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20.5 Enzyme Inhibition The structure of a noncompetitive inhibitor does not resemble the substrate and does not compete for the active site. Learning Goal Describe competitive and noncompetitive inhibition and reversible and irreversible inhibition. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Inhibitors Inhibitors • are molecules that cause a loss of catalytic activity. • prevent substrates from fitting into the active sites. • can be classified as either reversible inhibitors or irreversible inhibitors. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Reversible Inhibition Reversible inhibitors • cause a loss of enzyme activity that can be restored. • can act in different ways but do not form covalent bonds with the enzyme. Reversible inhibition can be competitive or noncompetitive. • Competitive inhibitors compete for the active site. • Noncompetitive inhibitors act on another site that is not the active site. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Competitive Inhibitors A competitive inhibitor • has a chemical structure and polarity similar to the substrate. • competes with the substrate for the active site. • has its effect reversed by increasing substrate concentration. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Antimetabolites: Competitive Inhibitors in Medicine • Some bacterial infections are treated with competitive inhibitors called antimetabolites. • Sulfanilamide competes with p-aminobenzoic acid (PABA), an essential metabolite in the growth cycle of bacteria. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Noncompetitive Inhibitors A noncompetitive inhibitor • has a structure that is much different from that of the substrate. • does not compete for the active site. • distorts the shape of the enzyme, which prevents the binding of the substrate at the active site. • cannot have its effect reversed by adding more substrate. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Irreversible Inhibition In irreversible inhibition, enzyme activity is destroyed when • the inhibitor covalently bonds with R groups of an amino acid that may be near the active site. • the inhibitor changes the shape of the enzyme, which prevents the substrate from entering the active site. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Irreversible Enzyme Inhibitors General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Enzyme Inhibition Summary General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Identify each description of an inhibitor that is either competitive or noncompetitive. A. Increasing substrate reverses inhibition. B. It binds to the enzyme’s surface but not to the active site. C. Its structure is similar to that of the substrate. D. Inhibition is not reversed by adding more substrate. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Identify each description of an inhibitor that is either competitive or noncompetitive. A. Increasing substrate reverses inhibition. B. It binds to the enzyme’s surface but not to the active site. C. Its structure is similar to that of the substrate. D. Inhibition is not reversed by adding more substrate. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake competitive noncompetitive competitive noncompetitive © 2016 Pearson Education, Inc.