Glycoside hydrolases cleave glycosidic bonds to release smaller sugars from oligo- or polysaccharides. Some bacteria simultaneously translocate and phosphorylate disaccharides via the phosphoenolpyruvate-dependent phosphotransferase system (PEP-PTS). After translocation, these phospho-disaccharides may be hydrolyzed by GH4 glycoside hydrolases. Other organisms (such as archaea and Thermotoga maritima) lack the PEP-PTS system, but have several enzymes normally associated with the PEP-PTS operon. GH4 family members include 6-phospho-beta-glucosidases, 6-phospho-alpha-glucosidases, alpha-glucosidases/alpha-glucuronidases (only from Thermotoga), and alpha-galactosidases. They require two cofactors, NAD+ and a divalent metal (Mn2+, Ni2+, Mg2+), for activity. Some also require reducing conditions. GH4 glycoside hydrolases are part of the NAD(P)-binding Rossmann fold superfamily, which includes a wide variety of protein families including the NAD(P)-binding domains of alcohol dehydrogenases, tyrosine-dependent oxidoreductases, glyceraldehyde-3-phosphate dehydrogenases, formate/glycerate dehydrogenases, siroheme synthases, 6-phosphogluconate dehydrogenases, aminoacid dehydrogenases, repressor rex, and NAD-binding potassium channel domains, among others.
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