Synthesis and catalytic properties of the pentapeptide, Thr-Ala-Cys-His-Asp
The pentapeptide, Thr-Ala-Cys-His-Asp, has been synthesised by use of S-diphenylmethyl-protected cysteine. The catalytic activity towards p-nitrophenyl acetate was comparable with those of cysteine hydrochloride and glutathione, but did not depend on the presence of the thiol group. The hydrolysis of p-nitrophenyl acetate by the pentapeptide obeyed Michaelis–Menten kinetics and it was further shown that an acetyl peptide intermediate was formed by reaction with the imidazole group of the histidine residue. However, the pentapeptide was only a slightly better catalyst than imidazole itself. It would appear therefore that for concerted action between groups, much greater orientational constraint is needed than is obtained by placing amino-acid residues adjacent to each other in a peptide chain.