Some poisons or toxins exert their damaging effects by acting as enzyme inhibitors. For example, the action of cyanide and carbon monoxide as poisons depends on their ability to inhibit enzyme important the utilization of oxygen in cellular respiration. Poisons and toxins typically act irreversibly by combining so strongly with enzymes, either covalently or noncovalently, that the inhibition is essentially permanent. Some irreversible poisons, rather than combining with the enzyme, destroy enzyme activity by chemically modifying critical amino acid side groups. The cell has built in mechanisms to control directly both enzyme concentration and activity.
First cells are able to regulate whether an enzyme is present at all.
This type of control regulates protein synthesis and will be discussed in a later chapter. Cells also have ways to control the level of activity of enzymes that have already been synthesized and are present in the cell. In noncompetitive inhibition, a molecule binds to an enzyme but not at the active site. The other binding site is called the allosteric. The molecule that binds to the allosteric site is an inhibitor because it causes a change in the 3-dimensional structure of the enzyme that prevents the substrate from binding to the active site.