Carbohydrates are ubiquitous modifications of eukaryotic cell surface proteins and lipids.

Carbohydrates are ubiquitous modifications of

eukaryotic cell surface proteins and lipids.

GlycoproteinGlycolipidOligosaccharides

Glycans

Localization of glycoconjugates in intracellular and extracellular compartments

Providing structural componentsCell wallsExtracellular matrix

Modifying protein propertiesSolubilityStability

Directing trafficking of glycoconjugatesIntracellularExtracellular

Mediating and modulating cell adhesionCell-cell interactionsCell-matrix interactions

Mediating and modulating signalingIntracellularExtracellular

Functions

Extrinsic functionsresulting from

glycan-lectin interactions

Intrinsic functionsperformed by glycans

Glycans

Three major classes of glycoconjugates

Attached to proteins * through a nitrogen atom of asparagine

(N-linked)

** through an oxygen atom of serine or threonine(O-linked)

*** Attached to lipids

Glycans: Structures

Glycans are composed of monosaccharides with related chemical structures

1

2

3

4

5

6

HC=O

HCOH

HCOH

HOCH

CH2OH

HOCH

1

2

3

4

5

6

HC=O

HCOH

HCOH

HCOH

HOCH

CH2OH

Epimerization at C4

Glucose Galactose

Hexoses are the most common constituents of glycans

Glycans: Structures

Two ways to form a pyranose (6-member) ring from glucose

Glycans: Structures

Substitution of the 2-hydroxyl group of glucose or galactose with acetamido group yields N-acetylhexosamines

Glycans: Structures

Common derivatives of the hexoses

Glycans: Structures

Structure of N-acetylneuraminic acid, the most common form of sialic acid

Glycans: Structures

Glycosidic linkages between monosaccharides exist in multiple configurations

Glycans: Structures

A typical N-linked glycan

Chemical structure

Word structure

Symbol structure

Glycans: Structures

Formation of glycosidic linkages require energy and catalyzed by specific enzymes, glycosyltransferases

Glycosyltransferase reaction

UDP-galactose, an example of

a nucleotide sugar donor

Uridine diphosphate (UDP)

Glycans: Structures

Understanding structure-function relationship for glycans can be more difficult than for other classes of biopolymers

* The functions of polypeptide and glycan portions of glycoproteins are potentially independent

Glycans: Structures

Understanding structure-function relationship for glycans can be more difficult than for other classes of biopolymers

** Monosaccharides generate more linkage variation than amino acids or nucleotides

Example: structure # of variations

3 nucleotide bases 6 3 amino acids 6 3 hexoses 1,056 - 27,648

Glycans: Structures

Glycans: Structures

Glycan structures are encoded indirectly in the genome






FunctionsExtrinsic functions

resulting from glycan-lectin interactions


Glycans

I. Heparan sulfate proteoglycans

Xylose

Galactose

GlcNAc

GlcA

Disaccharide unit

Core protein

Linkage region

Sulfate

IdoA

Large O-linked chains of as many as a hundred residues

Abnormalities in heparan sulfate biosynthesis is implicated in the human multiple exostoses syndrome

Drosophilagene

Function Vertebratehomologue

Associated phenotype (mammals)

dally Cell surface proteoglycan,effects on Wg and Dppsignaling

Glypicans Overgrowth, skeletal abnormalities,tumor-susceptibility syndrome,Wilm’s tumor of the kidney

sugarless Glycosaminoglycanbiosynthesis,effects on Wg, FGFR, andDpp signaling

UDP-glucosedehydrogenase

?

sulfateless Heparan sulfatemodification,effects on Wg, Hh, andEGFR signaling

N-deacetylaseN-sulfotransferase

Mast cells: loss of heparin anddefects in secretory granules

tout-velu Heparan sulfateco–polymerase, effects onHh signaling

EXT1 & EXT2 Multiple exostoses tumors

pipe Related toglycosaminoglycan2–O–sulfotransferases,effects on ventralizingsignal in embryogenesis

Heparan sulfate2–O–sulfotransferase

Renal agenesis,eye and skeletal abnormalities

II. Notch signaling

Notch proteins mediate a wide variety of cell fate decisions during development:

in Drosophila: - Organogenesis and pattern formation - Neurogenesis - Myogenesis - Oogenesis - etc.

Malfunctioning of Notch signaling causes:

- Human T-cell lymphoblastic leukaemia (TAN-1/Notch1)- Human stroke and dementia (CADASIL, Notch3)- Spondylocostal dysostosis (Dll3) - Alagille syndrome (Jagged1)- Murine breast cancer (int-3/Notch4)

in Vertebrates (4 Notch genes, 5 ligands): - Somitogenesis - Neurogenesis - T-cell development - etc.

NotchSerrateDelta

Notch

Serrate

Delta

Fringe

O-fucose

GlcNAc

+






FunctionsExtrinsic functions

resulting from glycan-lectin interactions


Glycans

III. O-mannosylation in muscular dystrophies

Normal muscle Muscular dystrophy

Extracellular

Intracellular Actin-filament

Dystrophin

Dystroglycan

Neurexin

Agrin

Laminin

Membrane

Basal Lamina

Actin-filament

Dystrophin

Dystroglycan

Neurexin

Agrin

Laminin

O-mannosyl glycans

Functions: III. O-mannosylation in muscular dystrophies

Compromised O-mannosylation of dystroglycan causes severe muscle and brain abnormalities

Example: Muscle-eye-brain disease:congenital muscular dystrophy,severe congenital myopia, hydrocephalus,

mental retardation

Muscle biopsy: Dystrophic changes

Cranial MRI: Cerebellar hypoplasiaNeuronal migration disorder

Yoshida et al., Dev. Cell 2001

Muscular dystrophies associated with mutations in glycosyltransferase genes.

Condition Gene Protein function Clinical features

Muscle-eye-brain disease (MEB)

O-mannose--1,2-N-acetylglucosaminyltransferase (POMGnT1)

O-mannosyl glycan synthesis Severe muscle weaknessMental retardationEpilepsyNeural migration disorderOcular abnormalities

Fukuyama congenital muscular dystrophy (FCMD)

Fukutin Putative glycosyltransferase Severe proximal and axial weaknessMental retardationEpilepsyNeural migration disorder

MDC1C and LGMD21 Fukutin related protein (FRP) Putative glycosyltransferase Variable proximal and axial muscle weaknessCardiomyopathy

Walker Warburg syndrome (WWS)

Protein O-mannosyltransferase 1 & 2 (POMT1 and POMT2)

O-mannosyltransferase Severe muscle weaknessDeath in infancyAbsent psychomotor developmentNeural migration disorderOcular abnormalities

Rotated abdomen (rt) and twisted (tw) in Drosophila

Protein O-mannosyltransferase 1 (POMT1)

O-mannosyltransferase Disrupted myogenesis

Myodystrophy mouse (myd) LARGE Putative glycosyltransferase Muscle weaknessNeural migration disorder

IV. ABO blood groups (1900s, Landsteiner et al.)

IO IA IBAlleles:

Encode glycosyltransferases: Inactive GalNAc-transferase Gal-transferase

Galactose

GalNAc

Fucose

Glycans: Functions

The glycan antigens are expressed on the surface of red blood cells and determine the compatibility in blood transfusions






StructureFunctions

Glycans

Carbohydrates are ubiquitous modifications of eukaryotic cell surface proteins and lipids.

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