Inhibition classical noncompetitive

Figure 9-12 shows a plot of 1 /v against 1 / [S] at a series of fixed values of [I]. For the case that fC, = K2 (classical noncompetitive inhibition), a family of reciprocal plots that intersect on the horizontal axis at a value of -1 / Km is obtained. On the other hand, if K1 and K2 differ (the general case of noncompetitive inhibition), the family of curves intersect at some other point to the left of the vertical axis and, depending upon the relative values of fC, and K2, either above or below the horizontal axis. The example illustrated is for K2 = 0.5/C, that is, for the binding of M to ES being twice as strong as that to E. 

All terms are identical to those of Equation (5.28). Graphical analysis of Equation (5.29) is shown in Figure 5.9B. Both the slope and the intercept on the VVQ axis are increased by the factor (1 + [I]/K,). If both increase by the same amount, the intercept on the 1/[S0] axis will remain the same and Km is unchanged. V is reduced because part of E and ES are bound as El and ESI. Because ES can bind I, cannot be restored to its value in the presence of inhibitor, regardless of how high the level of substrate may be. It should be noted that classical noncompetitive inhibition is attained only under rapid equilibrium conditions, where Km = Kd. 

Double reciprocal plots distinguish between competitive and noncompetitive inhibitors and simpbfy evaluation of inhibition constants Aj. v, is determined at several substrate concentrations both in the presence and in the absence of inhibitor. For classic competitive inhibition, the lines that connect the experimental data points meet at they axis (Figure 8-9). Since they intercept is equal to IIV, this pattern indicates that wben 1/[S] approaches 0, Vj is independent of the presence of inhibitor. Note, however, that the intercept on the X axis does vary with inhibitor concentration—and that since is smaller than HK, (the apparent... 

Sucrose (common table sugar) is hydrolyzed to glucose and fructose (Section 16.3) in a classic experiment in kinetics. The reaction is catalyzed by the enzyme invertase. Using the following data, determine, by the Lineweaver-Burk method, whether the inhibition of this reaction by 2 M urea is competitive or noncompetitive. 

The term should be used for enzymes that display Michaelis-Menten kinetics. Thus, it is not used with allosteric enzymes. Technically, competitive and noncompetitive inhibition are also terms that are restricted to Michaelis-Menten enzymes, although the concepts are applicable to any enzyme. An inhibitor that binds to an allosteric enzyme at the same site as the substrate is similar to a classical competitive inhibitor. One that binds at a different site is similar to a noncompetitive inhibitor, but the equations and the graphs characteristic of competitive and noncompetitive inhibition don t work the same way with an allosteric enzyme. 

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