Noncompetitive inhibitors pure inhibition

Noncompetitive inhibitors interact with both E and ES (or with S and ES, but this is a rare and specialized case). Obviously, then, the inhibitor is not binding to the same site as S, and the inhibition cannot be overcome by raising [S]. There are two types of noncompetitive inhibition pure and mixed. 

The slope of the reciprocal plot in the presence of a pure noncompetitive inhibitor is a linear function of [I] as shown earlier for pure competitive inhibition. The 1/v-axis intercept (1/Vroax,) is also a linear function of [I] as shown below. 

Figure 4.6 shows the relationship between K and [A TP] for various mechanisms of inhibition. For a competitive inhibitor, when ATP is present much below its Km value, K approximates to the inhibition constant, is. At [A TP] = Km, K = 2Idetermining potency and selectivity (See Section 4.4.2.1.). The selectivity at physiological [ATP] is influenced by which is an intrinsic property of the enzyme determining the relative ease of competitive inhibition. Uncompetitive inhibition may be barely detectable when [A TP] Km, because K Ki KJ[ATP. When [ATP] = Km, K = 2Kih and at [ATP] Km, K Ka. For pure noncompetitive inhibitors, K = K at all concentrations of ATP. It is noteworthy that when [ATP] = Km, K is between 1 and 2-times the value of the inhibition constant for each of these mechanisms. 

In noncompetitive inhibition (Figure 8.38), the inhibitor can combine with either the enzyme or the enzyme-substrate complex. In pure noncompetitive inhibition, the values of the dissociation constants of the inhibitor and enzyme and of the inhibitor and enzyme—substrate complex are equal (Section 8.5.1). The value of is decreased to a new value called V( i. and so the intercept on the vertical axis is increased. The new slope, which is equal to Km/ V( i. is larger by the same factor. In contrast with Vjjxix. is not affected by pure noncompetitive inhibition. The maximal velocity in the presence of a pure noncompetitive inhibitor, V ax. is given by... 

Fig. 9.18. Lineweaver-Burk plots of competitive and pure noncompetitive inhibition. A. lAi versus 1/[S] in the presence of a competitive inhibitor. The competitive inhibitor alters the intersection on the abscissa. The new intersection is 1/K , p (also called 1/K ). A compietitive inhibitor does not affect B. 1/Vj versus 1/[S] in the presence of a pure noncompetitive inhibitor. The noncompetitive inhibitor alters the intersection on the ordinate, Wmax.app W niax> But docs not offcct 1/K j. A pure noncompetitive inhibitor binds to E and ES with the same affinity. If the inhibitor has different affinities for E and ES, the lines will intersect above or below the abscissa, and the noncompetitive inhibitor will change both the and the V, .

The expressions for both the slope and the intercept in the equation for a Lineweaver-Burk plot of an uninhibited reaction have been replaced by more complicated expressions in the equation that describes noncompetitive inhibition. This interpretation is borne out by the observed results. With a pure, noncompetitive inhibitor, the binding of substrate does not affect the binding of inhibitor, and vice versa. Because the 7 is a measure of the affinity of the enzyme and substrate, and because the inhibitor does not affect the binding, the 7 does not change with noncompetitive inhibition. 

Non-competitive inhibitors bind reversibly to an allosteric site (see Appendix 7) on the enzyme. In pure non-competitive inhibition, the binding of the inhibitor to the enzyme does not influence the binding of the substrate to the enzyme. However, this situation is uncommon, and the binding of the inhibitor usually causes conformational changes in the structure of the enzyme, which in turn affects the binding of the substrate to the enzyme. This is known as mixed noncompetitive inhibition. The fact that the inhibitor does not bind to the active site of the enzyme means that the structure of the substrate cannot be used as the basis of designing new drugs that act in this manner to inhibit enzyme action. 

This equation predicts that both the slope and the l/v0 intercept of a Lineweaver-Burk plot will increase with increasing inhibitor concentration, but the intercept on the 1/[S]0 axis (-1 Km) will not change. A series of plots for several experiments with different concentrations of inhibitor will all pass through the l/[S]o intercept as shown in Fig. 9-4(6), indicating that pure noncompetitive inhibition does not alter Km. 

In noncompetitive inhibition (Figure 8.38). the inhibitor can combine with either the enzyme or the enzyme-substrate complex. In pure noncompetitive inhibition, the values of the dissociation constants of the inhibitor and enzyme and of the inhibitor and enzyme-substrate complex are equal (Section 8.5.1). The value of decreased to a new value... 

Depending on the inhibitor being considered, the values of these dissociation constants may or may not be equivalent. There are two forms of noncompetitive inhibition pure and mixed. In pure noncompetitive inhibition, a rare phenomenon, both Kz values are equivalent. Mixed noncompetitive inhibition is typically more complicated because the Kj values are different. 

The mechanism of inhibition is defined by the relative values of Ais and Ay, which are respectively the inhibition constants at [ATP] Km and >> Km.2S 29 Inhibition constants are measures of potency, because they equal the free inhibitor concentration when the rate is reduced by 50%. The mechanism is competitive if inhibition tends to zero when ATP is saturating dATP] Km). This mechanism is seen if Ais Ay. Conversely, the mechanism is uncompetitive if inhibition tends to zero when [ATP] Km, because As Ay. Inhibition is noncompetitive when it occurs both at [ATP] Km and [ATP] Am. Pure noncompetitive inhibition (Ais = Ay) arises when potency is independent of ATP-concentration. Mixed noncompetitive inhibition (Als A Ay) occurs if there is a tendency towards competitive or uncompetitive. 

The inhibitor may he such that the degree of inhibition is unaffected by the concentration of substrate, in which case we speak of pure noncompetitive inhibition. 

In pure noncompetitive inhibition, the inhibitor binds with equal affinity to the free enzyme and to the enzyme-substrate (ES) complex. In noncompetitive inhibition, the enzyme-inhibitor-substrate complex IES does not react to give product P. A kinetic scheme for noncompetitive inhibition is given in Figure 6.41... 

Lineweaver-Burk plots provide a good illustration of competitive inhibition and pure noncompetitive inhibition (Fig. 9.18). In competitive inhibition, plots of 1/v vs 1/[S] at a series of inhibitor concentrations intersect on the ordinate. Thus, at infinite substrate concentration, or 1/[S] = 0, there is no effect of the inhibitor. In pure noncompetitive inhibition, the inhibitor decreases the velocity even when [S] has been extrapolated to an infinite concentration. However, if the inhibitor has no effect on the binding of the substrate, the is the same for every concentration of inhibitor, and the lines intersect on the abcissa. 

Pure noncompetitive inhibition (decrease in V ax with no change in K ) is seldom observed in enzyme kinetics studies, except in the case of very small inhibitors, such as protons, metal ions, and small anions. For noncompetitive... 

Consider the pure noncompetitive inhibition by two nonexclusive inhibitors, depicted by Eq. (5.35). If there are two dead-end inhibitors I and X, the kinetic question is whether an EIX complex forms, and if so, whether the dissociation constant of I from EIX is the same as from El (and similarly for X from EIX and EX). To answer this, the substrate concentration is held constant, the concentration of the two inhibitors is varied, and initial velocities are determined (Yonetani Theorell, 1965 Yonetani, 1982). 

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