The prolyl aminopeptidase from Lactobacillus delbrueckii subsp. bulgaricus belongs to the alpha/beta hydrolase fold family

Prolyl aminopeptidase (PepIP) of Lactobacillus delbrueckii subsp. bulgaricus displays the Gly-x-Ser-x-Gly-Gly consensus motif surrounding the catalytic serine of the prolyl oligopeptidases family. Sequence comparison revealed that this motif and two other domains appear well conserved among bacterial PepIPs and members of the alpha/beta hydrolase fold family. Secondary structural predictions of PepIP were performed from amino acid sequence and corroborated by circular dichroism analysis. These predictions well matched the core structure of alpha/beta hydrolases organised in eight beta-sheets connected by alpha-helices. We obtained 26 mutants of PepIP by chemical or site-directed mutagenesis. Most substitutions associated with stable and inactive mutant proteins were mainly located in the three conserved boxes (including the catalytic serine motif). Taken together, our results strongly suggest that PepIP belongs to the alpha/beta hydrolase fold family and that Ser107, Asp246 and His273 constitute the catalytic triad of the enzyme.