glycan

Glycan is a carbohydrate polymer covalently attached to proteins or lipids, forming glycoproteins, proteoglycans, and glycolipids. Glycans range from small oligosaccharides to long polysaccharide chains and exhibit diverse monosaccharide compositions, linkages, and branching. Their biosynthesis is largely non-template driven, governed by a network of glycosyltransferases and glycosidases.

Major classes include N-linked and O-linked glycans, and glycosaminoglycans. N-linked glycans attach to asparagine within the consensus sequence Asn-X-Ser/Thr and are remodeled in the ER and Golgi to yield high-mannose, hybrid, or complex structures. O-linked glycans attach to serine or threonine, typically in the Golgi, and include mucin-type GalNAc glycans and others. Glycosaminoglycans are long, unbranched chains that form proteoglycans; examples are heparan sulfate and chondroitin sulfate.

Glycans are also key components of glycolipids and many glycoproteins. The exact structure of a glycan influences protein folding, stability, trafficking, and interactions with lectins and receptors.

Biological roles include mediating cell-cell and cell-matrix communication, modulating immune responses, and affecting development and pathogen recognition. Abnormal glycosylation is linked to diseases such as congenital disorders of glycosylation, cancer, and inflammatory conditions. Therapeutic antibodies and vaccines are impacted by glycan structures.

Glycobiology employs mass spectrometry, chromatography, NMR, and lectin-based assays to characterize glycans. The field, termed glycomics, seeks to map glycan structures and functions, with implications in biomedicine and biotechnology.