Exclusive inhibitors

If an enzyme is able to accommodate a molecule of substrate and a molecule of only one inhibitor at a time, but not the molecule of the other inhibitor, we have the case of two exclusive inhibitors thus, the two inhibitors are mutually exclusive. If the enzyme can bind a substrate and both inhibitors simultaneously, we have the case of two nonexclusive inhibitors. We assume that the presence of either inhibitor prevents the catalytic reaction. 

Table 3. Inhibition by a mixture of two exclusive inhibitors Case 1. Pure competitive inhibition by two exclusive inhibitors EX...

Case 3. Competitive and noncompetitive inhibition by two mutually exclusive inhibitors... 

Let us first examine the three cases of inhibition with a mixture of two exclusive inhibitors, derived from reaction (5.34) remember that exclusive inhibitors prevent the binding of each other (Table 3). 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

Jointing compounds such as neoprene tape or fabric strip impregnated with chromate inhibitor or other inhibited caulking compounds should be applied carefully to ensure complete exclusion of water. 

A number of methods of classifying inhibitors into types or groups are in use but none of these is entirely satisfactory since they are not mutually exclusive and also because there is not always general agreement on the allocation of an inhibitor to a particular group. Some of the main classifications — used particularly for inhibitors in near-neutral pH aqueous systems—are as follows. 

The HDACi s mentioned above inhibit exclusively Zn2+-dependent HDACs and not the NAD-dependent Sittuins. Researcher also focuses on development of sirtunin inhibitors and first successes like the compound siitinol have been repotted. Finally, the development of HAT inhibitors is also pursued and to date some compounds like the peptide-based inhibitor H3-CoA-20 or the small molecule MB-3 are among the first molecules to show HAT inhibition [3]. 

Inosine monophosphate dehydrogenase (EVDPDH) is a key enzyme of purine nucleotide biosynthesis. Purine synthesis in lymphocytes exclusively depends on the de novo synthesis, whereas other cells can generate purines via the so-called salvage pathway. Therefore, IMPDH inhibitors preferentially suppress DNA synthesis in activated lymphocytes. 

A strong acceptor TCNE undergoes [2+2] rather than [4+2] cycloaddition reactions even with dienes. 1,1-Diphenylbutadiene [20] and 2,5-dimethyl-2,4-hexadiene (Scheme 5) [21] afford mainly and exclusively vinyl cyclobutane derivatives, respectively. In the reactions of 2,5-dimethyl-2,4-hexadiene (1) the observed rate constant, is greater for chloroform solvent than for a more polar solvent, acetonitrile (2) the trapping of a zwitterion intermediate by either methanol or p-toluenethiol was unsuccessful (3) radical initiators such as benzyl peroxide, or radical inhibitors like hydroquinone, have no effect on the rate (4) the entropies of activation are of... 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

Disappearance of pairs of inhibitor radicals by disporportionation leads to the regeneration of a molecule of benzoquinone or a substituted quinone. This molecule may terminate a subsequent chain. Hence, if the inhibitor radicals disappear exclusively by disproportionation,... 

The rate of consumption of initiator should therefore be constant (i.e., of zero order) under given conditions. This deduction applies regardless of whether or not inhibitor radicals may undergo regeneration (reaction 63). It emphasizes again that the observation of a well-defined induction period of duration proportional to the amount of inhibitor initially present offers no assurance of a simple stoichiometric ratio between radicals stopped and inhibitor consumed. It will be observed that the rate of consumption of inhibitor, and therefore the length of the induction period for a given amount of inhibitor, depends exclusively on Ri if 2 = 0 if it depends also... 

These are usually mineral oils or synthetic basestocks containing a complex mixture of additives including corrosion inhibitors, extreme pressure additives and emulsifiers. They are almost exclusively oil-in-water emulsions, although in rare instances, invert emulsions may be used. They are normally used between l%-20% emulsions depending on the application. As these products are water extendible, they are subject to attack by micro-organisms. As a consequence, they are often formulated with one or more preservatives. 

We can now relate the kinetic constants kCM, Ku, and kcJKM to specific portions of the enzyme reaction mechanism. From our discussions above we have seen that the term kCM relates to the reaction step of ES conversion to ES. Hence experimental perturbations (e.g., changes in solution conditions, changes in substrate identity, mutations of the enzyme, and the presence of a specific inhibitor) that exclusively affect kCM are exerting their effect on catalysis at the ES to ES transition step. The term KM relates mainly to the dissociation reaction of the encounter complex ES returning to E + S. Conversely, the reciprocal of Ku (1IKU) relates to the association step of E and S to form ES. Inhibitors and other perturbations that affect the... 

An inhibitor that binds exclusively to the free enzyme (i.e., for which a = °°) is said to be competitive because the binding of the inhibitor and the substrate to the enzyme are mutually exclusive hence these inhibitors compete with the substrate for the pool of free enzyme molecules. Referring back to the relationships between the steady state kinetic constants and the steps in catalysis (Figure 2.8), one would expect inhibitors that conform to this mechanism to affect the apparent value of KM (which relates to formation of the enzyme-substrate complex) and VmJKM, but not the value of Vmax (which relates to the chemical steps subsequent to ES complex formation). The presence of a competitive inhibitor thus influences the steady state velocity equation as described by Equation (3.1) ... 

Figure 3.9 Apparent value of the dissociation constant (K,) for a labeled inhibitor, I, as a function of the concentration of a second inhibitor, J when measured by equilibrium binding methods. The solid circles represent the behavior expected when compounds I and J bind in a mutually exclusive fashion with one another. The other symbols represent the behavior expected when compounds I and J bind in a nonexclusive, but antagonistic (i.e., noncompetitive, a > 1) fashion, to separate binding sites. The data for mutually exclusive binding were fit to the equation (apparent)K, = A, 1 + ([f ] A",) I and that for nonexclusive binding were fit to the equation (apparent)Kt = ( [J] + Kj / Kj + f[I]/y) ) for y values of 5 (closed triangles), 10 (open squares), 20 (closed squares), and 50 (open circles).

Figure 3.10 Concentration of labeled compound I bound to an enzyme as a function of the concentration of a second inhibitor J. (A) Response of bound I to concentration of / when I and / bind in a mutually exclusive fashion. Note that here the concentration of the bound I is driven to zero at high concentrations of J. (B) Response of bound I to concentration of J when the two compounds bind in a nonexclusive, antagonistic manner to the target enzyme. Note that at high concentrations of J one does not drive the concentration of bound I to zero. Rather, the concentration of bound I at high concentrations of /reflects the concentration of ternary E I J complex. Condition of simulations I IK, = 1 (closed circles), 3 (open circles), and 5 (closed squares). For panel B, y = 5.

Mutual exclusivity can also be tested for by the effects of combinations of two inhibitors on the activity of a target enzyme. The advantage of this approach is that it does not require any special labeling of either compound, and only catalytic quantities of enzyme are required for the studies. There are a number of graphical methods that can be used to determine the effects of inhibitor combinations on enzyme velocity (see Copeland, 2000). The most popular of these was introduced by Yonetani and Theorell (1964) and is based on the following reciprocal equation ... 

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