Kinetics of Enzyme-Catalyzed Reactions

Examination of the change in reaction velocity as the reactant concentration is varied is one of the primary measurements in kinetic analysis. Returning to A ® P, a plot of the reaction rate as a function of the concentration of A yields a straight line whose slope is k (Figure 14.6).

Figure 14.6 •A plot of v versus [A] for the unimolecular chemical reaction, A ® P, yields a straight line having a slope equal to k.

The more A that is available, the greater the rate of the reaction, v. Similar analyses of enzyme-catalyzed reactions involving only a single substrate yield remarkably different results (Figure 14.7). At low concentrations of the substrate S, v is proportional to [S], as expected for a first-order reaction. However, v does not increase proportionally as [S] increases, but instead begins to level off. At high [S], v becomes virtually independent of [S] and approaches a maximal limit. The value of v at this limit is written V_max. Because rate is no longer dependent on [S] at these high concentrations, the enzyme-catalyzed reaction is now obeying zero-order kinetics; that is, the rate is independent of the reactant (substrate) concentration. This behavior is a saturation effect: when v shows no increase even though [S] is increased, the system is saturated with substrate. Such plots are called substrate saturation curves. The physical interpretation is that every enzyme molecule in the reaction mixture has its substrate-binding site occupied by S. Indeed, such curves were the initial clue that an enzyme interacts directly with its substrate by binding it.

Figure 14.7 • Substrate saturation curve for an enzyme-catalyzed reaction. The amount of enzyme is constant, and the velocity of the reaction is determined at various substrate concentrations. The reaction rate, v, as a function of [S] is described by a rectangular hyperbola. At very high [S], v = V_max. That is, the velocity is limited only by conditions (temperature, pH, ionic strength) and by the amount of enzyme present; v becomes independent of [S]. Such a condition is termed zero-order kinetics. Under zero-order conditions, velocity is directly dependent on [enzyme]. The H₂O molecule provides a rough guide to scale. The substrate is bound at the active site of the enzyme.

Date: 2016-01-03; view: 978