STRUCTURE AND BIOSYNTHESIS OF MAJOR GROUPS OF GLYCANS

Glycans, also known as carbohydrates, are essential biomolecules that play crucial roles in various biological processes. They are involved in cell recognition, signaling, and immune response modulation. The major groups of glycans include N-linked glycans, O-linked glycans, glycosaminoglycans (GAGs), and glycosphingolipids. Each group has distinct structures and biosynthetic pathways.

1) N-Linked Glycans: N-linked glycans are covalently attached to the nitrogen atom of asparagine residues in proteins. The biosynthesis of N-linked glycans begins in the endoplasmic reticulum (ER) with the assembly of a lipid-linked oligosaccharide precursor. This precursor is then transferred en bloc to the nascent polypeptide chain by the oligosaccharyltransferase enzyme. Subsequently, the glycoprotein is transported to the Golgi apparatus, where further processing and maturation of the N-linked glycans occur.

2) O-Linked Glycans: O-linked glycans are attached to the hydroxyl group of serine or threonine residues in proteins. The biosynthesis of O-linked glycans involves the stepwise addition of monosaccharides to the protein backbone. This process occurs in the Golgi apparatus and is catalyzed by specific glycosyltransferases. The diversity of O-linked glycans arises from variations in the types and linkages of monosaccharides added to the protein.

3) Glycosaminoglycans (GAGs): Glycosaminoglycans are linear polysaccharides composed of repeating disaccharide units. They are highly negatively charged due to sulfate or carboxyl groups present in their structure. GAGs are synthesized through a series of enzymatic reactions involving glycosyltransferases and sulfotransferases. The biosynthesis of GAGs occurs in the Golgi apparatus, where modifications such as sulfation and epimerization take place.

4) Glycosphingolipids: Glycosphingolipids are complex lipids containing a carbohydrate moiety. They are synthesized through a series of steps involving the assembly of ceramide and subsequent addition of sugars to form various glycosphingolipid structures. The biosynthesis of glycosphingolipids occurs in the Golgi apparatus and involves specific glycosyltransferases that add sugars with precise regio- and stereochemistry.

In summary, the biosynthesis of major groups of glycans involves intricate enzymatic processes occurring primarily in the endoplasmic reticulum and Golgi apparatus. These processes result in the diverse structures observed in N-linked glycans, O-linked glycans, glycosaminoglycans, and glycosphingolipids.