Asparagine 79 is an important amino acid for catalytic activity and substrate specificity of bile salt hydrolase (BSH)

Microbial bile salt hydrolases (BSHs), a member of cholylglycine hydrolase (CGH) family, catalyze the hydrolysis of glycine and taurine-linked bile salts in the small intestine of human. BSH is evolutionarily related to penicillin V acylase (PVA) which hydrolyses a penicillin V and is also a member of CGH family. Although, five of the six amino acids, C2, R16, D19, N170, N79 and R223, supposed to be responsible for catalytic activity of BSH enzyme, are strictly conserved in all CGH family members, N79 is partially conserved in this family. In this study, in order to analyze the correlation between N79 and catalytic activity or substrate specificity of BSH, the polar and acidic N79 was substituted for the aliphatic and hydrophobic V79 by PCR-based site directed mutagenesis and mutant recombinant BSH was expressed in E. coli BLR(DE3). While the effects of the mutation on catalytic activity and substrate specificity of BSH were detected by ninhydrin assay. The effect of this mutation on the stability of the BSH was observed by SDS-PAGE analysis. Although V79 mutation resulted in stable BSH, it reduced the catalytic activity and altered substrate specificity of BSH. The results suggested that N79 might be important for substrate binding and catalytic turnover of BSH.