Biochemistry - Chemical Reactions in the Living Cell, Volume 2 - D. Metzler 1980

Enzymes: Protein Catalysts of Cells
Enzyme Inhibition and Activation
Competitive Inhibitors

The activity of most Enzymes is inhibited by A wide variety of compounds. This process is frequently characterized by high Specificity, and investigating the relationship between an inhibitor's Structure and its inhibitory potency has proven exceptionally fruitful for elucidating The Nature of active sites and determining the surface complementarity of biological molecules. Enzyme Inhibition also forms The basis of action for most Pharmaceuticals.

Inhibition can be either reversible or irreversible. The latter involves reactions that result in the irreversible loss of enzymatic activity [33a]. An example of irreversible inhibition is the inactivation of the enzyme acetylcholinesterase by neurotoxic agents, namely organophosphorus compounds (Chapter 7, Section D, 1). The stage of irreversible inactivation is often preceded by the Reversible Binding of the inhibitor to a complementary site On the surface of the enzyme molecule. Here, we will not delve into the mathematical Treatment of kinetic data corresponding to irreversible inhibition, but instead limit our Discussion to the Quantitative Aspects of reversible inhibitors.

Inhibitors that are structurally analogous to the substrate are capable of binding to the substrate-binding site. In true competitive inhibition, direct competition must occur between the substrate and the inhibitor for binding to the same site; furthermore, the binding of either Ligand must preclude the binding of the other. The affinity of the inhibitor for the enzyme is quantified by the inhibition constant K₁, which represents the dissociation constant of the enzyme-inhibitor complex EI:

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By applying the steady-state assumption and incorporating not only the free enzyme and the ES complex, but also the EI complex into the enzyme mass balance equation, it is readily apparent that in the case of competitive inhibition, the dependence of velocity on Substrate Concentration is formally analogous to equation (6-15). However, the constant K_M must be replaced by the apparent Michaelis constant (K'_M), which depends on the inhibitor concentration:

The linear anamorphoses of the rate equation have the following form:

FIG. 6-6. Eadie–Hofstee plot (A) and double-reciprocal plot (B) for reactions proceeding in the presence of a competitive inhibitor. The apparent constant K_M increases with increasing inhibitor concentration in accordance with equation (6-44), whereas V_maх remains unchanged. I — in the absence of inhibitor; II — [I] = K₁; III — [I] = 2K₁.

To detect competitive inhibition, researchers typically construct plots of v/[S] versus v, described by equation (6-45), or 1/v versus 1/[S] [equation (6-46)] for reactions carried out in the absence of an inhibitor and in its presence at one or several fixed concentrations. In the case of competitive inhibition, a family of lines is obtained that intersect on one of the axes at the point 1/V_max (Fig. 6-6). In other words, the maximum velocity remains unaltered in the presence of a competitive inhibitor. By employing a sufficiently high substrate concentration, one can always saturate the enzyme with substrate and completely abolish inhibitor binding. From The change in slope with increasing inhibitor concentration, the value of K₁ can be readily calculated using equations (6-45) or (6-46).

At a fixed concentration of a competitive inhibitor, the plot of v versus lg[S] (Fig. 6-7) is simply shifted to the right along the abscissa (i.e., toward higher [S] values) without any change in the curve shape (or the V_mах value).