7.13: Enzyme Inhibition

Inhibitors are molecules that reduce enzyme activity by binding to the enzyme. In a normally functioning cell, enzymes are regulated by a variety of inhibitors. Drugs and other toxins can also inhibit enzymes. Some inhibitors bind to the enzyme’s active site, while others inhibit enzymatic activity by binding to other sites on the protein structure.

Competitive inhibitors occupy the active site of enzymes, making them unable to accommodate the substrate. However, sufficiently high concentrations of the substrate can outcompete the inhibitor; as a result, competitive inhibitors slow an enzymes initial reaction rate but do not impact the enzyme’s maximum rate. One example of a competitive inhibitor is the drug disulfiram, used to treat chronic alcoholism. When alcohol is ingested, it is normally converted to acetaldehyde, which is then converted to acetyl coenzyme A by acetaldehyde dehydrogenase. Disulfiram binds to and occupies the active site of acetaldehyde dehydrogenase, making the enzyme unable to perform this conversion. As a result, a patient taking disulfiram immediately begins to experience hangover-like symptoms, such as headache, thereby decreasing alcohol consumption.

Noncompetitive inhibitors bind to distinct sites on the enzyme, away from the active site. These are called allosteric sites and when molecules bind to them, the shape of the active site is changed such that the enzyme has a lower affinity for the substrate. Because noncompetitive inhibitors do not occupy the active site, the presence of additional substrate is unable to overcome noncompetitive inhibition and the enzyme is unable to achieve its maximum reaction rate.

Covalent binding between an inhibitor and an enzyme is usually irreversible, as in the case of some toxins. Most regulatory inhibitors normally active in the cell interact with enzymes by weak interactions. This type of binding is reversible and useful for the regulation of metabolic processes. The exploration of new molecules to competitively and non-competitively inhibit enzymes regulating cell growth in cancer is an active area of research.

Specific chemicals can regulate the action of enzymes by inhibiting, or blocking, their function. Enzyme inhibitors come in two different forms: competitive and non-competitive.

A competitive inhibitor is similar enough to the enzyme's specific substrate that it can bind to the active site and block the substrate from binding. This action essentially decreases the number of enzymes available to bind to the substrate.

In contrast, a non-competitive inhibitor will bind away from, but still influence, the active site by changing the enzyme's shape, for example, and greatly reducing the affinity for the substrate binding to the active site, preventing the enzyme from functioning properly.

In addition, both types of inhibitors differentially affect the rate of a chemical reaction.

Compared to a control, a normal enzymatic reaction rate, a reaction including a competitive inhibitor would take longer to reach Vmax, the maximum reaction rate, and would require more substrate to do so, as there must be enough substrate to consistently outcompete the inhibitor for access to the active sites.

On the other hand, a non-competitive inhibitor would not allow the rate to reach Vmax, because the number of enzymes available for binding is reduced.