Many different kinds of molecules inhibit enzymes and the inhibition may be reversible or irreversible. Reversible inhibition involves noncovalent binding of the inhibitor and can always be reversed, at least in principle, by removal of the inhibitor. Irreversible inhibition involves a covalent binding of a molecule to the enzyme, which truly incapacitates it.

Reversible Inhibition

Competitive inhibition - In this case, the inhibiting compound so closely resembles the substrate for the enzyme that it accepts the molecule to the substrate binding site. However, once bound, the inhibitor cannot be acted on and thus prevents the enzyme from catalyzing the intended reaction. This reaction scheme is depicted as

Here I stands for the inhibitory compound and KI is the dissociation constant for inhibitor binding - KI = [E][I]/[EI].

Now, [E]t = [E] + [ES] + [EI],

where [EI] is the concentration of inhibitor-enzyme complex.

Then, the velocity is

where is the apparent KM given by

= KM(1 + [I]/KI)

Thus, increasing [I] causes an apparent increase in the KM. The Vmax is unchanged, because as [S] increases relative to a fixed [I], the substrate molecules outcompete the inhibitor for the enzyme's active site. The effect of a competitive inhibitor on a graph of V versus [S] is shown in Figure 11.20a. The system still obeys an equation of the Michaelis-Menten form at a given [I], so the Lineweaver-Burk and Eadie-Hofstee plots are linear, but the KM is altered (Figure 11.20b). By plotting the apparent KM as a function of [I] (Figure 11.20c), one can obtain both KM and KI.

A variant of competitive inhibition is nonproductive binding. This occurs when a substrate molecule can fit into the enzyme's binding site in such a way that the normal catalytic event cannot occur. This scheme is as follows:

Here, ES' is the enzyme-substrate complex that cannot lead to product. In this situation, both KM and kcat are modified.

Noncompetitive inhibition - This occurs when a molecule or an ion binds to a site on the enzyme other than the active site and modifies kcat (Figure 11.22). Such a compound need not resemble the substrate at all. In fact, it only needs to have a strong affinity for the second binding site. Assuming the inhibitor has equal affinity for E and ES, the scheme is

Mathematical analysis yields equation 11.38. In this case, the KM is unaffected, but the apparent kcat is now given by

=kcat/([1+[I]/KI]). Thus, the apparent kcat decreases with increasing [I]. Vmax is therefore also changed (Figure 11.23a):

=kcat(apparent)[E]t = kcat[E]t/(1 + [I]/KI)

The effect of noncompetitive binding on a Lineweaver-Burk plot is shown in Figure 11.23b. Both the true kcat and KI may be determined by graphing

versus [I] (Figure 11.23c).

The situation is usually more complex than shown here. For example, the complex ESI may also be able to undergo the catalytic process slowly, or the binding of inhibitor may modify both kcat and KM. The latter case is called mixed inhibition.

Irreversible Inhibition

Irreversible inhibition occurs when substances combine covalently with enzymes so as to inactivate them irreversibly. Almost all irreversible enzyme inhibitors are toxic substances, either natural or synthetic. Some, such as cyanide and penicillin, are shown in Table 11.4. Figure 11.24 depicts the action of the competitive irreversible inhibitor, diisopropyl fluorophosphate (DFP), which reacts with serine groups on a protein to form a covalent adduct.

Irreversible inhibitors that strongly resemble the substrate rather than its transition state may be extremely selective. An example is TPCK in Table 11.4, which is an excellent inhibitor for chymotrypsin. When selective irreversible inhibitors are used to label active site residues of an enzyme to aid in their identification, they are called affinity labels. A suicide inhibitor, on the other hand, is an affinity label that is unreactive until it is acted upon by the enzyme, at which point it binds irreversibly.