Reactivity of sulfhydryl group in peptides and poly-peptides with p-nitrophenylacetate. S to N acyl shift in cysteine.
The activity of poly (Cys-Ser-Phe-Glu-Glu) and related polypeptides as models of cysteine-proteases towards p-nitrophenylacetate was studied. The reaction leads to the formation of an S-acetylated form which proved to be quite stable, except at very high pH values. The presence of imidazole groups in the neighbourhood of sulfhydryl functions seems to result in an enhancement of the activity, which we attributed to a cooperative interaction. However, this interaction has no effect upon the S-deacylation rate. In the case of cysteine we found that the reaction with NPA occurred through a S to N acyl shift. Our results lead us to suggest that, unless convincing proofs of deacylation are given,  the various models of cysteine-proteases studied so far do not behave as "true catalysts".
Reactivity of sulfhydryl group in peptides and poly-peptides with p-nitrophenylacetate. S to N acyl shift in cysteine. The activity of poly (Cys-Ser-Phe-Glu-Glu) and related polypeptides as models of cysteine-proteases towards p-nitrophenylacetate was studied. The reaction leads to the formation of an S-acetylated form which proved to be quite stable, except at very high pH values. The presence of imidazole groups in the neighbourhood of sulfhydryl functions seems to result in an enhancement of the activity, which we attributed to a cooperative interaction. However, this interaction has no effect upon the S-deacylation rate. In the case of cysteine we found that the reaction with NPA occurred through a S to N acyl shift. Our results lead us to suggest that, unless convincing proofs of deacylation are given,  the various models of cysteine-proteases studied so far do not behave as "true catalysts".