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Section 18.2

Occurrence and Functions of Carbohydrates

Carbohydrates

• The most abundant class of

bioorganic molecules on earth

• produced by the

photosynthetic activity of the

green plants

• also referred to as saccharides

because of the sweet taste of

many carbohydrates

• (Latin, saccharum, meaning

sugar)

• storehouse of chemical energy

(glucose, starch, glycogen)

– a gram of digested

carbohydrate gives about 4

kcal of energy

– complex carbohydrates are

best for diet

• supportive structural components

in plants and some animals

(cellulose, chitin)

• form part of the structural

framework of DNA & RNA

• carbohydrate “markers” on cell

surfaces play key roles in cell-cell

recognition processes.

Section 18.3

Classification of Carbohydrates

• Simpler Formula:

CnH2nOn– or Cn(H2O)n

(hydrates of C)

– n= number of atoms

• polyhydroxy aldehydes or

polyhydroxy ketones or

compounds that produce such

substances upon hydrolysis.

• Classification based on

products of acid hydrolysis:

• Monosaccharides

– the simple sugars

– contain a single polyhydroxy

aldehyde or polyhydroxy ketone

unit

– cannot be degraded into simpler

products by hydrolysis reactions

– pure monosaccharides are water-

soluble, white, crystalline solids

• Disaccharides

– contains 2 monosaccharide units

covalently bonded to each other

– crystalline and water soluble

substances

– upon hydrolysis they produce

monosaccharides

Section 18.3

Classification of Carbohydrates

• Oligosaccharides

– contains 2-10 monosaccharide

units - covalently bonded

– disaccharides are the most

common type

– trisaccharides (raffinose)

– tetrasaccharides (stachyose)

– free oligosaccharides, other than

disaccharides, are less common

in nature

– usually found associated with

proteins and lipids in complex

molecules that serve structural

and regulatory functions

• Polysaccharides

– consist of tens of thousands of

monosaccharide units covalently

bonded

– homopolysacchrides – polymers

of a single monosaccharide

(glycogen, cellulose, starch)

– heteropolysaccharides – contain

more than one kind of

monosaccharide (hyaluronic acid,

heparin, chondroitin sulfate)

• Derived carbohydrates

– those where carbohydrate

moieties have undergone some

reactions converting them into

other products

– sugar acids, sugar alcohols,

deoxysugars, and sugar amines

Section 18.8

Classification of Monosaccharides

• carbohydrates that have the

general formula CnH2nOn

– n varies from 3 – 8.

• grouped together according to

the number of carbons they

contain

– C3H6O3

– C4H8O4

– C5H10O5

– C6H12O6

– C7H14O7

– C8H16O8

triose

tetrose

pentose

hexose

heptose

octose

• may either be:

– an aldose – contains aldehyde

group

– a ketose – contains ketone group

-presence of a ketone group isusually indicated by using the

ending “ ulose” in naming the

sugar

- e.g., levulose

Section 18.8

Classification of Monosaccharides

Exercise

Classify each of the following monosaccharides according

to both the number of carbon atoms and the type of

carbonyl group present.

Answers:

a. ____________

b. ____________

c. ____________

d. ____________

Section 18.8

Classification of Monosaccharides

• Trioses

• the parent member of the

family of monosaccharides

• from them emanates the other

members of the

monosaccharide family.

• the final form of carbohydrate

into which all carbohydrates,

regardless of their complexity,

are degraded in the body

during carbohydrate

metabolism.

• glyceraldehyde

– an aldotriose

• Dihydroxyacetone

– a ketotriose

• Pentoses

• aldopentoses

– lyxose

• a constituent of the heart

muscle

– ribose

• ribose and 2-deoxyribose –

present as intermediates in

metabolic pathways and areblocksimportant building

of RNA and DNA

• ketopentoses

– ribulose

– xylulose

Section 18.8

Classification of Monosaccharides

• Hexoses

• the most common of all the

monosaccharides

• aldohexoses

– mannose

– glucose• A 5% (m/v) glucose solution

is often used in hospitals as

an intravenous source of

nourishment for patients who

cannot take food by mouth.

– galactose

• ketohexose

– fructose

• Mannose

– found in certain bacteria, fungi,

and plants

– converted to usable glucose in the

body, but has no real

physiological significance

Section 18.8

Classification of Monosaccharides

• Monosaccharides can be

classified based on their spatial

orientation (stereochemistry).

• A monosaccharide can be

classified as a D or L isomer,

depending on the spatial

orientation of the –H and –OH

groups attached to the carbon

atom adjacent to the terminal

primary alcohol group.

• The D isomer is represented when

the –OH is written to the right of

this carbon in the Fischer

projection formula. The L isomer is

represented when this –OH is

written to the left.

Section 18.9

Biochemically Important Monosaccharides

• Most abundant in nature

• Nutritionally most important

• Grape good source of glucose (20 - 30% by

mass)

• also named grape sugar, dextrose and

blood sugar (70 - 100 mg/100 mL of blood)

• Six membered cyclic form

Section 18.9

Biochemically Important Monosaccharides

• Ketohexose

• Sweetest of all sugars; the fruit

sugar

• Found in many fruits and in honey

• Good dietary sugar-- due to higher

sweetness

• Five membered cyclic form

Section 18.9

Biochemically Important Monosaccharides

Galactose and Ribose

1. A component of milk sugar

2. Synthesized in human

3. Also called brain sugar-- part of brain and

nerve tissue

4. Used to differentiate between blood types

5. Six membered cyclic form

6. Galactosemia

- a result of genetic deficiency in the infant – the gene responsible for

the enzyme that converts D-galactose to D-glucose. Such infants cannot

metabolize galactose and it builds up in the blood and tissue.

1. Part of RNA

2. Part ofATP

3. Part of DNA

4. Five membered cyclic form

Section 18.10

Cyclic Forms of Monosaccharides

Hemiacetals and Hemiketals

• The dominant form of

monosaccharides with 5 or more

C atoms is cyclic

• Hemiacetals and hemiketals are

formed from the reaction between

two functional groups: aldehyde or

ketone and alcohol

– may take place either

intermolecularly or

intramolecularly as in the case of

sugars, provided there are

sufficient number of carbons

between the aldehyde or ketone

and the alcohol group to permit a

stable ring formation

– five- or six-membered hemiacetal

rings are stable

• Two types of ring structures are

possible:– five-membered ring, or furanose ring,

derived from parent compound furan

– six-membered ring, or pyranose ring,

derived from parent compoundpyran

Section 18.10

Cyclic Forms of Monosaccharides

Cyclic Hemiacetal Forms of D-Glucose

• In the cyclic hemiacetals of

glucose, C1*, is now a chiral

center (an anomeric carbon)

– two anomers of D-glucose: -D-

glucose & -D-glucose

• The cyclic hemiacetals are

readily interconvertible in

aqueous solution

– this intercoversion of - and -

anomers in solution is

accompanied by a change in

specific rotation called

MUTAROTATION.

– only sugars that form hemiacetal

or hemiketal structure mutarotate.

Section 18.10

Cyclic Forms of Monosaccharides

• 2 anomeric forms of D-

glucose:

– Alpha-form: -OH of C1 and

CH2OH of C5 are on

opposite sides

– Beta-form: -OH of C1 and

CH2OH of C5 are on same

sides

• Anomers: Cyclic

monosaccharides that differ

only in the position of the

substituents on the anomeric

carbon atom.

• Any —OH group at a chiral center

that is to the right in a Fischer

projection formula points down in

the Haworth projection formula

and any —OH group to the left in

a Fischer projection formula points

up in the Haworth projection

formula.

O

OH OH

O

OH

OH

OH

-D-Glucose

OH

-D-Glucose

2

3

4

5

OH

6CH2OH

1

2

3

4

5

OH

6CH2OH Anomeric

Carbon

1

Anomeric Carbon

Section 18.10

Cyclic Forms of Monosaccharides

• All aldoses with five or more

carbon atoms establish similar

equilibria, but with different

percentages of the alpha, beta,

and open-chain forms

• Fructose and other ketoses

with a sufficient number of

carbon atoms also cyclize

Section 18.11

Haworth Projection Formulas

Practice Exercise

• Which of the monosaccharides glucose, fructose, galactose, and

ribose has each of the following structural characteristics? (There

may be more than one correct answer for a given characteristic)

a. It is a pentose.

b. It is a ketose.

c. Its cyclic form has a 6-membered ring.

d. Its cyclic form has two carbon atoms outside the ring.

Answers:

a. _________

b. _________

c. _________d. _________

Section 18.12

Reactions of Monosaccharides

• Five important reactions of monosaccharides:

– Oxidation to acidic sugars

– Reduction to sugar alcohols

– Phosphate ester formation

– Amino sugar formation

– Glycoside formation

• These reactions will be considered with respect to

glucose; other aldoses, as well as ketoses, undergo

similar reactions.

Section 18.12

Reactions of Monosaccharides

Oxidation

• Gives three different types of

acidic sugars depending on the

type of oxidizing agent used:

– Weak oxidizing agents like

Tollens and Benedict’s solutions

oxidize the aldehyde end to give

an aldonic acid.

– Strong oxidizing agents can

oxidize both ends of a

monosaccharide at the same time

to produce aldaric acid.

– In biochemical systems enzymes

can oxidize the primary alcohol

end of an aldose such as

glucose, without oxidation of the

aldehyde group, to produce an

alduronic acid.

Section 18.12

Reactions of Monosaccharides

Reduction

• The carbonyl group in a

monosaccharide (either an

aldose or a ketose) is reduced

to a hydroxyl group using

hydrogen as the reducing

agent.

– product is the

corresponding polyhydroxy

alcohol, sugar alcohol

– e.g., Sorbitol (glucitol) -

used as moisturizing

agents in foods and

cosmetics and as a

sweetening agent in

chewing gum

Section 18.12

Reactions of Monosaccharides

Redox Reactions of Monosaccharides

Copyright © Cengage Learning. All rights reserved 21

• Under prescribed conditions,

some sugars reduce silver ions

to free silver and copper(II)

ions to copper(I) ions. Such

sugars are called reducing

sugars.

• A reducing sugar will have one

of the following groups.

• an aldehyde group

(as in glyceraldehyde)

• a hydroxyketone

(as in fructose)

• a cyclic hemiacetal group

(as in glucose and maltose)

• The Benedict, Barfoed, and

Fehling tests are based on the

formation of a brick red copper(I)

oxide precipitate as a positive

result while the Tollens test is

based on the formation of a silver

mirror.

• The Barfoed test is more sensitive

in that it can distinguish a reducing

monosaccharide from a reducing

disaccharide.

• The sugars are oxidized to

carboxylic acids and the metal

ions are reduced

Section 18.12

Reactions of Monosaccharides

Section 18.12

Reactions of Monosaccharides

• Many clinical tests that monitor

color change are based on the

oxidation reaction shown here.

Reducing sugars

• Sugars with the hemiacetal

structure can be reducing sugars

under alkaline conditions because

the ring opens forming an

aldehyde group.

Section 18.12

Reactions of Monosaccharides

Phosphate Ester Formation

• The hydroxyl groups of a

monosaccharide can

react with inorganic

oxyacids to form

inorganic esters.

• Phosphate esters of

various monosaccharides

are stable in aqueous

solution and play

important roles in the

metabolism of

carbohydrates.

Section 18.12

Reactions of Monosaccharides

Amino Sugar Formation

• One of the hydroxyl groups of

a monosaccharide is replaced

with an amino group

• In naturally occurring amino

sugars the carbon 2 hydroxyl

group is replaced by an amino

group

• Amino sugars and their N-

acetyl derivatives are important

building blocks of

polysaccharides such as chitin

and hyaluronic acid

Section 18.12

Reactions of Monosaccharides

Glycoside Formation

• The cyclic forms of

monosaccharides, the

hemiacetals, react with

alcohols to form acetals (also

called glycosides)

• A glycoside is an acetal

formed from a cyclic

monosaccharide by

replacement of the hemiacetal

carbon —OH group with an —

OR group to form a double

ether

• A glycoside produced from

glucose - glucoside

• from galactose – galactoside

Section 18.12

Reactions of Monosaccharides

Section 18.12

Reactions of Monosaccharides

Antigens used in the ABO blood group classification

Section 18.13

Disaccharides

• The two monosaccharides are

linked together by acetal

formation to form disaccharide

• One monosaccharide act as a

hemiacetal and other as

alcohol and the resulting ether

bond is a glycosidic linkage.

• Condensation of the hydroxyl

function of the hemiacetal

group of one monosaccharide

with the hydroxyl group of

another monosaccharide forms

the bond, called a glycosidic

bond, joining the 2 saccharide

units.

Section 18.13

Disaccharides

Maltose (reducing sugar)• Malt sugar, found in corn syrup,

malt, and germinating seeds

• consists of two molecules of

glucose joined by -1,4-glycosidic bond

– -1,4-glycosidic bond means that

the first sugar is in -configuration

and its C#1 is linked to C#4 of the

second sugar component

– the second sugar may be either

an α- or a β-anomer.

Section 18.13

Disaccharides

Cellobiose (reducing disaccharide)

• one of the major fragments

isolated after extensive

hydrolysis of cellulose

• Maltose is digested easily by

humans because we have

enzymes that can break α-

(1→4) linkages but not β-

(1→4) linkages of cellobiose

• the 2 glucose units are joined

by a -1,4-glycosidic linkage

Section 18.13

Disaccharides

Lactose (reducing disaccharide)

• Milk sugar

– human - 7%–8% lactose

– cow’s milk - 4%–5% lactose

• consists of -galactose with a

-1,4-glycosidic linkage to -

glucose (or -glucose)

• Lactose intolerance: a condition in

which people lack the enzyme

lactase needed to hydrolyze

lactose to galactose and glucose.

• Lactose intolerance is unpleasant,

but its effects can be avoided by a

diet that rigorously excluded milk

and milk products.

Section 18.13

Disaccharides

Sucrose (nonreducing disaccharide)

• the common table sugar & the

most abundant of all

disaccharides found in plants.

• produced commercially from

the juice of sugar cane and

sugar beets.

• the -anomeric carbon 1 of

glucose joins the -anomeric

carbon 2 of fructose (-1,2-

glycosidic bond)

Section 18.14

Oligosaccharides

• Commonly found in onions, cabbage, broccoli and wheat

• In humans, intestinal bacteria action on the undigestable raffinose

and stachyose present in beans produces gaseous products that

can cause discomfort and flatulence.

Section 18.14

Oligosaccharides

• Solanin - a potato toxin, is a oligosaccharide found in association

with an alkaloid

• bitter taste of potatoes is due to relatively higher levels of solanin.

Section 18.15

General Characteristics of Polysaccharides

The Polymer Chain

• many monosaccharide

units bonded with

glycosidic linkages

• branched or unbranched

• homopolysaccharide or

heteropolysaccharides

Section 18.15

General Characteristics of Polysaccharides

• alternate name is glycan

• not sweet and don’t show

positive tests with Tollen’s and

Benedict’s solutions

• limited water solubility

• Storage polysaccharides

– starch

– glycogen

• Structural polysaccharides

– cellulose

– chitin

• Acidic polysaccharides

– heparin

– hyaluronic acid

• Homopolysaccharides

– starch

– glycogen

– cellulose

– chitin

– carageenan

• Heteropolysaccharides

– hyaluronic acid

– heparin

– chondroitin sulfate

– alginic acid

Section 18.16

Storage Polysaccharides

Starch• the chief caloric distributor in the diet; the reserve carbohydrates for plants

• Amylose - straight chain polymer; 15 - 20% of the starch; water-soluble fraction; 60

– 300 glucose units joined by -1,4-glycosidic bonds

• experimental evidence indicates that the molecule is actually coiled like a spring and

is not a straight chain of glucose units.

• When coiled in this fashion the molecule has just enough room in its core to

accommodate an iodine molecule.

• The characteristic blue color that starch gives when treated with iodine is due to the

formation of the amylose-I2 complex.

Section 18.16

Storage Polysaccharides

Starch

• Amylopectin

– branched chain polymer

– 80 - 85 % of the starch

– the water-insoluble fraction

– composed of 300 – 6000

glucose units joined

primarily by -1,4-

glucosidic bonds and

occasionally by -1,6-

glucosidic bonds

– -1,6 bonds are

responsible for branching

which occurs about once

every 25-30 units.

Section 18.16

Storage Polysaccharides

Glycogen

• the animal starch

• glucose storage molecule of

animals

• stored in granules in liver and

muscle cells

• like amylopectin, is a nonlinear

polymer of glucose units joined

by -1,4- and -1,6-glycosidic

bonds but has lower molecular

weight

• more highly branched structure

• its branches are shorter

• gives red-brown color with I2

Section 18.17

Structural Polysaccharides

Cellulose

• a fibrous carbohydrate found in all

plants where it serves as the

structural component of the plant’s

cell wall

• a linear polymer of glucose units

joined by -1,4-glucosidic bonds

• linear nature of chains allows

close packing into fibers, making it

difficult for solvent molecules to

pull the chains apart, thus

cellulose is inert towards most

solvents

• Cotton ~95% cellulose and wood

~50% cellulose

• It serves as dietary fiber in food--

readily absorbs water and results

in softer stools

• 20 - 35 g of dietary fiber is desired

everyday

Section 18.17

Structural Polysaccharides

Cellulose

• yields D-glucose upon hydrolysis

yet man & the carnivorous animals

can’t utilize cellulose as a source

of glucose.

• human‘s digestive juices lack the

enzyme cellulase that hydrolyze -

1,4-glucosidic linkages.

• ruminants (cows, goats) and

termites have, within their

digestive tracts, microorganisms

that produce cellulase

Section 18.17

Structural Polysaccharides

Chitin

• Similar to cellulose in both

function and structure

• polymer of N-acetyl-D-

glucosamine bound by β-1 → 4

glycosidic linkages (has a linear

extended structure like cellulose)

• Function is to give rigidity to the

exoskeletons of crabs, lobsters,

shrimp, insects, and other

arthropods

• itself is inert and practically

insoluble in most solvents. Its

derivative, chitosan, can be

prepared by simple alkali-

catalyzed deacylation. Chitosan

derivatives are commercially used

as films, fibers, surface coatings

and ultrafiltration membranes

Section 18.17

Structural Polysaccharides

Carageenan

• occurs as hydrocolloid extracted

from selected species of red algae

• locally obtained from Eucheuma

• striatum, Eucheuma spinosum

and Acanthapora

• sulphated polysaccharides,

consisting of polymers of

sulphated D-galactopyranose

bonded through alternating α-

1→3 and β-1→4 glycosidic

linkages

• widely used in food industry

• its gelling property is used in

enhancing the texture of various

dairy products and in preventing

oiling off in caramel and toffee

during hot weather

• also serve as coating to retard

moisture loss from foods and fresh

produce like fruits and vegetables

Section 18.18

Acidic Polysaccharides

Hyaluronic acid

• repeating unit is a disaccharide

composed of -D-glucuronic acid

and N-acetyl-D-glucosamine in a

-(1→3)-linkage.

• each disaccharide is attached to

the next by -(1→4)-linkage

• alternating -(1→3) and -(1→4)-

linkages

• highly viscous - serve as

lubricants in the fluid of joints and

part of vitreous humor of the eye

• when some insects sting, they

inject an enzyme called

hyaluronidase, which breaks

hyaluronic acid linkages and

facilitates the spread of the venom

Section 18.18

Acidic Polysaccharides

Heparin

• consists of repeating units of D- glucuronic acid and D- glucosamine

• an anticoagulant in blood that inhibits blood clot formation

• used in open-heart surgery

Section 18.20

Glycolipids and Glycoproteins: Cell Recognition

• A glycolipid is a lipid molecule that has one or more

carbohydrate (or carbohydrate derivative) units

covalently bonded to it.

• A glycoprotein is a protein molecule that has one or

more carbohydrate (or carbohydrate derivative) units

covalently bonded to it.

• Such carbohydrate complexes are very important in

cellular functions such as cell-cell recognition, cell

adhesion and cellular communication.