If we were to extract the chemicals from a living cell and watch them react with one another, we'd probably have to wait a long time—perhaps longer than the normal life of the cell—to see many reactions take place. On their own, many of the chemicals react too slowly to support an organism's metabolism. Living cells circumvent this problem by producing biological catalysts, called enzymes. Enzymes lower the amount of energy required (activation energy) to trigger chemical reactions, making reactions more likely to occur and thereby increasing reaction rates. In the accompanying animation, we look at a reaction in which an enzyme holds a substrate molecule at the enzyme's active site and facilitates the molecule's conversion into a product. We will examine the relative rates of this reaction in the presence or absence of the enzyme, as well as in the presence of enzyme inhibitors.
This test tube contains substrate molecules that have a tendency to convert to another form by splitting into product molecules. However, this reaction occurs only very slowly without an enzyme.
Enzymes speed up the rate of a chemical reaction. The active site of the enzyme provides a pocket that holds the substrate molecule in position. The enzyme may put strain on the molecule, or perhaps electrical charges on the enzyme facilitate the chemical reaction.
Enzymes can be regulated by various chemicals that act as inhibitors to the reaction. A competitive inhibitor is similar to the enzyme's natural substrate and binds to the active site of the enzyme, preventing substrate molecules from binding. By blocking the active site, a competitive inhibitor significantly decreases the rate of the enzyme-catalyzed reaction. The binding of a competitive inhibitor is reversible. Also, with a competitive inhibitor, if the substrate concentration is increased, the substrate is more likely to bind, and the enzyme is active again.
An uncompetitive inhibitor is a type of regulator that binds after the substrate binds to the enzyme. The binding of substrate to enzyme creates a binding site for the uncompetitive inhibitor, which binds and then prevents the release of products. Unlike competitive inhibition, uncompetitive inhibition cannot be overcome by adding more substrate, because the inhibitor and substrate don't compete for the same binding site. However, uncompetitive inhibitors can become unbound, allowing some product formation. Uncompetitive inhibitors decrease the rates of enzyme-catalyzed reactions.
A noncompetitive inhibitor is a type of regulator that binds to an enzyme at a place other than the active site. Upon binding to the noncompetitive inhibitor, the enzyme changes shape so that normal substrate binding cannot occur. Noncompetitive inhibitors decrease the rates of enzyme-catalyzed reactions. Noncompetitive inhibitors can become unbound, so their effects are reversible.
A variety of molecules can affect the rates of enzyme-catalyzed reactions. This animation focuses on chemical inhibitors that cause reversible inhibition. That is, they can bind and thereby inhibit the action of the enzyme, and then they can become unbound, allowing the enzyme to function again.
Competitive inhibitors bind to the active site of the enzyme, preventing substrate binding. This type of inhibition can be overcome by increasing the concentration of substrate molecules, because the inhibitor and substrate compete for the same binding site.
Uncompetitive inhibitors bind to the enzyme after the substrate has already bound. Uncompetitive inhibitors prevent the release of products. This type of inhibition is not overcome by an increase in substrate concentration.
Noncompetitive inhibitors bind at a site other than the active site, changing enzyme structure so that normal substrate binding cannot occur. This type of inhibition is not overcome by an increase in substrate concentration.