Simple Enzyme Substrate Interactions
The difference between an enzyme-catalyzed reaction and a reaction that is not enzyme-catalyzed is that the reactant (or substrate) must form a complex (or bind) with the enzyme before the reaction can occur. Unlike receptor binding measurements, the substrate concentration (which is analogous to the ligand or agonist concentration) will not be static in an enzyme-catalyzed reaction. The enzyme and substrate combine to form the ES complex. Then, the substrate is converted to product in the enzyme active site. Finally, the enzyme-product complex dissociates. Each of the steps in this mechanism is reversible. The measurement of an enzyme-catalyzed reaction is in actuality a measurement of a physiological effect of the enzyme interacting with its substrate. Enzymes can be inhibited or activated in the same ways in which receptors can be inhibited or activated. Therefore, most of the relationships discussed in Chapter 8 are also valid when applied to enzyme kinetics.