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Publication Abstract from PubMed

Asn-linked oligosaccharides are extensively modified during transit through the secretory pathway, first by trimming of the nascent glycan chains and subsequently by initiating and extending multiple oligosaccharide branches from the trimannosyl glycan core. Trimming and branching pathway steps are highly ordered and hierarchal based on the precise substrate specificities of the individual biosynthetic enzymes. A key committed step in the synthesis of complex-type glycans is catalyzed by N-acetylglucosaminyltransferase II (MGAT2), an enzyme that generates the second GlcNAcbeta1,2- branch from the trimannosyl glycan core using UDP-GlcNAc as the sugar donor. We determined the structure of human MGAT2 as a Mn(2+)-UDP donor analog complex and as a GlcNAcMan3GlcNAc2-Asn acceptor complex to reveal the structural basis for substrate recognition and catalysis. The enzyme exhibits a GT-A Rossmann-like fold that employs conserved divalent cation-dependent substrate interactions with the UDP-GlcNAc donor. MGAT2 interactions with the extended glycan acceptor are distinct from other related glycosyltransferases. These interactions are composed of a catalytic subsite that binds the Man-alpha1,6- monosaccharide acceptor and a distal exosite pocket that binds the GlcNAc-beta1,2Man-alpha1,3Manbeta- substrate "recognition arm." Recognition arm interactions are similar to the enzyme-substrate interactions for Golgi alpha-mannosidase II, a glycoside hydrolase that acts just before MGAT2 in the Asn-linked glycan biosynthetic pathway. These data suggest that substrate binding by MGAT2 employs both conserved and convergent catalytic subsite modules to provide substrate selectivity and catalysis. More broadly, the MGAT2 active-site architecture demonstrates how glycosyltransferases create complementary modular templates for regiospecific extension of glycan structures in mammalian cells.

Human N-acetylglucosaminyltransferase II substrate recognition uses a modular architecture that includes a convergent exosite.,Kadirvelraj R, Yang JY, Sanders JH, Liu L, Ramiah A, Prabhakar PK, Boons GJ, Wood ZA, Moremen KW Proc Natl Acad Sci U S A. 2018 Apr 16. pii: 1716988115. doi:, 10.1073/pnas.1716988115. PMID:29666272^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.