Activating effect of the Ile-Val dipeptide on the catalytic properties of bovine trypsinogen

The dependence of pre-steady-state and steady-state kinetics for the trypsinogen-catalyzed hydrolysis of ZArgONp on the concentration (up to 2.0 M) of the Ile-Val effector dipeptide has been investigated at pH 8.0 and 21 +/- 0.5 degrees C. Kinetic parameters for the hydrolysis of ZArgONp catalyzed by the Ile-Val:zymogen adduct are more favorable than those observed for the free proenzyme but lower than those reported for beta-trypsin; these data indicate that the effector dipeptide induces only a partial activation of the zymogen even under saturating Ile-Val concentrations. From the dependence of kinetic parameters of proenzyme catalysis on the effector dipeptide concentration, values of the equilibrium constants for binding of Ile-Val to the free trypsinogen, to the reversible zymogen-ZArgONp complex and to the proenzyme-ZArg acyl intermediate have been obtained. Thermodynamics of binding of Ile-Val to trypsinogen, in the presence and absence of substrates and inhibitors, are indicative of the presence of different activation levels of the proenzyme, none of which is superimposable on that of beta-trypsin. On this basis, it is suggested that some of these different states could correspond to those involved in the zymogen-to-active-enzyme transition, which should be considered as a multistep process, rather than an all-or-nothing event.