and it isthe most abundant organic substance in plantkingdom. It is a predominant constituent ofplant cell wall. Cellulose is totally absent inanimal body.

Cellulose is composed of -D-glucose unitslinked by (1 4) glycosidic bonds (Fig.2.15).Cellulose cannot be digested by mammals—including man—due to lack of the enzyme thatcleaves -glycosidic bonds ( amylase breaks bonds only). Certain ruminants and herbivorousanimals contain microorganisms in the gut whichproduce enzymes that can cleave -glycosidicbonds. Hydrolysis of cellulose yields adisaccharide cellobiose, followed by -D-glucose.

Fig. 2.14 : Structure of glycogen (A) General structure(B) Enlarged at a branch point.

Cellulose, though not digested, has greatimportance in human nutrition. It is a majorconstituent of fiber, the non-digestable carbo-hydrate. The functions of dietary fiber includedecreasing the absorption of glucose andcholesterol from the intestine, besides increasingthe bulk of feces. (For details, Chapter 23)

ChitinChitin is composed of N-acetyl D-

glucosamine units held together by (1 4)glycosidic bonds. It is a structural polysaccharidefound in the exoskeleton of some invertebratese.g. insects, crustaceans.

HETEROPOLYSACCHARIDES

When the polysaccharides are composed ofdifferent types of sugars or their derivatives, theyare referred to as heteropolysaccharides orheteroglycans.

MUCOPOLYSACCHARIDESMucopolysaccharides are heteroglycans made

up of repeating units of sugar derivatives, namelyamino sugars and uronic acids. These are morecommonly known as glycosaminoglycans(GAG). Acetylated amino groups, besides sulfateand carboxyl groups are generally present inGAG structure. The presence of sulfate andcarboxyl groups contributes to acidity of themolecules, making them acid mucopoly-saccharides.

Some of the mucopolysaccharides are foundin combination with proteins to formmucoproteins or mucoids or proteoglycans(Fig.2.16). Mucoproteins may contain up to 95%carbohydrate and 5% protein.

Fig. 2.15 : Structure of cellulose (The repeating unit ‘n’may be several thousands).

(A)

(B) O

O

OCH2OH

1O

4

OCH2

1O

4

OCH2OH

1O

4O

O

CH

2OH

O

CH

2OH

6

4

1

1

OCH2OH

H

OH

H

H

HH

OH

1 O

OCH2OH

H

OH

H

H

HH

OH

4 O

n-D-Glucose -D-Glucose

Chapter 2 : CARBOHYDRATES 23

Mucopolysaccharides are essential componentsof tissue structure. The extracellular spaces oftissue (particularly connective tissue-cartilage,skin, blood vessels, tendons) consist of collagenand elastin fibers embedded in a matrix or groundsubstance. The ground substance is predominantlycomposed of GAG.

The important mucopolysaccharides includehyaluronic acid, chondroitin 4-sulfate, heparin,dermatan sulfate and keratan sulfate (Fig.2.17).

Hyaluronic acid

Hyaluronic acid is an important GAG foundin the ground substance of synovial fluid of jointsand vitreous humor of eyes. It is also present asa ground substance in connective tissues, andforms a gel around the ovum. Hyaluronic acidserves as a lubricant and shock absorbant injoints.

Fig. 2.16 : Diagrammatic representation of aproteoglycan complex.

+ Glucose is the most important energy source of carbohydrates to the mammals (exceptruminants). The bulk of dietary carbohydrate (starch) is digested and finally absorbed asglucose into the body.

+ Dextrose (glucose in solution in dextrorotatory form) is frequently used in medicalpractice.

+ Fructose is abundantly found in the semen which is utilized by the sperms for energy.

+ Several diseases are associated with carbohydrates e.g., diabetes mellitus, glycogenstorage diseases, galactosemia.

+ Accumulation of sorbitol and dulcitol in the tissues may cause certain pathologicalconditions e.g. cataract, nephropathy.

+ Inulin, a polymer of fructose, is used to assess renal function by measuring glomerularfiltration rate (GFR).

+ The non-digestible carbohydrate cellulose plays a significant role in human nutrition.These include decreasing the intestinal absorption of glucose and cholesterol, andincreasing bulk of feces to avoid constipation.

+ The mucopolysaccharide hyaluronic acid serves as a lubricant and shock absorbant injoints.

+ The enzyme hyaluronidase of semen degrades the gel (contains hyaluronic acid) aroundthe ovum. This allows effective penetration of sperm into the ovum.

+ The mucopolysaccharide heparin is an anticoagulant (prevents blood clotting).

+ The survival of Antarctic fish below –2°C is attributed to the antifreeze glycoproteins.

+ Streptomycin is a glycoside employed in the treatment of tuberculosis.

Hyaluronic acid

Link protein

Core proteinChondroitin

sulfate

Keratan sulfate

BIOCHEMISTRY24

Hyaluronic acid is composed of alternateunits of D-glucuronic acid and N-acetylD-glucosamine. These two molecules formdisaccharide units held together by (1 3)glycosidic bond (Fig.2.16). Hyaluronic acidcontains about 250–25,000 disaccharide units(held by 1 4 bonds) with a molecular weightup to 4 million.

Hyaluronidase is an enzyme that breaks( 1 4 linkages) hyaluronic acid and otherGAG. This enzyme is present in highconcentration in testes, seminal fluid, and incertain snake and insect venoms. Hyaluronidaseof semen is assigned an important role infertilization as this enzyme clears the gel(hyaluronic acid) around the ovum allowing abetter penetration of sperm into the ovum.Hyaluronidase of bacteria helps their invasioninto the animal tissues.

Chondroitin sulfates

Chondroitin 4-sulfate (Greek : chondros-cartilage) is a major constituent of variousmammalian tissues (bone, cartilage, tendons,heart, valves, skin, cornea etc.). Structurally, it iscomparable with hyaluronic acid. Chondroitin4-sulfate consists of repeating disaccharide unitscomposed of D-glucuronic acid and N-acetylD-galactosamine 4-sulfate (Fig.2.17).

Heparin

Heparin is an anticoagulant (prevents bloodclotting) that occurs in blood, lung, liver, kidney,spleen etc. Heparin helps in the release of theenzyme lipoprotein lipase which helps inclearing the turbidity of lipemic plasma.

Heparin is composed of alternating units ofN-sulfo D-glucosamine 6-sulfate and glucuronate2-sulfate (Fig.2.17).

Dermatan sulfate

Mostly found in skin, dermatan sulfate isstructurally related to chondroitin 4-sulfate. Theonly difference is that there is an inversion in theconfiguration around C5 of D-glucuronic acid toform L-iduronic acid (Fig.2.17).

Fig. 2.17 : Structures of common glycosaminoglycans –the disaccharides as repeating units.

O OH

H

HH

COO–

H O O

O

CH2 O SO3

HO

H

H

HH

NH SO3 n

1 4H

–

H

–

OH

O SO3–

Heparin

D-Glucuronate-2-sulfate N-Sulfoglucosamine6-sulfate

n

Hyaluronic acid

OOH

H

H

OH

H

COO–

H O O

O

CH2OH

H

HO HO

H

H

HH

NH CO CH3

1

D-Glucuronic acid N-Acetylglucosamine

3

n

OOH

H

H

OH

H

COO–

H O O

O

CH2OH

O

HO

H

H

HH

NH CO CH3

13H

SO3–

Chondroitin 4-sulfate

D-Glucuronic acid N-Acetylgalactosamine4-sulfate

n

Dermatan sulfate

OH

H

H

OH

H

COO–H O O

O

CH2OH

O

HO

H

H

HH

NH CO CH3

3H

SO3–

N-Acetylgalactosamine4-sulfate

L-Iduronic acid

nH

H

OH

H

O O

O

CH2OH

HO

H

H

HH

NH CO CH3

HHOH

CH2 O SO3–

N-Acetylglucosamine6-sulfate

D-Galactose

Keratan sulfate

4 1

4 1

Chapter 2 : CARBOHYDRATES 25

TABLE 2.3 A summary of glycosaminoglycans – composition, distribution and functions

Glycosaminoglycan Composition Tissue distribution Function(s)

Hyaluronic acid D-Glucuronic acid, Connective tissue, synovial fluid, Serves as a lubricant, andN-acetylglucosamine vitrous humor shock absorber. Promotes

wound healing

Chondroitin sulfate D-Glucuronic acid, Cartilage, bone, skin, blood vessel Helps to maintain the structureN-acetylgalactosamine walls and shapes of tissues4-sulfate

Heparin D-Glucuronate 2-sulfate, Blood, lung, liver, kidney, spleen Acts as an anticoagulantN-sulfoglucosamine6-sulfate

Dermatan sulfate L-Iduronic acid, N-acetyl- Blood vessel valves, heart valves, Maintains the shapes of tissuesgalactosamine 4-sulfate skin

Keratan sulfate D-Galactose, N-acetyl- Cartilage, cornea, connective Keeps cornea transparentglucosamine 6-sulfate tissues

Keratan sulfate

It is a heterogeneous GAG with a variablesulfate content, besides small amounts ofmannose, fructose, sialic acid etc. Keratansulfate essentially consists of alternating units ofD-galactosamine and N-acetylglucosamine6-sulfate.

AGAR AND PECTINS

Agar, mostly found in sea weeds, is a polymerof galactose sulfate and glucose. Since agar isnot digested, it serves as a dietary fiber (ReferChapter 23). Agarose (with galactose andanhydrogalactose) is useful in the laboratory as amajor component of microbial culture media,and in electrophoresis.

Pectins, found in apples and citrus fruits,contain galactouronate and rhamnose. Pectins,being non-digestible, are useful as dietary fiber.They are also employed in the preparation ofjellies.

GLYCOPROTEINS

Several proteins are covalently bound tocarbohydrates which are referred to as glyco-proteins. The carbohydrate content ofglycoprotein varies from 1% to 90% by weight.

Sometimes the term mucoprotein is used forglycoprotein with carbohydrate concentrationmore than 4%. Glycoproteins are very widelydistributed in the cells and perform variety offunctions. These include their role as enzymes,hormones, transport proteins, structural proteinsand receptors. A selected list of glycoproteinsand their major functions is given in Table 2.4.

The carbohydrates found in glycoproteinsinclude mannose, galactose, N-acetyl-glucosamine, N-acetylgalactosamine, xylose,

TABLE 2.4 A selected list of glycoproteins and

their major functions

Glycoprotein(s) Major function(s)

Collagen Structure

Hydrolases, proteases, Enzymesglycosidases

Ceruloplasmin Transport

Immunoglobulins Defense against infection

Synovial glycoproteins Lubrication

Thyrotropin, erythropoietin Hormones

Blood group substances Antigens

Fibronectin, laminin Cell-cell recognition andadhesion

Intrinsic factor Absorption of vitamin B12Fibrinogen Blood clotting

BIOCHEMISTRY26

L-fucose and N-acetylneuraminic acid (NANA).NANA is an important sialic acid (See Fig.2.11).

Antifreeze glycoproteins : The Antarctic fishlive below –2°C, a temperature at which theblood would freeze. It is now known that thesefish contain antifreeze glycoprotein which lowerthe freezing point of water and interfere with thecrystal formation of ice. Antifreeze glycoproteinsconsist of 50 repeating units of the tripeptide,alanine-alanine-threonine. Each threonine

1. Carbohydrates are the polyhydroxyaldehydes or ketones, or compounds which producethem on hydrolysis. The term sugar is applied to carbohydrates soluble in water andsweet to taste. Carbohydrates are the major dietary energy sources, besides theirinvolvement in cell structure and various other functions.

2. Carbohydrates are broadly classified into 3 groups—monosaccharides, oligosaccharidesand polysaccharides. The monosaccharides are further divided into different categoriesbased on the presence of functional groups (aldoses or ketoses) and the number ofcarbon atoms (trioses, tetroses, pentoses, hexoses and heptoses).

3. Glyceraldehyde (triose) is the simplest carbohydrate and is chosen as a reference towrite the configuration of all other monosaccharides (D- and L- forms). If twomonosaccharides differ in their structure around a single carbon atom, they are knownas epimers. Glucose and galactose are C4 – epimers.

4. D-Glucose is the most important naturally occurring aldose/monosaccharide.Glucose exists as and anomers with different optical rotations. The interconversionof and anomeric forms with change in the optical rotation is known as mutarotation.

5. Monosaccharides participate in several reactions. These include oxidation, reduction,dehydration, osazone formation etc. Formation of esters and glycosides bymonosaccharides is of special significance in biochemical reactions.

6. Among the oligosaccharides, disaccharides are the most common. These include thereducing disaccharides namely lactose (milk sugar) and maltose (malt sugar) and thenon-reducing sucrose (cane sugar).

7. Polysaccharides are the polymers of monosaccharides or their derivatives, held togetherby glycosidic bonds. Homopolysaccharides are composed of a single monosaccharide(e.g., starch, glycogen, cellulose, inulin). Heteropolysaccharides contain a mixture offew monosaccharides or their derivatives (e.g., mucopolysaccharides).

8. Starch and glycogen are the carbohydrate reserves of plants and animals respectively.Cellulose, exclusively found in plants, is the structural constituent. Inulin is utilized toassess kidney function by measuring glomerular filtration rate (GFR).

9. Mucopolysaccharides (glycosaminoglycans) are the essential components of tissuestructure. They provide the matrix or ground substance of extracellular tissue spaces inwhich collagen and elastin fibers are embedded. Hyaluronic acid, chondroitin 4-sulfate,heparin, are among the important glycosaminoglycans.

10. Glycoproteins are a group of biochemically important compounds with a variablecomposition of carbohydrate (1-90%), covalently bound to protein. Several enzymes,hormones, structural proteins and cellular receptors are in fact glycoproteins.

residue is bound to -galactosyl (1 3) N-acetylgalactosamine.

Blood group substancesThe blood group antigens (of erythrocyte

membrane) contain carbohydrates as glyco-proteins or glycolipids. N-Acetylgalactosamine,galactose, fucose, sialic acid etc. are found inthe blood group substances. The carbohydratecontent also plays a determinant role in bloodgrouping.

Chapter 2 : CARBOHYDRAT