This is the N-terminal domain found in N-acetyl-beta-D-glucosaminidase (GcnA) present in Streptococcus gordonii. GcnA is a family 20 glycosidase that cleaves N-acetyl-beta-D-glucosamine and N-acetyl-beta-D-galactosamine from 4-methylumbelliferylated substrates. Similar N-terminal domains have been observed in all family 20 glycosidases although the number of beta-sheet strands may vary from five [1].

1: Structure of N-acetyl-beta-D-glucosaminidase (GcnA) from the endocarditis pathogen Streptococcus gordonii and its complex with the mechanism-based inhibitor NAG-thiazoline. Langley DB, Harty DW, Jacques NA, Hunter N, Guss JM, Collyer CA; J Mol Biol. 2008;377:104-116. PMID:18237743