ES Complex and Pre-steady-state Kinetics

Abstract

Enzymes are well-defined chemical structures at the molecular level. They cannot perform their “catalytic fete” from a distance – enzymes have to come in intimate contact with their substrates (reactants). Interaction of an enzyme (E) and its cognate substrate (S) begins the moment the two come together through diffusion. Specific interactions and binding result in the formation of enzyme-substrate (ES) complex. All the transition state(s) and intermediate(s) are represented in this simplified version of ES complex:Changes in the concentrations of reaction participants with time, for an enzyme-catalyzed reaction, are shown in Fig. 11.1. When E and S are brought together, there is an initial buildup of ES complex. The ES complex breaks down to form product (P) and regenerates E. Being a catalyst, normally there will be fewer enzyme molecules in the reaction compared to those of substrate. This sets up a distribution of enzyme molecules between E and ES forms. The amount of P formed increases with time. Eventually the reaction equilibrium is attained. Although the enzyme continues to convert S → P and back, there will be no net change in the concentration of S or P, at equilibrium.