Secondary interactions in mesentericopeptidase-catalyzed hydrolysis of peptide ester and 4-nitroanilide substrates.

Five substrate series with the formulae Z-(Gly)n-Phe-OMe, Z-(Ala)n-Phe-OMe, Ac-(Ala)n-Phe-OMe, Z-(Gly)n-Phe-NA, and Suc-(Gly)n-Phe-NA (n = 0-2) (Z-benzyloxycarbonyl) were synthesized and used to study the active site of mesentericopeptidase (EC 3.4.21). The elongation of the peptide chain in all series leads to a 100- to 300-fold increase of kcat/Km. This indicates an extended substrate binding site, comprising at least three subsites (S1-S3). The sequence P1-P3 that fits these subsites is Phe-Ala-Ala. Mesentericopeptidase responds to the elongation of the peptide chain in the series Ac-(Ala)n-Phe-OMe in a way similar, but not identical, to subtilisin Carlsberg and subtilisin BPN'. The poor amidolytic activity of mesentericopeptidase and subtilisins toward 4-nitroanilides with peptide sequences matching the S1-S3 subsites is discussed in terms of unfavorable S'1-P'1 interaction.