The Kinetics of the Conversion of Chicken Pepsinogen to Chicken Pepsin

The kinetics of the conversion of chicken pepsinogen to chicken pepsin was studied at 25°C by following the appearance of peptic activity using a pH‐static method and by following the changes in the fluorescence emission of the protein. It was found that in pH range of 2–4 both processes were first‐order reactions, and that the change in fluorescence emission was faster than the appearance of activity. The results are interpreted in terms of a unimolecular mechanism comprising two steps. In this pH range the zymogen undergoes a rapid conformational change to form an intermediate, and the subsequent conversion of this intermediate to the active enzyme is the rate‐determining step. In this step the amino‐terminal peptide of the zymogen is split by intramolecular catalysis which requires the protonation of a group, presumably a carboxyl group, with a pK of 3.1 in the active site of the enzyme. Above pH 4 the conformational change becomes the rate‐deter mining step in the unimolecular activation process, and a bimolecular pathway in which chicken pepsinogen in its native conformation is activated by active chicken pepsin becomes apparent. It was found that the peptide fragment formed during the conversion of chicken pepsinogen to chicken pepsin, though not an inhibitor of chicken pepsin activity, can combine with chicken pepsin and protect it against denaturation at pH 10.