Inhibition competitive versus noncompetitive

Noncompetitive inhibitions result from combination of the inhibitor with an enzyme form other than the one the substrate combines with, and one that is present at both high and low levels of the substrate. An example is a dead-end inhibitor resembling the first substrate in an ordered mechanism. It is competitive versus A, but noncompetitive versus B, because B cannot prevent the binding of the inhibitor to free enzyme. In a random mechanism, an inhibitor binding at one site is noncompetitive versus a substrate binding at another site. 

Competitive and noncompetitive inhibitions are the most common types, especially for product inhibitors. The first product (P) released in an ordered mechanism, for example, gives noncompetitive inhibition versus either substrate A or B as the result of partially reversing the reaction. This result can occur at either low or high levels of substrate, and thus V/K as well as V is affected. A dead-end inhibitor combining with EQ in the same fashion, however, gives uncompetitive inhibition because it cannot reverse the reaction. 

Douhle-reciprocal plots are especially useful for distinguishing between competitive and noncompetitive inhibitors. In competitive inhibition, the intercept on they-axis of the plot of tV q versus 1/[S] is the same in the presence and in the absence of inhibitor, although the slope is increased (Figure 8,37). That the intercept is unchanged is because a competitive inhibitor does not alter sufficiently high concentration, virtually all the active sites are filled by... 

Three mies can be formulated to predict product or dead-end inhibition patterns in such mechanisms. (1) When an inhibitor occupies the same portion of an active site as the variable substrate, the inhibition is competitive. (2) When an inhibitor combines in a different portion of the same active site as the variable substrate, the inhibition is noncompetitive. Thus, with pyruvate carboxylase, MgADP and P are both noncompetitive inhibitors versus bicarbonate, although in accordance with Rule 1 they are competitive versus MgATP. (3) When an... 

The only study that has not been consistent with the proposed random mechanism has been that of Viebrock 47). He used an impure rat liver preparation that likely was the basic isozyme based on the Michaelis constants observed. The results of that study were interpreted as being consistent with an ordered mechanism in which GTP, IMP, and aspartate add in that order with release of GDP, Pi, and adenylosuccinate in that order. This mechanism is clearly ruled out by the data of Ogawa et al. 49) who show inhibition patterns for AMP and GDP in which these compounds were competitive versus IMP and GTP, respectively, and noncompetitive relative to the other two substrates. 

Plots devised by Dixon to determine K, for tight-binding inhibitors, (a) A primary plot of v versus total inhibitor present ([/Id yields a concave line. In this example, [S] = 3 x Km and thus v = 67% of Straight lines drawn from Vo (when [/It = 0) through points corresponding to Vq/2, Vq/3, etc. intersect with the x-axis at points separated by a distance /Cj app/ when inhibition is competitive. When inhibition is noncompetitive, intersection points are separated by a distance equivalent to K. The positions of lines for n = 1 and n = 0 can then be deduced and the total enzyme concentration, [EJt, can be determined from the distance between the origin and the intersection point of the n = 0 line on the x-axis. If inhibition is competitive, this experiment is repeated at several different substrate concentrations such that a value for K, app is obtained at each substrate concentration. (b) Values for app are replotted versus [S], and the y-intercept yields a value for /Cj. If inhibition is noncompetitive, this replot is not necessary (see text)... 

Effect on Lineweaver-Burke plot Noncompetitive inhibition is readily differentiated from competitive inhibition by plotting 1/v0 versus 1/[S] and noting that Vmax decreases in the presence of a noncompetitive inhibitor, whereas Km is unchanged (see Figure 5.14). 

To be used effectively, the mechanisms through which inhibitors exert their effects need to be understood. For example, competitive inhibitors compete with physiological substrates for binding sites. Their effect depends on the relative concentrations of substrates and the inhibitor and the degree of inhibition depends on the number of active sites occupied by the inhibitor versus the metabolic substrate. In contrast, noncompetitive inhibitors bind to parts of the enzyme other than the substrate binding site, so the degree of inhibition depends only on the inhibitor and not the substrate concentration. This type of inhibition is typically irreversible and reduces the amount of total enzyme available to catalyze a particular reaction. Uncompetitive inhibition occurs when the inhibitor binds to the enzyme-substrate complex and prevents the reaction from being catalyzed. 

Reversible inhibition that produces complete loss of catalytic activity is referred to as linear inhibition because the plots of K IV or 1/y versus [I] are straight lines. When some catalytic activity remains, even at saturating amounts of inhibitor, it is referred to as hyperbohc inhibition because these plots are nonlinear (this case will not be considered here). Both of these types of reversible inhibition are further classified according to the various apparent Michaelis-Menten parameters that are affected by the inhibitor. The two limiting cases are competitive inhibition and uncompetitive inhibition a third type is mixed inhibition, which includes as a special case noncompetitive inhibition. 

Figure 4-33 Hanes-Woolf Plot [S]/u versus [S] (a) Competitive inhibition, (i) Noncompetitive inhibition, (c)... 

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... 

The answer is c. (Murray, pp 48-73. Scriver, pp 4571-4636. Sack, pp 3-17. Wilson, pp 287-317.) Allosteric enzymes, unlike simpler enzymes, do not obey Michaelis-Menten kinetics. Often, one active site of an allosteric enzyme molecule can positively affect another active site in the same molecule. This leads to cooperativity and sigmoidal enzyme kinetics in a plot of [S] versus V The terms competitive inhibition and noncompetitive inhibition apply to Michaelis-Menten kinetics and not to allosteric enzymes. 

In an ordered sequential mechanism where C is an alternate substrate for the second substrate, B, one observes competitive inhibition by C versus B and noncompetitive inhibition versus A. for the competitive inhibition is given by an equation analogous to Eq. (25) containing ia, and A in place of ib, b(c). and B. For the noncompetitive inhibition, is given by an equation analogous to Eq. (26), with KaKc K (e), and B replacing K, Ka,... 

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, .

To derive the Lineweaver-Burk equations, we proceed by taking the reciprocals of each side of Eqs. (5.25), (5.29), and (5.32). The corresponding graphs of 1/vq versus l/[S]o have various characteristic changes in slopes and intercepts as [I] is varied. Competitive inhibition gives lines that all intersect on the ordinate. Pure noncompetitive inhibition (in which K, = K,) gives lines that all intersect on the abscissa. For anti- or uncompetitive inhibition, the telltale feature is that the set of lines are all parallel to each other. For mixed inhibition [K K in Eq. (5.32)], both the slopes and the intercepts on the ordinate and abscissa differ for different values of [I] see Fig. 5-31. 

However, this case is extremely rare in nature. An example is the noncompetitive inhibition of phenyllactate versus an amide substrate for carboxypeptidase. In this case, the initial collision complex of substrate and enzyme has an interaction with the terminal carboxyl and the arginine on the enzyme, as well as with the rest of the polypeptide chain, but the aromatic group of the terminal amino add is not in Ae specificity pocket. For it to seat itself requires twisting of the amide bond, which is the rate limiting and energy requiring step of the reaction. Thus, phenyllactate can slip into this pocket and prevent proper seating of the substrate. With an ester substrate, where rotation of the ester bond is not hindered, the collision complex has the specificity pocket filled, and phenyllactate is a competitive inhibitor (Auld Holmquist, 1974). 

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