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2.7. ENZYME INHIBITORS Enzyme inhibitors are species that cause a decrease in the activity of an enzyme. Inhibitors usually interact with the enzyme itself, forming enzyme inhibitor (E  I) complexes, but in a few cases, the inhibition mechanism involves reaction with one of the substrates. Inhibition is considered to be reversible if the enzyme recovers its activity when the inhibitor is removed, and irreversible if the inhibitor causes a permanent loss of activity. Reversible inhibition affects the speci c activity and apparent Michaelis Menten parameters for the enzyme, while irreversible inhibition (where the E  I complex formation is irreversible) simply decreases the concentration of active enzyme present in the sample. A well-known example of irreversible inhibition is the effect of nerve gas on the enzyme cholinesterase. Inhibitors may be quantitated using enzyme activity assays if the unknown inhibitor sample does not contain the enzyme employed in the inhibition assay. Standards containing constant enzyme and saturating substrate concentrations are prepared with varying concentrations of inhibitor. The resulting activity versus [inhibitor] calibration curves, for reversible and irreversible inhibitors, are shown below in Figure 2.12. The curvature seen for reversible inhibition [Fig. 2.12 (a)] indicates that an inhibitor-binding equilibrium precedes the conversion of substrate to product. Three types of reversible inhibition may be distinguished. (1) Competitive inhibition occurs when the degree of inhibition decreases as substrate concentration increases and Vmax is unaffected. (2) Noncompetitive inhibition exists when the degree of inhibition does not vary with substrate concentration, and Km is unaffected. (3) Uncompetitive inhibition exists if the degree of inhibition increases as substrate concentration increases; both Vmax and Km are affected. Uncompetitive inhibition is often thought of as a mixture of competitive and noncompetitive behavior.
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Figure 2.12. Calibration curves for (a) reversible and (b) irreversible inhibitors.
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2.7.1. Competitive Inhibition Competitive inhibitors compete with the substrate for the enzyme s active site, but are not converted to products after they are bound. They block the active site from substrate, and their effectiveness is described by their inhibition constant, KI, which is the dissociation constant of the enzyme inhibitor complex (k 3 =k3 ): ! E S $ ES E P
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In this model, the enzyme inhibitor complex is completely inactive, but is in equilibrium with the active form of the enzyme. For a simple one-substrate reaction, the effect of a competitive inhibitor on the initial rate of the reaction is described by Eq. 2.40. n Vmax =f1 Km = S 1 I =KI g 2:40
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It can be seen from this equation that competitive inhibitors have no effect on the Vmax of the enzyme, but alter the apparent Km . In the presence of inhibitor, Km will be increased by a factor of 1 I =KI . Lineweaver Burk plots constructed at various inhibitor concentrations provide a useful diagnostic for this type of inhibition. Figure 2.13 shows that identical y intercepts (1=Vmax ) are obtained at different inhibitor concentrations, while x intercepts (reciprocal of apparent Km ) decrease with increasing [I], and are equal to 1=fKm 1 I =Ki g.
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Figure 2.13. Lineweaver Burk plots for competitive inhibitors.
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2.7.2. Noncompetitive Inhibition Noncompetitive inhibitors interact reversibly with enzymes to form an inactive species, effectively removing active enzyme and thus interfering with the rate of conversion of substrate to product. The inhibitor may interact with free enzyme, or with the enzyme substrate complex. The key feature of noncompetitive inhibition that distinguishes it from competitive inhibition is that inhibition does not affect the apparent af nity of the enzyme for its substrate (i.e., the apparent Km ). For example, a noncompetitive inhibitor may bind in a region remote from the active site to cause a reversible change in enzyme tertiary structure that completely prevents substrate binding and product formation. In this type of inhibition, the quantity of active enzyme appears to decrease as inhibitor concentration increases, so that the apparent Vmax for the reaction decreases. n Vmax =f 1 I =Ki 1 Km = S g 2:41
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where Vmax;app Vmax =f1 I =Ki g. Figure 2.14 shows Lineweaver Burk plots that are typical of noncompetitive inhibition. 2.7.3. Uncompetitive Inhibition Uncompetitive inhibitors bind with the ES complex, affecting both the apparent Km and the apparent Vmax of an enzymatic reaction. Their behavior is approximated by Eq. 2.42: n Vmax =f Km = S I =Ki 1g 2:42
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