Protein Glycosylation: The Clash of the Titans

Enzymes catalyze a variety of transformations with spectacular rate enhancements while demonstrating exquisite . Information from complementary structural mechanistic studies has yielded insight into the inner of many enzyme-catalyzed reactions. In particular, of transformations that involve the interaction one or more small-molecule substrates with soluble have met with notable success. There remain, , a number of reactions, including those involving covalent modification of macromolecular substrates, confound us by their complexity. One such transformation, asparagine-linked glycosylation, involves the enzyme-catalyzed modification of an asparagine side in a nascent polypeptide with a triantennary tetradecasaccharide (GlcNAc_2Man_9Glc_3) moiety. This first step in the biosynthesis of N-linked glycoproteins catalyzed by oligosaccharyl transferase (OT), a membrane-associated enzyme complex, found the lumen of the endoplasmic reticulum (ER) of cells. Glycosyl transfer to the nitrogen of the side chain of an asparagine occurs through intermediacy of a dolichol-linked pyrophosphate as illustrated in Figure 1. The primary peptide requirements for the glycosylation process a minimum -Asn-Xaa-Ser/Thr- tripeptide recognition where Xaa can be any of the 20 natural amino except proline. Thus, while OT exhibits rather substrate requirements, the enzyme catalyzes an and specific reaction wherein the nucleophilicity the asparagine side chain must be greatly enhanced to a covalent linkage with the oligosaccharide. After of the initial tetradecasaccharide, subsequent of the primary glycoprotein conjugates from enzyme-catalyzed processing steps that occur both the ER and the Golgi apparatus. Specifically, the oligosaccharide structures are subject to the action an elaborate array of glycosyl hydrolase and glycosyl enzymes. The latter class of enzymes catalyzes incorporation of a broader diversity of carbohydrate including fucose, sialic acid, and galactose. The action of these enzymes generates the structural associated with mature N-linked glycoproteins eukaryotic cells.