Effect of substrate concentration

In a simple chemical reaction involving a single substrate, the rate at which product is formed increases linearly as the concentration of the substrate increases. When more substrate is available, more will undergo reaction.

With enzyme-catalysed reactions, the change in the rate of formation of product with increasing concentration of substrate is not linear, but hyperbolic, as shown in Figure 2.8. At relatively low concentrations of substrate (region A in Figure 2.8), the catalytic site of the enzyme may be empty at times, until more substrate binds to undergo reaction. Under these conditions, the rate of formation of product is limited by the time taken for another molecule of substrate to bind to the enzyme. A relatively small change in the concentration of substrate has a large effect on the rate at which product is formed in this region of the curve.

At high concentrations of substrate (region B in Figure 2.8), as product leaves the catalytic site, another molecule of substrate binds more or less immediately, and the enzyme is saturated with substrate. The limiting factor in the formation of product is now the rate at which the enzyme can catalyse the reaction, and not the availability of substrate. The enzyme is acting at or near its maximum rate (or maximum velocity, usually abbreviated to Vmax). Even a relatively large change in the concentration of substrate has little effect on the rate of formation of product in this region of the curve.

Figure 2.8 The substrate dependence of an enzyme-catalysed reaction. In region A the enzyme is very unsaturated with substrate, and the rate of reaction increases sharply with increasing concentration of substrate. In region B the enzyme is almost saturated with substrate, and there is little change in the rate of reaction with increasing substrate.

[substrate]

Figure 2.8 The substrate dependence of an enzyme-catalysed reaction. In region A the enzyme is very unsaturated with substrate, and the rate of reaction increases sharply with increasing concentration of substrate. In region B the enzyme is almost saturated with substrate, and there is little change in the rate of reaction with increasing substrate.

From a graph of the rate of formation of product versus the concentration of substrate (Figure 2.8), it is easy to estimate the maximum rate of reaction that an enzyme can achieve (Vmax) when it is saturated with substrate. However, it is not possible to determine from this graph the concentration of substrate required to achieve saturation, because the enzyme gradually approaches Vmax as the concentration of substrate increases.

It is easy to estimate the concentration of substrate at which the enzyme has achieved half its maximum rate of reaction. The concentration of substrate to achieve half V

max is called the Michaelis constant of the enzyme (abbreviated to Km), to commemorate Michaelis, who, together with Menten, first formulated a mathematical model of the dependence of the rate of enzymic reactions on the concentration of substrate.

The K of an enzyme is not affected by the amount of the enzyme protein that is present. It is an (inverse) index of the affinity of the enzyme for its substrate. An enzyme which has a high Km has a relatively low affinity for its substrate compared with an enzyme which has a lower Km. The higher the value of Km, the greater is the concentration of substrate required to achieve half-saturation of the enzyme.

In general, enzymes that have a low Km compared with the normal concentration of substrate in the cell are likely to be acting at or near their maximum rate, and hence to have a more or less constant rate of reaction despite (modest) changes in the concentration of substrate. By contrast, an enzyme which has a high K compared with the normal concentration of substrate in the cell will show a large change in the rate of reaction with relatively small changes in the concentration of substrate.

If two enzymes in a cell can both act on the same substrate, catalysing different reactions, the enzyme with the lower Km will be able to bind more substrate, and therefore its reaction will be favoured at relatively low concentrations of substrate. Thus, knowing the values of K and V for two enzymes, for example at a branch point in a metabolic pathway (see Figure 2.18), it is possible to predict whether one branch or the other will predominate in the presence of different amounts of the substrate.

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