Structural and functional characterization of a novel α/β hydrolase from cariogenic pathogen Streptococcus mutans

The protein Smu.1393c from Streptococcus mutans is annotated as a putative α/β hydrolase, but it has low sequence identity to the structure-known α/β hydrolases. Here we present the crystal structure of Smu.1393c at 2.0 Å resolution. Smu.1393c has a fully open alkaline substrate pocket, whose conformation is unique among other similar hydrolase structures. Three residues, Ser101, His251, and Glu125, were identified as the active center of Smu.1393c. By screening a series of artificial hydrolase substrates, we demonstrated Smu.1393c had low carboxylesterase activity towards short-chain carboxyl esters, which provided a clue for exploring the in vivo function of Smu.1393c.