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Noncompetitive Inhibitors

The other type of inhibition is noncompetitive inhibition . In noncompetitive inhibition, a molecule binds to an enzyme somewhere other than the active site. This changes the enzyme's three-dimensional structure so that its active site can still bind substrate with the usual affinity, but is no longer in the optimal arrangement to stabilize the transition state and catalyze the reation.

On the macroscopic scale, noncompetitive inhibition lowers the V_max. Thus, the enzyme simply cannot catalyze the reaction with the same efficiency as the uninhibited enzyme. Note that noncompetitive inhibition cannot be overcome by raising the substrate concentration like competitive inhibition can.

Select either uninhibited or inhibited from the boxes below. Then click in the image area to see the course of an uninhibited or a noncompetitively inhibited enzymatic reaction.

Do you want to see the uninhibited or inhibited reaction? Make your choice from the radio boxes above.

For example, the amino acid alanine noncompetitively inhibits the enzyme pyruvate kinase. Alanine is one product of a series of enzyme-catalyzed reactions, the first step of which is catalyzed by pyruvate kinase.

Why does it make sense for the product of an enzymatic chain of reactions to inhibit one of the enzymes earlier in the chain? Type your answer in the space provided, then click on the Check button.

This prevents the unnecessary buildup of molecules. Once the cell has enough alanine, for example, it uses alanine to shut off the chain that produces more.

Some inhibitors have the effects of both competitive and noncompetitive inhibition, i.e., they affect both the enzyme's affinity for substrate and the maximal rate of catalysis. Such inhibitors are called mixed inhibitors.