Substrate Concentration and Reversible Reactions

At a given level of enzyme concentration, the rate of product formation will increase as the substrate concentration increases. Eventually, however, a point will be reached where additional increases in substrate concentration do not result in comparable increases in reaction rate. When the relationship between substrate concentration and reaction rate reaches a plateau of maximum velocity, the enzyme is said to be saturated. If we think of enzymes as workers in a plant that converts a raw material (say, metal ore) into a product (say, iron), then enzyme saturation is like the plant working at full capacity, with no idle time for the workers. Increasing the amount of raw material (substrate) at this point cannot increase the rate of product formation. This concept is illustrated in figure 4.6.

Some enzymatic reactions within a cell are reversible, with both the forward and backward reactions catalyzed by the

■ Figure 4.6 The effect of substrate concentration on the rate of an enzyme-catalyzed reaction. When the reaction rate is at a maximum, the enzyme is said to be saturated.

same enzyme. The enzyme carbonic anhydrase, for example, is named because it can catalyze the following reaction:

H2CO3 ^ H2O + CO2 The same enzyme, however, can also catalyze the reverse reaction:

The two reactions can be more conveniently illustrated by a single equation with double arrows:

The direction of the reversible reaction depends, in part, on the relative concentrations of the molecules to the left and right of the arrows. If the concentration of CO2 is very high (as it is in the tissues), the reaction will be driven to the right. If the concentration of CO2 is low and that of H2CO3 is high (as it is in the lungs), the reaction will be driven to the left. The principle that reversible reactions will be driven from the side of the equation where the concentration is higher to the side where the concentration is lower is known as the law of mass action.

Although some enzymatic reactions are not directly reversible, the net effects of the reactions can be reversed by the action of different enzymes. Some of the enzymes that convert glucose to pyruvic acid, for example, are different from those that reverse the pathway and produce glucose from pyruvic acid. Likewise, the formation and breakdown of glycogen (a polymer of glucose) are catalyzed by different enzymes.

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