N-terminal catalytic domain of putative prokaryotic class II alpha-mannosidases; glycoside hydrolase family 38 (GH38)
This mainly bacterial subfamily corresponds to a group of putative class II alpha-mannosidases, including various proteins assigned as alpha-mannosidases, Streptococcus pyogenes (SpGH38) encoded by ORF spy1604. Escherichia coli MngB encoded by the mngB/ybgG gene, and Thermotoga maritime TMM, and similar proteins. SpGH38 targets alpha-1,3 mannosidic linkages. SpGH38 appears to exist as an elongated dimer and display alpha-1,3 mannosidase activity. It is active on disaccharides and some aryl glycosides. SpGH38 can also effectively deglycosylate human N-glycans in vitro. MngB exhibits alpha-mannosidase activity that catalyzes the conversion of 2-O-(6-phospho-alpha-mannosyl)-D-glycerate to mannose-6-phosphate and glycerate in the pathway which enables use of mannosyl-D-glycerate as a sole carbon source. TMM is a homodimeric enzyme that hydrolyzes p-nitrophenyl-alpha-D-mannopyranoside, alpha -1,2-mannobiose, alpha -1,3-mannobiose, alpha -1,4-mannobiose, and alpha -1,6-mannobiose. The GH38 family contains retaining glycosyl hydrolases that employ 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. Divalent metal ions, such as zinc or cobalt ions, are suggested to be required for the catalytic activities of typical class II alpha-mannosidases. However, TMM requires the cobalt or cadmium for its activity. The cadmium ion dependency is unique to TMM. Moreover, TMM is inhibited by swainsonine but not 1-deoxymannojirimycin, which is in agreement with the features of cytosolic alpha-mannosidase.
Jiyao W et al.(2023). "The conserved domain database in 2023.",