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Lecture 12

BCH2011: Textbook summary - Lecture 12

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Monash University

LECTURE 12 Enzymes Are Subject to Reversible or Irreversible Inhibition: Enzymes inhibitors are molecules that interfere with catalysis, slowing or halting enzymatic reactions. One common type of reversible inhibition is called competitive. A competitive inhibitor competes with the substrate for the active site of an enzyme. While the inhibitor (I) occupies the active site, it prevents binding of the substrate to the enzyme. Many competitive inhibitors are structurally similar to the substrate and combine with the enzyme to form an EI complex, but without leading to catalysis. Because the inhibitor binds reversibly to the enzyme, the competition can be biased to favor the substrate simply by adding more substrate. When [S] far exceeds [I], the probability that an inhibitor molecule will bind to the enzyme is minimized and the reaction exhibits a normal Vmax. However, the [S] at which Vo = 1/2Vmax, the apparent Km, increases in the presence of inhibitor by the factor of alpha. Two other types of reversible inhibition, uncompetitive and mixed, can be defined in terms of one-substrate enzymes, but are in practice observed only with enzymes having two or more substrates. An uncompetitive inhibitor binds at a site distinct from the substrate active site and, unlike a competitive inhibitor, binds only to the ES complex. An uncompetitive inhibitor lowers the measured Vmax. Apparent Km also decreases, because the [S] required to reach one-half Vmax decreased by the factor alpha’. A mixed inhibitor also binds at a site distinct from the substrate active site, but it binds to either E or ES. A mixed inhibitor usually affects both Km and Vmax. The irreversible inhibitors bind covalently with or destroy a functional group on an enzyme that is essential for the enzyme’s activity, or form a particularly stable non-covalent association. Regulatory Enzymes: Regulatory enzymes exhibit increased or decreased catalytic activity in response to certain signals. The activities of regulatory enzymes are modulated in a variety of ways. Allosteric enzymes function through reversible, non-covalent binding of regulatory compounds called allosteric modulators or allosteric effectors, which are generally small metabolites or cofactors. Allosteric
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