Effect of N-methylation on the modulation by synthetic peptides of the activity of the complement-factor-B-derived serine proteinase CVFBb

Although they share the active-site catalytic triad of less-specific enzymes such as trypsin and chymotrypsin, the serine proteinases of the complement and coagulation cascades each cleave a highly restricted set of substrates. Peptides with sequences similar to that at which C3 is cleaved by the alternative-pathway complement proteinase CVFBb were synthesized by solid-phase methodology and examined for their effects on the activity of this enzyme as measured by three different types of assays. It was found that a peptide methylated at the scissile bond was a far more effective inhibitor of the cleavage of the protein substrate C5 and of the lysis of guinea-pig erythrocytes by the alternative pathway than was the equivalent unmethylated peptide. Whereas the unmethylated peptide inhibited cleavage of the peptide substrate, the methylated peptide actually stimulated cleavage in this assay. This stimulation was found to be due to a 2.8-fold increase in kcat; the dissociation constant for the substrate was not altered significantly. One model consistent with this behaviour is that the binding of the activator peptide in the extended substrate-recognition region stabilizes a catalytically more active conformation of the active site. A small peptide substrate may have access to such an activated active site, whereas the larger substrate, C5, may be excluded from the site. These results demonstrate that the observed effect of a given compound on activity of an enzyme with an extended substrate-recognition region may depend upon the substrate.