Double reciprocal plot uncompetitive inhibition

At very low substrate concentration ([S] approaches zero), the enzyme is mostly present as E. Since an uncompetitive inhibitor does not combine with E, the inhibitor has no effect on the velocity and no effect on Vmsa/Km (the slope of the double-reciprocal plot). In this case, termed uncompetitive, the slopes of the double-reciprocal plots are independent of inhibitor concentration and only the intercepts are affected. A series of parallel lines results when different inhibitor concentrations are used. This type of inhibition is often observed for enzymes that catalyze the reaction between two substrates. Often an inhibitor that is competitive against one of the substrates is found to give uncompetitive inhibition when the other substrate is varied. The inhibitor does combine at the active site but does not prevent the binding of one of the substrates (and vice versa). 

Rule 1. Upon obtaining a double-reciprocal plot of 1/v vx. 1/[A] (where [A] is the initial substrate concentration and V is the initial velocity) at varying concentrations of the inhibitor (I), if the vertical intercept varies with the concentration of the reversible inhibitor, then the inhibitor can bind to an enzyme form that does not bind the varied substrate. For example, for the simple Uni Uni mechanism (E + A EX E -P P), a noncompetitive or uncompetitive inhibitor (both of which exhibit changes in the vertical intercept at varying concentrations of the inhibitor), I binds to EX, a form of the enzyme that does not bind free A. In such cases, saturation with the varied substrate will not completely reverse the inhibition. 

The characteristics of the double reciprocal plots given by Equation (5.149), Equation (5.154), and Equation (5.155) determine what kind of enzyme inhibition may occur competitive, noncompetitive, or uncompetitive. In a given concentration of enzyme and inhibitor, the substrate concentration is changed and the double reciprocal plot of 1/V against 1/[A] is drawn. Figure 5.24a illustrates the double... 

FIGURE 5.24 The double reciprocal plots of (a) competitive inhibition, (b) noncompetitive inhibition, and (c) uncompetitive inhibition. [Graph reconstructed from data by Nnane et al., Br. J. Cancer, 83, 74 (2000).]... 

Figure 5.9 Double-reciprocal plots showing different types of inhibition. (A) Competitive inhibition [Equation (5.28)J (B) noncompetitive inhibition [Equation (5.29)] (C) uncompetitive inhibition (Equation 5.30). Km and Vmax are estimated from the slopes of uninhibited reactions, and K, from the slopes and/or intercepts of the inhibited reactions.

Figure 11-12 Double reciprocal plot of uncompetitive inhibition.

The L-phenylalanine inhibition of rat (G5) or of (Fig. 12) human intestinal alkaline phosphatase and of human placental (G6) enzyme is of the uncompetitive type, because the double reciprocal plots of velocity and substrate concentration were all straight lines parallel to those obtained without the inhibitor. Consequently, the extent of the inhibition was greatly dependent on substrate (Fig. 11) and inhibitor concentrations (Fig. 10). Detailed studies have appeared elsewhere (G5). 

Double-reciprocal plots are especially useful for distinguishing between competitive, uncompetitive, and noncompetitive inhibitors. In competitive inhibition, the intercept on they-axis of the plot of I/Vq versus 1/fS] is the... 

Figure 8.21 Uncompetitive inhibition illustrated by a double-reciprocal plot.

In uncompetitive inhibition (f igure 8.21), the inhibitor combines only with the enzyme—substrate complex. The equation that describes the double-reciprocal plot for an uncompetitive inhibitor is... 

Inhibition can be analysed visually by double reciprocal plots (Figure 5.12). In the case of competitive inhibition, 1/v versus 1/[S] plots at various values of P] give a series of lines all intersecting at 1 /[S] = 0, 1/v = 1/ Fmax, whereas in the case of anticompetitive inhibition, the 1/v versus 1/[S] plots at various values of P] are all parallel. The fascination of double reciprocal plots was such that a further form of inhibition, where the lines all intersected at 1/v = 0, 1/[S] = — 1/ Am was defined and confusingly termed non-competitive inhibition the physical situation, rarely encountered in practice, was that inhibitor bound equally to E and ES. The rate law is that of eqn. (5.30). In practice, inhibition which is neither cleanly competitive nor cleanly uncompetitive is best described by the mixed inhibition eqn. (5.31). 

Double reciprocal plots for an inhibited enzyme. The black line shows the double reciprocal plot for an enzyme in the absence of inhibitor. The other lines show the effect of an uncompetitive inhibitor (gray), a noncompetitive inhibitor (light purple), or a competitive inhibitor (purple). 

Fig. 3.5 Graphical representation of inhibition mechanisms in Lineweaver-Burke double reciprocal plots. Cl competitive inhibition NCI non-competitive inhibition UCI uncompetitive inhibition MTI mixed-type inhibition...

Figure 4 shows the survey of different t)q)es of nonlinear hyperbolic inhibition mechanisms and their characteristics. A basic property of aU nonlinear mechanisms, shown in Fig. 4, is that the double reciprocal plot of i/uq versus /A, in the presence of different constant concentrations of an inhibitor is a family of straight lines with a common intersection point. This common intersection point is found either in the I, in, or in the IV quadrant, depending on the mechanism only in Case 5 (hyperbolic uncompetitive type), the double reciprocal plot is a family of parallel straight lines without a common intersection point. 

The double reciprocal plot (Fig. 2.30c) shows that in the presence of an uncompetitive inhibitor, both the maximum velocity, V, and Km are changed but not the ratio of Km/V. Hence the slopes of the lines are equal and in the presence of increasing amounts of inhibitor, the lines plotted are parallel. Uncompetitive inhibition is rarely found in single-substrate reactions. It occurs more often in two-substrate reactions. 

Fig. 6.49 Double reciprocal plot for uncompetitive inhibition. (From http //images.tutorvi5ta.com/cms/ images/81/uncompetitive-inhibition.png).

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