alpha-Galacturonidase(s): A new class of Family 4 glycoside hydrolases with strict specificity and a unique CHEV active site motif

The catalytic activity of the Family 4 glycosidase LplD protein, whose active site motif is CHEV, is unknown despite its crystal structure having been determined in 2008. Here we identify that activity as being an alpha-galacturonidase whose natural substrate is probably alpha-1,4-di-galacturonate (GalUA(2)). Phylogenetic analysis shows that LplD belongs to a monophyletic clade of CHEV Family 4 enzymes, of which four other members are also shown to be galacturonidases. Family GH 4 enzymes catalyze the cleavage of the glycosidic bond, via a non-canonical redox-assisted mechanism that contrasts with Koshland's double-displacement mechanism. Published by Elsevier B. V. on behalf of the Federation of European Biochemical Societies.