Abstract
Cellobiose 2-epimerase (CE, EC 5.1.3.11) catalyzes the reversible epimerization of cellobiose to 4-O-β-D-glucopyranosyl-D-mannose. In this study, we found a CE gene in the genome sequence of non-cellulolytic Bacteroides fragilis NCTC 9343. The recombinant enzyme, expressed in Escherichia coli cells, catalyzed a hydroxyl stereoisomerism at the C-2 positions of the reducing terminal glucose and at the mannose moiety of cello-oligosaccharides, lactose, β-mannobiose (4-O-β-D-mannopyranosyl-D-mannose), and globotriose [O-α-D-galactopyranosyl-(1→4)-O-β-D-galactopyranosyl-(1→4)-D-glucose]. The CE from B. fragilis showed less than 40% identity to reported functional CEs. It exhibited 44–63% identities to N-acyl-D-glucosamine 2-epimerase-like hypothetical proteins of unknown function in bacterial genome sequences of the phyla Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi, and Verrucomicrobia. On the other hand, it showed less than 26% identity to functional N-acyl-D-glucosamine 2-epimerases. Based on the amino acid homology and phylogenetic positions of the functional epimerases, we emphasize that many genes for putative N-acyl-D-glucosamine 2-epimerases and related hypothetical proteins of unknown function reported to date in the bacterial genomes should be annotated as CE-like proteins or putative CEs.
