N-terminal catalytic domain of Golgi alpha-mannosidase II and similar proteins; glycoside hydrolase family 38 (GH38)
This subfamily is represented by Golgi alpha-mannosidase II (GMII, also known as mannosyl-oligosaccharide 1,3- 1,6-alpha mannosidase, EC 3.2.1.114, Man2A1), a monomeric, membrane-anchored class II alpha-mannosidase existing in the Golgi apparatus of eukaryotes. GMII plays a key role in the N-glycosylation pathway. It catalyzes the hydrolysis of the terminal of both alpha-1,3-linked and alpha-1,6-linked mannoses from the high-mannose oligosaccharide GlcNAc(Man)5(GlcNAc)2 to yield GlcNAc(Man)3(GlcNAc)2(GlcNAc, N-acetylglucosmine), which is the committed step of complex N-glycan synthesis. GMII is activated by zinc or cobalt ions and is strongly inhibited by swainsonine. Inhibition of GMII provides a route to block cancer-induced changes in cell surface oligosaccharide structures. GMII has a pH optimum of 5.5-6.0, which is intermediate between those of acidic (lysosomal alpha-mannosidase) and neutral (ER/cytosolic alpha-mannosidase) enzymes. GMII is a retaining glycosyl hydrolase of family GH38 that employs a two-step mechanism involving the formation of a covalent glycosyl enzyme complex; two carboxylic acids positioned within the active site act in concert: one as a catalytic nucleophile and the other as a general acid/base catalyst.
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