Enzyme Catalysis


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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.


Enzymes speed up chemical reactions. In a cell, enzymes may be regulated by the binding of molecules that either activate or inhibit the function of the enzyme. This animation focuses on chemical inhibitors, specifically two types that cause reversible inhibition. These inhibitors can bind to an enzyme and inhibit its function, 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.

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.