Glycosaminoglycans and glycoproteins

Glycosaminoglycans and glycoproteins

A. Hyaluronic acid 

As constituents of proteoglycans ,

the glycosaminoglycans—a group of acidic

heteropolysaccharides—are important structural

elements of the extracellular matrix.

Glycosaminoglycans contain amino sugars

as well as glucuronic acid and iduronic acid as

characteristic components . In addition,

most polysaccharides in this group are

esterified to varying extents by sulfuric acid,

increasing their acidic quality. Glycosaminoglycans

can be found in free form, or as components

of proteoglycans throughout the organism.

Hyaluronic acid, an unesterified glycosaminoglycan

with a relatively simple structure,

consists of disaccharide units in which Nacetylglucosamine

and glucuronic acid are

alternately β1
4-linked and β1
3-linked.

Due to the unusual β1
3 linkage, hyaluronic

acid molecules–which may contain several

thousand monosaccharide residues—are

coiled like a helix. Three disaccharide units

form each turn of the helix. The outwardfacing

hydrophilic carboxylate groups of the glucuronic

acid residues are able to bind Ca2+

ions. The strong hydration of these groups

enables hyaluronic acid and other glycosaminoglycans

to bind water up to 10 000 times

their own volume in gel form. This is the

function which hyaluronic acid has in the vitreous

body of the eye, which contains approximately

1% hyaluronic acid and 98% water.

B. Oligosaccharide in immunoglobulin G(IgG) 

Many proteins on the surface of the plasma

membrane, and the majority of secreted proteins,

contain oligosaccharide residues that

are post-translationally added to the endoplasmic

reticulum and in the Golgi apparatus

. By contrast, cytoplasmic proteins

are rarely glycosylated. Glycoproteins can

contain more than 50% carbohydrate; however,

the proportion of protein is generally

much greater.

As an example of the carbohydrate component

of a glycoprotein, the structure of one of

the oligosaccharide chains of immunoglobulin

G (IgG; is shown here. The

oligosaccharide has an N-glycosidic link to

the amide group of an asparagine residue in

the Fc part of the protein. Its function is not

known.

Like all N-linked carbohydrates, the oligosaccharide

in IgG contains a T-shaped core

structure consisting of two N-acetylglucosamines

and three mannose residues (shown

in violet). In addition, in this case the structure

contains two further N-acetylglucosamine

residues, as well as a fucose residue

and a galactose residue. Glycoproteins show

many different types of branching. In this

case, we not only have β1
4 linkage, but

also β1
2, α1
3, and α1
6 bonds.

C. Glycoproteins: forms 

On the cell surface of certain glycoproteins,

O-glycosidic links are found between the carbohydrate

part and a serine or threonine residue,

instead of N-glycosidic links to asparagine

residues. This type of link is less common

than the N-glycosidic one.

There are two types of oligosaccharide

structure with N-glycosidic links, which arise

through two different biosynthetic pathways.

During glycosylation in the ER, the protein is

initially linked to an oligosaccharide,which in

addition to the core structure contains six

further mannose residues and three terminal

glucose residues . The simpler

from of oligosaccharide (the mannose-rich

type) is produced when only the glucose residues

are cleaved from the primary product,

and no additional residues are added. In other

cases, the mannose residues that are located

outside the core structure are also removed

and replaced by other sugars. This produces

oligosaccharides such as those shown on the

right (the complex type). At the external end

of the structure, glycoproteins of the complex

type often contain N-acetylneuraminic acid

residues,which give the oligosaccharide components

negative charges.