Compete with the substrate for the active site of an

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compete with the substrate for the active site of an enzyme. The rate of the reaction in the presence of an inhibitor (fixed conc.) will increase with increasing substrate concentration. E + S E.S E + I E. I competition Those active sites blocked by E. I are unavailable for the catalyzed reaction. Noncompetitive inhibitors bind to the enzyme and cannot be displaced. There is no kinetic competition for the active site. The rate of the reaction decreases with added inhibitor. E + S E.S E + P E + I E. I This amount of enzyme is kinetically deactivated.
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Example of a Competitive Inhibitor Succinate anion is dehydrogenated to fumarate anion by the enzyme succinate dehydrogenase . Added malonate anion competes reversibly for the active site with succinate. - O 2 CCH 2 CH 2 CO 2 - + E succinate E. - O 2 CCH 2 CH 2 CO 2 - succinate dehydrogenase - O 2 CCH 2 CO 2 - + E malonate E. - O 2 CCH 2 CO 2 - bound malonate - O 2 CCH=CHCO 2 - E.H 2 + fumarate Note the structural similarity of the substrate and inhibitor. An Example of an Irreversible, Noncompetitive Inhibitor Iodoacetamide is an irreversible inhibitor of many enzymes that contain a cysteine residue in the active site. The inhibitor chemically binds to the thiol group of the cysteine thereby irreversibly blocking the active site. E CHCH 2 SH NH C=O cysteine residue + ICH 2 CNH 2 O = iodoacetamide E CHCH 2 S NH C=O (-HI) CH 2 CNH 2 O = alkylated enzyme (inactive)
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