Cells tightly regulate their enzymatic reactions, allowing necessary reactions to proceed while inhibiting unnecessary or damaging ones. One means of regulating enzyme action is allosteric regulation, in which a molecule binds to an enzyme at a site other than the active site and changes the enzyme's activity. Such a molecule is an allosteric regulator. Some allosteric regulators activate enzymes, while others inhibit them.
In this example of allosteric regulation, an enzyme has a site for the binding of an inhibitor. When the inhibitor binds, it causes the active site to change shape, preventing the substrate from binding. When the inhibitor releases, the enzyme returns to its original conformation, allowing the substrate to bind to the active site and convert into products.
Most enzymes exist in more than one shape, converting on their own between active and inactive conformations. If a substrate molecule encounters the active form, it can bind to the active site. Additionally, the active form can be stabilized by the binding of an activator to another site on the enzyme, making the binding of a substrate molecule more likely.
An allosteric inhibitor can bind to the inactive state and stabilize it in the inactive conformation. When an inhibitor is present, fewer of the enzymes are in a conformation available for substrate binding.
Allosteric regulators bind reversibly. The inhibitor can leave the enzyme, and the enzyme can then continue to toggle between its two conformations. Substrate molecules can fit into the active sites again when the enzyme toggles back to its active conformation.
Enzymes speed up chemical reactions, making reactions occur at the right time and place in a cell. Although enzymes are essential for life, they must be tightly regulated so that cells focus their energy and resources on appropriate cellular reactions. In this tutorial, we examine a means of regulating enzyme activity—allosteric regulation—in which regulators bind to the enzyme at a site different from the active site. Allosteric regulators are either inhibitors or activators. The binding of the regulator affects the activity of the enzyme.
Cells also use other means of regulating enzymes. For example, a cell may regulate the expression of a gene that codes for a particular enzyme. Also, a cell may modify an enzyme's activity by covalently attaching molecules, such as phosphate groups, to them. Cells use these and other regulation mechanisms to keep metabolic reactions in check.