Intramolecular Catalysis of Acylation and Deacylation in Peptides Containing Cysteine and Histidine

A series of peptides having various arrangements of cysteine and histidine was synthesized to determine the potential of thiol-imidazole intramolecular systems for catalysis of hydrolytic reactions. With substituted phenyl acetates as substrates, no cooperative effect of histidine was observed for the acylation of the cysteine residue. In the deacylation of the S-acylated peptides, again the intramolecular histidine had little or no effect. However, in the presence of the thiol specific reagents. 5,5′-dithiobis(2-nitrobenzoic acid) (Ellman's reagent) and N-ethylmaleimide, the rates of appearance of free SH from S-acylated peptides were much greater than the observed deacylation rates in the absence of these reagents. For the peptide N-Ac-Gly-Arg-Phe-Cys(Ac)-Phe-His-Gly-COOH, the rate constant for the appearance of free SH in the presence of Ellman's reagent was3.1 X 10^-2min-1 under conditions for which the observed deacylation rate was 3.2 X 10-⁴min^-1 (pH 8.1, 25 °C).This trapping effect was not observed for cysteine peptides lacking histidine or for mixtures of N.S-diacetylcysteirie and imidazole free in solution. These observations indicate the existence of a rapid, reversible intramolecular transfer of the acetyl group between the cysteine and histidine residues which greatly favors cysteine. A mechanism inhibiting back-attack by cysteine of the acyl histidine would allow efficient deacylation to occur through intramolecular nucleophilic catalysis by the imidazole group. The cysteine proteases may have evolved such a mechanism.