FIGURE 3-25 Schematic model of an enzyme’s reaction mechanism. Enzyme kinetics suggest that enzymes (E) bind substrate molecules (S) at a fixed and limited number of sites –the enzymes’ active sites. The bound species is known as an enzyme-substrate (ES) complex. The ES complex is in equilibrium with the unbound enzyme and substrate (double arrows) and is an intermediate step in the conversion of substrate to products (P).