Cells regulate their biochemical processes by inhibiting or activating enzymes.

The cell uses specific molecules to regulate enzymes in order to promote or inhibit certain chemical reactions. Sometimes it is necessary to inhibit an enzyme to reduce a reaction rate, and there is more than one way for this inhibition to occur including competitive, non-competitive, mixed, and uncompetitive.

In competitive inhibition, an inhibitor molecule is similar enough to a substrate that it can bind to the enzyme’s active site to stop it from binding to the substrate. It “competes” with the substrate to bind to the enzyme.

In non-competitive inhibition, inhibitor molecules bind to an enzyme at the allosteric site. Their binding induces a conformational change that reduces the affinity of the enzyme’s active site for its substrate. The binding of this allosteric inhibitor changes the conformation of the enzyme and its active site, so the substrate is not able to bind. This prevents the enzyme from lowering the activation energy of the reaction, and the reaction rate is reduced.

Mixed inhibition is a type of enzyme inhibition in which the inhibitor may bind to the enzyme whether or not the enzyme has already bound the substrate. It does, however, have a greater affinity for one state or the other. It is called “mixed” because it can be seen as a conceptual “mixture” of competitive inhibition and uncompetitive inhibition. In mixed inhibition, the inhibitor binds to an allosteric site, i.e. a site different from the active site where the substrate binds. However, not all inhibitors that bind at allosteric sites are mixed inhibitors.

In uncompetitive inhibition, the inhibitor binds only to the enzyme-substrate complex. The inhibitor-bound complex forms mostly under concentrations of high substrate and the ES-I complex cannot release product while the inhibitor is bound.

Practice Questions

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Key Points

• Feedback inhibition involves the use of a reaction product to regulate its own further production.

• Enzymes can be inhibited and have their rate of reaction decreased in several ways.

• In competitive inhibition, an inhibitor molecule competes with a substrate by binding to the enzyme ‘s active site so the substrate is blocked.

• In noncompetitive inhibition (also known as allosteric inhibition), an inhibitor binds to an allosteric site; the substrate can still bind to the enzyme, but the enzyme is no longer in an optimal position to catalyze the reaction.

• Mixed inhibition is a type of enzyme inhibition in which the inhibitor may bind to the enzyme whether or not the enzyme has already bound the substrate.

• In uncompetitive inhibition, the inhibitor binds only to the enzyme-substrate complex.

Key Terms

Allosteric Site: A site other than the active site on an enzyme.

Active site: The active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed.

Substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.