oligosackaryltransferas

Oligosaccharide transferases, commonly referred to as oligosaccharyltransferases (OSTs), are enzymes that catalyze the en bloc transfer of a preassembled oligosaccharide onto specific proteins. In eukaryotes, the principal reaction is N-linked glycosylation, in which the oligosaccharide Glc3Man9GlcNAc2 is transferred from a dolichol‑linked donor to an asparagine residue within the sequence Asn-X-Ser/Thr (X ≠ Pro).

Location and mechanism: The reaction takes place in the endoplasmic reticulum, typically cotranslationally as the nascent polypeptide enters the ER lumen. OST is a membrane‑embedded multi‑subunit complex, and its catalytic core is a STT3 subunit. In mammals, two catalytic isoforms, STT3A and STT3B, function with distinct regulatory partners.

Variations and analogs: In prokaryotes, a single‑component oligosaccharyltransferase such as PglB fulfills a similar role in periplasmic glycosylation. Across domains, OSTs have conserved mechanisms to recognize the consensus sequon and to catalyze glycosidic bond formation.

Biological and clinical relevance: Proper N‑linked glycosylation by OST is essential for protein folding, stability, and trafficking. Defects in OST subunits or their assembly cause congenital disorders of glycosylation and developmental abnormalities. OST activity is also a focus in glycoengineering and antiviral research, with inhibitors used to study glycosylation pathways and to explore therapeutic approaches.