Glycosyl hydrolases family 38 N-terminal domain. Glycosyl hydrolases are key enzymes of carbohydrate metabolism.

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.

alpha-mannosidase

N-terminal catalytic domain of lysosomal alpha-mannosidase and similar proteins; glycoside hydrolase family 38 (GH38). The subfamily is represented by lysosomal alpha-mannosidase (LAM, Man2B1, EC 3.2.1.114), which 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. LAM is expressed in all tissues and in many species. In mammals, the absence of LAM can cause the autosomal recessive disease alpha-mannosidosis. LAM has an acidic pH optimum at 4.0-4.5. It is stimulated by zinc ion and is inhibited by cobalt ion and plant alkaloids, such as swainsonine (SW). LAM catalyzes hydrolysis by a double displacement mechanism in which a glycosyl-enzyme intermediate is formed and hydrolyzed via oxacarbenium ion-like transition states. A carboxylic acid in the active site acts as the catalytic nucleophile in the formation of the covalent intermediate while a second carboxylic acid acts as a general acid catalyst. The same residue is thought to assist in the hydrolysis (deglycosylation) step, this time acting as a general base.