Action of Enzymes

An enzyme functions by associating in a highly specific way with its substrate, the molecule whose reaction it catalyzes. The enzyme bears an active site located within a cleft or pocket and contains a unique molecular configuration. The active site has a flexible surface that enfolds and conforms to the substrate (Figure 4-4). The binding of enzyme to substrate forms an enzyme-substrate complex (ES complex), in which the substrate is secured by covalent bonds to one or more points in the active site of the enzyme. The ES complex is not strong and will quickly dissociate, but during this fleeting moment the enzyme provides a unique chemical environment that stresses certain chemical bonds in the substrate so that much less energy is required to complete the reaction.

If the formation of an enzyme-substrate complex is so rapidly followed by dissociation, how can biochemists be certain that an ES complex exists? The original evidence offered by Leonor Michaelis in 1913 is that, when the substrate concentration is increased while the enzyme concentration is held constant, the reaction rate reaches a maximum velocity.This saturation effect is interpreted to mean that all catalytic sites become filled at high substrate concentration. It is not seen in uncatalyzed reactions. Other evidence includes the observation that the ES complex displays unique spectroscopic characteristics not displayed by either the enzyme or the substrate alone. Furthermore, some ES complexes can be isolated in pure form, and at least one kind (nucleic acids and their polymerase enzymes) has been directly visualized with the electron microscope.

Enzymes that engage in important main-line sequences—such as the crucial energy-providing reactions of the cell that proceed constantly—seem to operate in sets rather than in isolation. For example, conversion of glucose to carbon dioxide and water proceeds through 19 reactions, each requiring a specific enzyme. Main-line enzymes are found in relatively high concentrations in the cell, and they may implement quite complex and highly integrated enzymatic sequences. One enzyme carries out the first step, then passes the product to another enzyme that catalyzes another step, this process continuing until the end of the enzymatic pathway is reached. The reactions may be said to be coupled. Coupled reactions will be explained in a following section on chemical energy transfer by ATP.