glycans

Glycans are large, diverse carbohydrates composed of monosaccharide units linked by glycosidic bonds. They occur as free polymers or as covalent modifications of proteins (glycoproteins) and lipids (glycolipids). Glycans include several major classes: N-linked glycans attached to asparagine residues; O-linked glycans attached to serine or threonine residues; glycosaminoglycans, long unbranched chains that form proteoglycans in the extracellular matrix; and glycan moieties on glycolipids and other glycoconjugates.

N-linked glycans are attached to the amide nitrogen of asparagine in the consensus motif N-X-S/T, where X is any amino acid except proline. O-linked glycans are assembled in the Golgi on serine or threonine residues and show diverse monosaccharide sequences. Glycosaminoglycans are repeating disaccharides often sulfated, producing highly polyanionic chains that confer hydration and resilience to tissues; proteoglycans bear these GAGs to core proteins. Glycans modulate protein folding and trafficking, influence receptor-ligand interactions, and shape immune recognition.

Biosynthesis begins in the endoplasmic reticulum and continues in the Golgi, using activated sugar donors (nucleotide sugars). The process involves glycosyltransferases and, for many glycans, subsequent trimming and remodeling enzymes. Because there is no single template, glycan structures are highly heterogeneous, enabling a wide range of biological information and functions.

Glycans are essential for development and homeostasis; alterations in glycosylation are associated with diseases and may affect pathogen recognition, blood group antigens, and cell signaling. Techniques in glycomics and glycoproteomics seek to characterize their structures and functions.