N-terminal catalytic domain of eukaryotic class II alpha-mannosidases; glycoside hydrolase family 38 (GH38). The family corresponds to a group of eukaryotic class II alpha-mannosidases (AlphaMII), which contain Golgi alpha-mannosidases II (GMII), the major broad specificity lysosomal alpha-mannosidases (LAM, MAN2B1), the noval core-specific lysosomal alpha 1,6-mannosidases (Epman, MAN2B2), and similar proteins. GMII catalyzes the hydrolysis of the terminal 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. LAM is a broad specificity exoglycosidase hydrolyzing all known alpha 1,2-, alpha 1,3-, and alpha 1,6-mannosidic linkages from numerous high mannose type oligosaccharides. Different from LAM, Epman can efficiently cleave only the alpha 1,6-linked mannose residue from (Man)3GlcNAc, but not (Man)3(GlcNAc)2 or other larger high mannose oligosaccharides, in the core of N-linked glycans. Members in this family are retaining glycosyl hydrolases 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.

N-terminal catalytic domain of Golgi alpha-mannosidase IIx, and similar proteins; glycoside hydrolase family 38 (GH38). This subfamily is represented by human alpha-mannosidase 2x (MX, also known as mannosyl-oligosaccharide 1,3- 1,6-alpha mannosidase, EC 3.2.1.114, Man2A2). MX is enzymatically and functionally very similar to GMII (found in another subfamily), and as an isoenzyme of GMII. It is thought to also function in the N-glycosylation pathway. MX specifically hydrolyzes the same oligosaccharide substrate as does MII. It specifically removes two mannosyl residues from GlcNAc(Man)5(GlcNAc)2 to yield GlcNAc(Man)3(GlcNAc)2(GlcNAc, N-acetylglucosmine).

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.

alpha-mannosidase

N-terminal catalytic domain of Golgi alpha-mannosidase II, Spodoptera frugiperda Sf9 alpha-mannosidase III, 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 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. This subfamily also includes human alpha-mannosidase 2x (MX, also known as mannosyl-oligosaccharide 1,3- 1,6-alpha mannosidase, EC 3.2.1.114, Man2A2). MX is enzymatically and functionally very similar to GMII, and is thought to also function in the N-glycosylation pathway. Also found in this subfamily is class II alpha-mannosidase encoded by Spodoptera frugiperda Sf9 cell. This alpha-mannosidase is an integral membrane glycoprotein localized in the Golgi apparatus. It shows high sequence homology with mammalian Golgi alpha-mannosidase II(GMII). It can hydrolyze p-nitrophenyl alpha-D-mannopyranoside (pNP-alpha-Man), and it is inhibited by swainsonine. However, the Sf9 enzyme is stimulated by cobalt and can hydrolyze (Man)5(GlcNAc)2 to (Man)3(GlcNAc)2, but it cannot hydrolyze GlcNAc(Man)5(GlcNAc)2, which is distinct from that of GMII. Thus, this enzyme has been designated as Sf9 alpha-mannosidase III (SfManIII). It probably functions in an alternate N-glycan processing pathway in Sf9 cells.