Glycoproteins

Glycoproteins are proteins that bear carbohydrate chains, or glycans, attached covalently to amino acid residues. The glycans are diverse in composition and linkages and can account for a substantial portion of the molecule in many cases. Glycosylation—the process of adding glycans—occurs in the secretory pathway and on membrane proteins, making it one of the most common post-translational modifications.

Two major classes of glycosylation are N-linked and O-linked. N-linked glycosylation attaches glycans to the amide nitrogen of asparagine residues within a consensus sequence and begins in the endoplasmic reticulum with the transfer of a preassembled oligosaccharide, followed by processing in the Golgi. O-linked glycosylation attaches glycans to serine or threonine residues and occurs mainly in the Golgi, where sugars are added stepwise and further modified. The resulting glycan structures can be highly branched and capped with diverse sugars.

Functions of glycoproteins are manifold. They influence protein folding, stability, solubility, and half-life, and mediate cell-cell interactions, signaling, and immune recognition. Examples include antibodies with Fc glycosylation that affects effector functions, mucins contributing to mucus viscosity, and extracellular matrix proteins like collagen. Viral glycoproteins, such as influenza hemagglutinin or HIV gp120, are often heavily glycosylated, which can shield epitopes from the host immune system.

Clinical and research relevance includes congenital disorders of glycosylation, altered glycosylation in cancer, and the design of glycoprotein therapeutics. Analytical methods for glycoproteins include mass spectrometry, chromatography, and lectin-based assays.