PP: Enzyme Regulation

The ability to control enzyme activity is essential in cells in order to produce molecules when needed and to conserve energy/resources. One way to regulate enzymes is through activators and inhibitors. These molecules alter the enzyme's conformation or block the active site, but they are not involved in the reaction in any way. Enzymes can also be regulated through covalent modification by phosphorylation. The reversible addition of a phosphate group alters the conformation of the enzyme, increasing or decreasing its activity by affecting substrate binding and/or its ability to produce products. In phosphorylation, kinases add phosphate groups while phosphatases remove them. Another form of enzyme regulation is the cleavage of an inactive enzyme, where catalytically inactive precursors are cut to create the active enzyme. The activation of chymotrypsinogen to alpha-chymotrypsin is an example of this.

Irreversible inhibitors also regulate enzymes by permanently impairing enzyme activity, typically via covalent modification. Irreversible inhibitor usually, but not always, result in the complete loss of enzyme activity. On the other hand, reversible inhibitors are not permanent, and they come in three forms: competitive, uncompetitive, and noncompetitive/mixed. Competitive inhibitors bind to the active site and prevent substrate binding while uncompetitive inhibitors only bind to the enzyme-substrate complex at a location other than the active site; however, noncompetitive/mixed inhibitors are able to bind to the free enzyme and the enzyme-substrate complex at a location other than the active site, although not at the same time.