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.


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.

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Textbook Reference: Concept 3.4 Regulation of Metabolism Occurs by Regulation of Enzymes