GlycosaminoglycansProteoglycans Glycoproteins

Iqra Jabbar

School of Biological Sciences, PU

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CONTENTS

▫Glycosaminoglycans

▫Glyco-conjugates

Proteoglycans

Glycoproteins

GLYCOSAMINOGLYCANS

(GAGs)

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Introduction to GAGs

GAGs (free or linked to proteins to form proteoglycans)

Gel-like matrix

Fibrous proteins

(Collagen and elastin

fibers)

Extracellular matrix• Extracellular spaces• connective tissues• Basement membrane

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Structure of GAGs • Linear polymers

• Heteropolysaccharides…. repeating disaccharide units, (AB)n

Uronic acid

Glycosamine

Repeating unit of glycosaminoglycans

D-glucuronic acid or

L-iduronic acid (except in

keratan sulfate)

N-acetylglucosamine

Or N-

acetylgalactosamine

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Sulfate groups are

also present

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Properties and roles of GAGs

Volume occupied by various macromolecules

Albert’s Molecular biology of the cell: 4th ed.

• Highly negatively charged

• Extended conformations

• High viscosity and elasticity

• Slimy, mucus-like consistency

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• ideal as a lubricating fluid in the joints

• provide structural integrity to the cells

• Attached to proteins.. cell and tissue development and

physiology

Negative charges of

GAGs

Attracts cations(Na+)

Sucks in water and

creates pressure

Enable ECM to with stand compressive

forces

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Types of GAGs

GAGs

1.

Hyaluronic acid/hyalurona

n

2. Chondroitin-sulfate and Dermatan-

sulfate

3.

Heparin and heparan-sulfate

4.

Keratan-sulfate

Basis

1. Sugars

2. the type of linkage 

3. the number and

location of sulfate

groups

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1. Hyaluronic acid

• Greek “Hyalos” means “glass”

• Viscous clear solutions

Structure

▫ GlcA and GlcNAc

▫ β (1 3) and β (1 4) linkages

▫ 50,000 repeats..

▫ Mol. wt. … 105 – 107 KDa

• Highly hydrated

• Degradation by hyaluronidase

breaks… β(14) linkages

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Cont…

Location

• ECM of Cartilage and tendons

▫ Provides strength and elasticity

• synovial fluid

▫ biological lubricant and shock

absorber

• Vitreous of the eye

▫ Clear gel

• Early embryos

▫ cell migration and tissue

morphogenesis

• Bacterial capsule

vitreous humor of the eye

Cross section of a healthy joint

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Hyaluronic acid differ from other GAGs

Hyaluronic acid

Other GAGs

Disaccharide units

upto 5o,000 50 to 1000 disaccharide

units

Sulfation Non-sulfated Sulfated

Covalently bound to proteins

No Yes

Monomeric units

GlcA and GlcNAc

Different

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2. Chondroitin sulfate and Dermatan sulfate

Chondroitin sulfate

Greek “chondros” means

“cartilage”

▫ Cartilage, tendons,

ligaments, walls of the aorta

• Most abundant GAG in ECM

• Provides strength

• Binds collagen fibers

• Loss can cause osteoarthritis

Dermatan sulfate

Greek “derma” means “skin”

▫ skin, blood vessels, cornea,

heart valves

• Coagulation

• wound repair

• Pliability of skin

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2. Chondroitin sulfate and Dermatan sulfate

β(1 3) and β(1 4) linkages

Structures

α(1 3) and β(1 4) linkages

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3. Keratan-sulfate

• Greek “keras” means “horn”

• Occurs in…▫ Cornea, Cartilage, Bone▫ horny structures

• Most heterogeneous of GAGs

▫ Variable sulfate content

▫ different sugars are present

Fucose, mannose, NAG, etc

• usually associated with proteins

β (14) linkages

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4. Heparin and heparan sulfate

Greek “hepar” means liver

Heparin

• Most negatively charged

• 15-90 DS units per chain

• occurs as granules in mast

cells

• arterial walls, lungs, liver and

skin

• Anticoagulant

▫ complexes with anti-

thrombin III

▫ Binds and inactivates

thrombin

α (14) linkages

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Heparan sulfate

• fewer sulfate groups than heparin

• more N-acetyl groups

• ubiquitous

• Found associated with proteins … HSPG

• Acts as receptor

▫ Cell-cell communication

▫ Cell-growth

PROTEOGLYCANS

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Tetrasaccharide bridge

GAG

Introduction to proteoglycans

Basic structure

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at least one covalently

attached GAG

A core protein

Lehninger principles of biochemistry: 5th ed.

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Proteoglycans• Functions:

▫ Act as tissue organizers

▫ influence various cellular activities such as growth factor activation and adhesion

▫ Selective filters

• At least 30 types in mammals

• Present as: Integral membrane protein

▫Examples: Syndecan and glypican ECM component

▫Example: Aggrecan

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Membrane Heparan sulfate proteoglycans: “Syndecan” and “Glypican”

Lehninger principles of biochemistry: 5th ed.

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NS and NA domains in heparin sulfate chain

NS = highly sulfated domainsNA = N-acetylated domains (unmodified GlcNAc and GlcA residues)

Lehninger principles of biochemistry: 5th ed.

a. Conformational activation

c. Co-receptor for extracellular ligands

b. Enhanced protein-protein interactions

d. Cell-surface localization or concentration

NS domain bind to extracellular proteins and signaling molecules to alter their activity…

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2. Aggrecan

• the main proteoglycan component of cartilage ECM

Structure

• Many PGs…. non-covalently attached to a single

hyaluronan molecule

• Bottle-brush like architecture

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Aggrecan structure

Lehninger principles of biochemistry: 5th ed.

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Aggrecan- core protein structure

N-terminal domain (binds hyaluronan)

Cen

tral

dom

ain

N-terminal region(binds few carbohydrate chains, N-linked)

Central region(rich in oligosaccharides which serve as anchor points for keratan sulfate chains, O-linked)

C-terminal region(rich in chondroitin sulfate chains)

C-terminal domain(bind monosaccharides

Biochemistry (Voet): 4th ed.

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2. Aggrecan

• A strand of hyaluronic acid binds▫ 100 associated aggrecan chains

• Total mol.wt …. (Mr > 2 x 108 kDa)

• Functions:▫ increased water retention▫ Interact strongly with collagen fibrils in the ECM of

cartilage contributing to its high tensile strength

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Role of PG aggregates in Cartilage flexibility

Water molecules in ECM

Compression of cartilage due to

movement

Release of pressure

Water squeezes

out of ECM

Water reconstitute

dReabsorbtion of water molecules

Lippincott‘s : 4th ed.

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MUCOPOLYSACCHARIDOSES (MPS)

• Rare inborn errors in the degradation of

glycosaminoglycans

• Heretidy disorders

▫ autosomal recessive except Hurler’s( X-linked)

Skeletal and extracellular

matrix deformities and mental retardation

Accumulation of GAGs

Deficiency of lysosomal

hydrolase for GAGA

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Cont…

• Major features:

▫ Dysotosis multiplex (severe abnormalities in

development of cartilage and bone)

▫ mental retardation

▫ Organomegaly

▫ Abnormal facial features

▫ Abnormalities in hearing, vision and cardiovascular

system

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Type Syndrome Enzyme defect Accumulation

I Hurler’s α-L-iduronidase DS, HS

II Hunter’sL-iduronate-2-sulfatase DS, HS

III Sanfilippo’s heparan sulfatase HS

IV Morquio‘sGalactosamine sulphatase KS, CS

V Scheie’s L-iduronidase DS

VI Maroteaux-Lamy’sN-acetylglucosamine-4-sulfatase

DS

VII Sly’s β-glucuronidase DS, HS

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CHONDRODYSTROPHIES

• Autosomal recessive diseases

• Affect proper development of cartilage and

maintenance of skeletal system

• Defect in sulphation of growing GAG chain

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Association of GAGs and PGs with cancer• Hyaluronic acid

▫ tumor cell… increased production of hyaluronic acid

▫ cell migration through ECM… aid in tumor spread

• Heparan sulfate

▫ Role in adhesiveness

▫ Tumor cells.. less HS at their surface …. lack of

adhesiveness

GLYCOPROTEINS

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Introduction to glycoproteins• one or several oligosaccharides of varying complexity

joined covalently to a protein.

Glycoproteins Proteoglycans

Composition Carbohydrate<<

protein (1 to 70 %)

Carbohydrate>>Protein(95%)

Carbohydrate chain length

Smaller (2-10 sugar residues)

Very long

Serial disaccharide repeats

No (Very

heterogeneous)yes

Branching of carbohydrate chains

yes no

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Structure of glycoproteins

• Linkage b/w oligosaccharide and

protein

▫ O-linked

▫ N-linked

O-linked

Anomeric carbon of NAG …

attached to O of serine or threonine▫ cell surface glycoproteins▫ mucins ▫ viral glycoproteins

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N-linked

• Anomeric C of NAG ---- attached to amide nitrogen of an

Asn

• Specific sequence … Asn-X-Ser or Asn-X-Thr (X=any

a.a exept proline)

• 5 times more abundant than O-linked

▫ Immunoglobulins G and M

▫ Ovalbumin

▫ Peptide hormones, etc.

• Posttranslational additions of N-linked oligosaccharides

▫ direct selected proteins to various compartments

▫ Help in proper folding

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Types of N-linked glycans

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Difference b/w glycosylation and glycationGlycation (or non-enzymatic glycosylation)

• covalent bonding of a protein or lipid molecule with a sugar molecule

without the controlling action of an enzyme.

• For example.. Glucose, or sugar, binds to a protein called heme within the

cells in a non-reversible process called glycation.

Glycosylation

• It is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached

to a hydroxyl or other functional group of another molecule (a glycosyl

acceptor).

• It is an enzyme-directed site-specific process

• Glycosylation is a form of co-translational and post-translational

modification.

• The majority of proteins synthesized in the rough ER undergo glycosylation.

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• N-linked glycosylation

• The most common type of glycosylation and is important

for the folding of some eukaryotic proteins and for cell-cell

and cell-extracellular matrix attachment.

• In eukaryotes… occurs in the lumen of the endoplasmic

reticulum 

• O-linked glycosylation is a form of glycosylation that

occurs in eukaryotes in the golgi apparatus.

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Functions of glycoproteinsStructural

• receptors on cell surfaces

• strength and support to a matrix

• slime layer of bacteria, and flagella

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Protection

• Mucin … form a highly viscous gel

▫ Protect internal epithelial surfaces

• Act as a lubricant

▫ Human lacrimal glands produce a glycoprotein which protects

the corneal epithelium

Reproduction

• Glycoproteins on surface of sperm cell

▫ increase attraction for the egg by altering the electrophoretic

mobility of the plasma membrane.

• Hen ovalbumin …. serves as a food storage unit for the embryo.

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Adhesion:

• cells to cells ..

▫ development of tissues..

i.e N-CAM (nerve cell adhesion molecule)

on nerve cells and muscle cells… form myoneural junctions

▫ Bacterial infection

• cells to substratum

▫ cell surface receptors for certain adhesion ligands

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Hormones

• Erythropoietin which regulates erythrocyte production

Enzymes

▫ Oxidoreductases

▫ Transferases

▫ Hydrolases

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Oligosaccharidoses

• glycoprotein storage diseases

• a deficiency of one of the degradative enzymes

• accumulation of partially degraded structures

• After cell death, appear in the urine