For uncompetitive inhibition

An inhibitor that binds exclusively to the ES complex, or a subsequent species, with little or no affinity for the free enzyme is referred to as uncompetitive. Inhibitors of this modality require the prior formation of the ES complex for binding and inhibition. Hence these inhibitors affect the steps in catalysis subsequent to initial substrate binding that is, they affect the ES —> ES1 step. One might then expect that these inhibitors would exclusively affect the apparent value of Vm and not influence the value of KM. This, however, is incorrect. Recall, as illustrated in Figure 3.1, that the formation of the ESI ternary complex represents a thermodynamic cycle between the ES, El, and ESI states. Hence the augmentation of the affinity of an uncompetitive inhibitor that accompanies ES complex formation must be balanced by an equal augmentation of substrate affinity for the El complex. The result of this is that the apparent values of both Vmax and Ku decrease with increasing concentrations of an uncompetitive inhibitor (Table 3.3). The velocity equation for uncompetitive inhibition is as follows ... 

For uncompetitive inhibition, the value of kobs will increase as a rectangular hyperbola with increasing substrate concentrations according to Equation (6.17) ... 

These three classes of inhibition can be distinguished by virtue of the effect of variations in inhibitor concentration on the slopes and intercepts of reciprocal plots. For competitive inhibition only the slope varies. For uncompetitive inhibition only the intercept varies, while for noncompetitive inhibition both the slope and the intercept vary. 

It can be observed that when InES is not at all formed, the above rate law reduces as equation (6.56), while if InE is not formed at all, the rate law reduces to that for uncompetitive inhibition as equation (6.58). 

Thus, for uncompetitive inhibition, the apparent values of both K and Fmax are modified. 

Fig. 11.13 Eadie-Hofstee plots (A) for noncompetitive inhibition and (B) for uncompetitive inhibition.

MichaeUs-Menten kinetics predict that as the concentration of the substrate increases, the rate increases hyperbolically. However, some enzymes exist in which a maximum velocity is obtained at low substrate concentration, but further increases in the substrate concentration lead to a decrease in velocity. This effect is known as substrate inhibition and can eventually lead to complete enzyme inhibition or partial enzyme inhibition. It is thought that substrate inhibition occurs if two substrate molecules bind to the enzyme simultaneously in an incorrect orientation and produce an inactive E S S complex, analogous to that discussed for uncompetitive inhibition. The rate of the enzyme reaction that undergoes substrate inhibition is given by Equation 17, where K represents the... 

In other words, an uncompetitive inhibitor decreases and Km to the same extent. The v versus [S] plot is shown in Figure 4-29. The reciprocal form of the velocity equation for uncompetitive inhibition is ... 

As with competitive inhibition, two additional reaction steps are addr the Michaelis-Menten kinetics for uncompetitive inhibition as shown in i tion Steps 4 and 5. 

PP y S pseudo-sieady-siate hypothesis to the intermediate I E S), we arrive at the rate law for uncompetitive inhibition... 

Figure 7 11 Lineweaver-Burk piot for uncompetitive inhibition.

Figure 6.47. Dependence of the reaction rate on substrate concentration for uncompetitive inhibition.

The Lineweaver-Burk plot is a diagnostic for uncompetitive inhibition. The slope of the lines is independent on the concentration of inhibitor, while the y-intersept increases with an increase of [I] (Figure 6.48). 

In a similar fashion as for other types of inhibition the Hanes-Woolf and the Eadie -Hofstee equations for uncompetitive inhibition could be used respectively for analysis if this type of inhibition is present... 

As with the other types of inhibition, equations have been derived that permit estimation of the inhibition constant as well as and Unax for uncompetitive inhibition (Eq. 4.19). 

Inhibitors structurally related to the substrate may be bound to the enzyme active center and compete with the substrate (competitive inhibition). If the inhibitor is not only bound to the enzyme but also to the enzyme-substrate complex, the active center is usually deformed and its function is thus impaired. In this case the substrate and the inhibitor do not compete with each other (noncompetitive inhibition). Competitive and noncompetitive inhibitions affect the enzyme kinetics differently. A competitive inhibitor does not change but increases the on the contrary, a noncompetitive inhibition results in an unchanged and in a decrease in In the case of mixed inhibition, the inhibitor binds the enzyme and the enzyme-substrate complex with a different affinity. For uncompetitive inhibition, the inhibitor binds only when the enzyme-substrate complex is formed [21]. 

Singh et al. also indicate that all enzyme kinetics parameters viz. maximum oxygen consumption rate, and Michaelis-Menten constants for oxygen consumption, competitive inhibition of consumption by CO and for uncompetitive inhibition of consumption... 

The Lineweaver-Burk plot is a diagnostic for uncompetitive inhibition. 

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

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