Substrate Concentration Affects Reaction Rate

In what follows, enzyme reactions are treated as if they had only a single substrate and a single product. While most enzymes have more than one substrate, the principles discussed below apply with equal validity to enzymes with multiple substrates.

For a typical enzyme, as substrate concentration is increased, vi increases until it reaches a maximum value Vmax (Figure 8-3). When further increases in substrate concentration do not further increase vi, the enzyme is said to be "saturated" with substrate. Note that the shape of the curve that relates activity to substrate concentration (Figure 8-3) is hyperbolic. At any given instant, only substrate molecules that are combined with the enzyme as an ES complex can be transformed into product. Second, the equilibrium constant for the formation of the enzyme-substrate complex is not infinitely large. Therefore, even when the substrate is present in excess (points A and B of Figure 8-4), only a fraction of the enzyme may be present as an ES complex. At points A or B, increasing or decreasing [S] therefore will increase or decrease the number of ES complexes with a corresponding change in vi. At point C (Figure 8-4), essentially all the enzyme is present as the ES complex. Since no free enzyme remains available for forming ES, further increases in [S] cannot increase the rate of the reaction. Under these saturating conditions, v; depends solely on—and thus is limited by— the rapidity with which free enzyme is released to combine with more substrate.

Figure 8-3. Effect of substrate concentration on the initial velocity of an enzyme-catalyzed reaction.

Figure 8-2. Effect of pH on enzyme activity. Consider, for example, a negatively charged enzyme (EH-) that binds a positively charged substrate (SH+). Shown is the proportion (%) of SH+ [\\\] and of EH- [///] as a function of pH. Only in the cross-hatched area do both the enzyme and the substrate bear an appropriate charge.

High pH

High pH

Figure 8-4. Representation of an enzyme at low (A), at high (C), and at a substrate concentration equal to Km (B). Points A, B, and C correspond to those points in Figure 8-3.

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