Optical and chemical identification of kinetic steps in trypsin- and chymotrypsin-catalysed reactions

Abstract

Previous interpretations of the mechanism of trypsin- and chymotrypsin-catalyzed reactions in terms of 2 intermediates, the Michaelis complex and an acyl-enzyme, were based on steady-state studies and on the observation of individual steps under sub-optimum conditions. In the present paper new methods for the rapid analysis of chemical events and for the spectrophotometric detection of individual steps are applied to these 2 enzymes. These methods can be used to study reactions with specific amino acid ester substrates. It can be shown that there are at least 3 distinct steps between the Michaelis complex and the release of ethanol; the latter is likely to correspond to acyl-enzyme formation. The relative rates of these 3 steps are measured by rapid-flow techniques from observations of the displacement of chromophoric inhibitors and reactions with specific substrates containing chromophores, as well as from ethanol analyses during a single turnover of the enzyme reactions. It is concluded that the reactions of trypsin and chymotrypsin with their specific substrates involve the formation of a specially reactive conformation of the enzyme- substrate complex and that the rate constants involved in this rearrangement are at least as important for the overall reaction as those of the subsequent formation and decomposition of the acyl-enzyme.