Catalytic domain of glycoside hydrolase (GH) families 38 and 57, lactam utilization protein LamB/YcsF family proteins, YdjC-family proteins, and similar proteins.
The superfamily possesses strong sequence similarities across a wide range of all three kingdoms of life. It mainly includes four families, glycoside hydrolases family 38 (GH38), heat stable retaining glycoside hydrolases family 57 (GH57), lactam utilization protein LamB/YcsF family, and YdjC-family. The GH38 family corresponds to class II alpha-mannosidases (alphaMII, EC 3.2.1.24), which contain intermediate Golgi alpha-mannosidases II, acidic lysosomal alpha-mannosidases, animal sperm and epididymal alpha -mannosidases, neutral ER/cytosolic alpha-mannosidases, and some putative prokaryotic alpha-mannosidases. AlphaMII possess a-1,3, a-1,6, and a-1,2 hydrolytic activity, and catalyzes the degradation of N-linked oligosaccharides by employing a two-step mechanism involving the formation of a covalent glycosyl enzyme complex. GH57 is a purely prokaryotic family with the majority of thermostable enzymes from extremophiles (many of them are archaeal hyperthermophiles), which exhibit the enzyme specificities of alpha-amylase (EC 3.2.1.1), 4-alpha-glucanotransferase (EC 2.4.1.25), amylopullulanase (EC 3.2.1.1/41), and alpha-galactosidase (EC 3.2.1.22). This family also includes many hypothetical proteins with uncharacterized activity and specificity. GH57 cleaves alpha-glycosidic bond by employing a retaining mechanism, which involves a glycosyl-enzyme intermediate, allowing transglycosylation. Although the exact molecular function of LamB/YcsF family and YdjC-family remains unclear, they show high sequence and structure homology to the members of GH38 and GH57. Their catalytic domains adopt a similar parallel 7-stranded beta/alpha barrel, which is remotely related to catalytic NodB homology domain of the carbohydrate esterase 4 superfamily.
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