Kinetics of the Interaction of α‐Chymotrypsin with Trypsin Kallikrein Inhibitor (Kunitz) in Which the Reactive‐Site Peptide Bond Lys‐15–Ala‐16 is Split

Abstract
Modified trypsin kallikrein inhibitor (I), with the reactive-site peptide bond Lys-15-Ala-16 split, reacts with .alpha.-chymotrypsin (E) via an intermediate X to the stable tetrahedral complex C: E + I .dblarw. X .fwdarw. C. Formation of X constitutes a fast pre-equilibrium (equilibrium constant Kx = 7 .times. 10-5 M, association rate constant kx = 4 .times. 103 M-1 s-1) to the slow reaction X .fwdarw. C (rate constant kc = 2 .times. 10-3 s-3), all values at pH 7.5. No intermediate X is observed when .alpha.-chymotropsin [EC 3.4.21.1] reacts with I-OMe in which the carboxyl group of Lys-15 is esterified by methanol. The observation as well as the different pH dependence of the overall association rate constants in the case of I and I-OMe indicate that formation of X precedes formation of the acyl enzyme in the catalytic pathway. The data are compared to the similar results obtained with .beta.-trypsin and I or I-OMe.
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