Glycoprotein

Glycoprotein is a protein that has carbohydrate chains covalently attached. The glycans are typically N-linked, attached to asparagine residues within the sequon Asn-X-Ser/Thr, or O-linked, attached to serine or threonine residues; other linkages occur. N-linked glycosylation begins in the endoplasmic reticulum with en bloc transfer of a preassembled oligosaccharide, followed by extensive processing and branching in the Golgi apparatus to yield diverse structures.

Glycoproteins perform a wide range of functions: they act as enzymes, structural components, hormones, receptors, or antibodies; on cell surfaces they mediate adhesion, signaling, and immune recognition; in the plasma they influence half-life and stability.

Examples include immunoglobulins, glycosylated hormones like erythropoietin, plasma proteins such as transferrin, and viral surface proteins like influenza hemagglutinin. Mucins are heavily glycosylated secreted or membrane-associated glycoproteins that protect and lubricate epithelia.

Clinical relevance: defects in glycosylation cause congenital disorders of glycosylation. Abnormal glycosylation patterns are associated with cancer and inflammatory diseases. Biopharmaceuticals such as many monoclonal antibodies and recombinant hormones are glycoproteins, and their glycosylation can affect efficacy and pharmacokinetics.

Detection and study often use lectin binding, glycan sequencing, or mass spectrometry.