KINETIC STUDIES OF THE ALKALINE MESENTERICOPEPTIDASE

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.

A study of the alkaline mesentericopeptidase active site by means of peptide chloromethyl ketones

The kinetics of inactivation of alkaline mesentericopeptidase was studied using chloromethyl ketone derivatives of amino acids and peptides. It was shown that Tos-LysCH2Cl and Tos-PheCH2Cl did not influence the enzyme activity, while the inhibitory effect of Cbz-Ala-Gly-PheCH2Cl was 35 times that of Cbz-Ala-PheCH2Cl. The dependence of the pseudo-first order rate constant of the enzyme inactivation by Cbz-Ala-Gly-PheCH2Cl on pH and temperature indicated that a group with a pK of 6.59 and deltaHi of 7.7 kcal/mol was the site of the inhibitor's attack. Amino acid analysis of the modified totally inactive enzyme revealed a definite loss of histidine and after performic acid oxidation a recovery of 3-carboxymethyl histidine. The whole set of experimental data is convincing evidence on behalf of a selective alkylation of the N-3 of the active site histidine after treatment with Cbz-Ala-PheCH2Cl and Cbz-Ala-Gly-PheCH2Cl. Alkaline mesenteriocopeptidase possesses an extended active site and only a peptide chloromethyl ketone, covering a determined sequence of theenzyme molecule (S3, S2, S1, S'1, S'2, S'3 ...) is able to provide effective inhibition. The values of the inactivation constant (kinact) for Cbz-Ala-PheCH2Cl and Cbz-Ala-Gly-PheCH2Cl are close to the corresponding values reported for subtilisin Amylosacchariticus.