Enzyme inhibitors are molecules that interact in certain ways with the enzyme to prevent it from functioning in a normal manner. They can alter the catalytic action of the enzyme and consequently slow down, or even stop catalysis. Inhibitors can act in various modes when interacting with the enzyme: reversible or irreversible inhibition, covalent binding or non-covalent binding, as well as specific or nonspecific inhibition. Based on the inhibition kinetics, enzyme inhibition can be categorized into three major types: competitive inhibition, non-competitive inhibition, and uncompetitive inhibition.
Competitive inhibition occurs when an inhibitor and a substrate both tend to bind to the enzyme in an exclusive manner. A competitive inhibitor could be any compound that closely resembles the chemical structure and molecular geometry of the substrate. The inhibitor competes for the same active site with the substrate molecule. Upon binding, interactions between the inhibitor and the enzyme may be strong in the active site, but no reaction would happen because the inhibitor does not carry the same chemical reactivity. As a result, the inhibitor is "stuck" on the enzyme and prevents any substrate molecules from reacting with the enzyme, and thus the reaction rate is decreased. However, a competitive inhibition is usually temporary and reversible. Therefore, the level of inhibition depends upon the relative concentrations of the substrate and inhibitor, as well as the relative affinities of the inhibitor and substrate to the active site, since they are competing for places in the enzyme active sites.
Non-competitive inhibitors bind to the enzyme regardless of whether the active site is already occupied by the substrate. In fact, the enzyme could be in complexes with either the substrate or inhibitor, or both during non-competitive inhibition. A common form of non-competitive inhibition is called allosteric inhibition, where the inhibitor does not bind to the active site, but the allosteric site. Binding to an allosteric site distorts the 3-dimensional tertiary structure of the enzyme, such that it can no longer catalyze a reaction. There are also other mechanisms of non-competitive inhibition than allosteric inhibition. For example, both the substrate and inhibitor could bind to the active site at the same time. Some non-competitive inhibitors are irreversible and permanent because they denature the enzymes effectively. However, there are also non-competitive inhibitors reversibly and non-permanently stops the enzymatic reaction, which are vital in controlling metabolic functions in organisms.
Uncompetitive inhibition is not commonly seen. An uncompetitive inhibitor binds to the enzyme and enhances the binding affinity of the substrate, but the resultant enzyme-inhibitor-substrate complex undergoes reaction to form the product very slowly. It should be noted that uncompetitive inhibition requires the formation of an enzyme-substrate complex prior to binding to the inhibitor. To the contrary, non-competitive inhibition can occur with or without the substrate present.
Application of the Enzyme Inhibitors
Enzyme inhibitors play important roles in pharmaceutical and biochemical industries. They can be widely used in metabolic control, as metabolic poisons and medicines. For example, many poisons work by inhibiting the action of enzymes involved in metabolic processes, which defends a plant or animal against predators. In addition, some enzyme inhibitors can be used as drugs in the treatment of various diseases. Some antimicrobial drugs are enzyme inhibitors that deactivate the enzymes that are needed for the survival of pathogens.
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