Critical F129 and L138 in loop III of bile salt hydrolase (BSH) in Lactobacillus plantarum B14 are essential for the catalytic activity and substrate specificity

Bile salt hydrolase (BSH) is a gut-bacterial enzyme that influences human health by altering the host fat digestion and cellular energy generation. BSH is essential for deconjugation of the glycine or taurine-conjugated bile salts in the small intestine of humans. Therefore, BSH may be a key microbiome target for the designing of new measures to control some diseases in humans. BSHs, a member of the N-terminal nucleophile (Ntn) hydrolase superfamily, exhibit higher variation in substrate specificity. The phenylalanine-129 (F129) and leucine-138 (L138) in loop III of BSH, thought to be responsible for substrate specificity, are partially conserved in this superfamily. In this study, the aromatic-hydrophobic F129 and aliphatic-hydrophobic L138 of C-terminally His-tagged BSH from Lactobacillus plantarum B14 (LbBSH) was substituted for aliphatic-hydrophobic isoleucine (I) and negatively charged polar glutamate (E) amino acid, respectively, by site-directed mutagenesis and characterized using an Escherichia coli BLR(DE3) expression system. Although both mutations resulted in an assembled and stable recombinant BSHs (rBSHs), they altered the catalytic activity and substrate specificity of rBSH. This is the first experimental finding which confirmed that F129 and L138 were critical amino acids for the catalytic activity and substrate specificity turnover of BSH.