A competitive inhibitor is a compound that binds to the active site of the enzyme in competition with the substrate. Commonly, but not always, such compounds are chemical analogues of the substrate. Although a competitive inhibitor binds to the active site, it does not undergo reaction, or, if it does, not to yield the product that would have been obtained by reaction of the normal substrate.
A competitive inhibitor reduces the rate of reaction because at any time some molecules of the enzyme have bound the inhibitor, and therefore are not free to bind the substrate. However, the binding of the inhibitor to the enzyme is reversible, and therefore there is competition between the substrate and the inhibitor for the enzyme. This means that the sequence of the reaction in the presence of a competitive inhibitor can be shown as:
Figure 2.12 shows the s/v and double-reciprocal plots for an enzyme incubated with various concentrations of a competitive inhibitor. If the concentration of substrate is increased, it will compete more effectively with the inhibitor for the active site of the enzyme. This means that at high concentrations of substrate the enzyme will achieve the same maximum rate of reaction (V ) in the presence or absence of inhibitor.
It is simply that in the presence of inhibitor the enzyme requires a higher concentration of substrate to achieve saturation - in other words, the Km of the enzyme is higher in the presence of a competitive inhibitor.
The effect of a drug that is a competitive inhibitor is that the rate at which product is formed is unchanged, but there is an increase in the concentration of the substrate of the inhibited enzyme in the cell. As the inhibitor acts, so it will cause an increase in the concentration of substrate in the cell, and eventually this will rise high enough for the enzyme to reach a more or less normal rate of reaction. This means that a competitive inhibitor is appropriate for use as a drug where the aim is to increase the available pool of substrate (perhaps so as to allow an alternative reaction to proceed), but inappropriate if the aim is to reduce the amount of product formed.
Figure 2.12 Substrate/velocity and Lineweaver—Burk double-reciprocal plots for an enzyme incubated with varying concentrations of a competitive inhibitor.
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