An enzyme inhibitor is a molecule that binds to an enzyme and decreases its rate of reaction. Many drugs are enzyme inhibitors and inhibitor discovery and improvement is an active area of biochemistry and pharmacology.

Some chemicals, such as anti-epileptic drugs drugs alter enzyme activity by causing more or less of the enzyme to be produced, this process is not related to enzyme inhibition and is called enzyme induction.

Types of inhibitors

Reversible inhibitors

There are three kinds of reversible enzyme inhibitors. They are classified according to the effect of varying the concentration of the enzyme's substrate on the inhibitor.

• Competitive inhibition is where the substrate and the inhibitor cannot bind to the enzyme at the same time. This usually results from the inhibitor having an affinity for the active site of an enzyme where the substrate also binds. This type of inhibition can therefore be overcome by high concentrations of substrate out-competing the inhibtor. Competitive inhibitors are often similar in structure to the real substrate.

• Mixed inhibition is where the inhibitor can bind to the enzyme at the same time as the enzyme's substrate. However, the binding of the inhibitor affects the binding of the substrate, and vice versa. This type of inhibition can be reduced, but not overcome by increasing concentrations of substrate. Although it is possible for mixed-type inhibitors to bind in the active site, this type of inhibition can also be the result of an allosteric effect. Here, an inhibitor binds to an allosteric site of an enzyme. This is a location on the enzyme's structure separate from the active site that, when a molecule binds to it, can alter the shape of the enzyme. When an inhibitor binds to the allosteric site of an enzyme, it changes the conformation or three-dimensional shape of the enzyme so that the affinity of the substrate for the active site is reduced.

• Non-competitive inhibition is a sub-set of mixed inhibition where the binding of the inhibitor to the enzyme is entirely independent of the binding of substrate.

These types of inhibition can also be described more quantitatively.

In figure 1, an enzyme binds to its substrate S to form the enzyme-substrate complex ES. This complex then breaks down to release product P and free enzyme. The inhibitor (I) can bind to either E or ES with the disassociation constants K_i or K_i', respectively.

• Competitive inhibitors can bind to E, but not ES. Competitive inhibition causes the K_m value to increase, but does not effect V_max.
• Mixed-type inhibitors can bind to E and ES, but their affinities for these two forms of the enzyme are different (K_i ≠ K_i'). Both a decrease in V_max and an increase in the K_m value are seen in mixed inhibition.
• Non-competitive inhibitiors have identical affinities for E and ES (K_i = K_i'). Non-Competitive inhibition causes a decrease in V_max, but does not change the K_m value.

Special cases

• Partially competitive inhibition The mechanism of partially competitive inhibition is similar to that of non-competitive, except that the EIS-complex has catalytic activity, which may be lower or even higher (partially competitive activation) than that of the enzyme-substrate (ES) complex. This inhibition typically displays a lower V_max, but an unaffected K_m value.

• Uncompetitive inhibition Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme-substrate complex, not to the free enzyme; the EIS complex is catalytically inactive. This mode of inhibition is rare and causes a decrease in both V_max and the K_m value.

• Substrate and product inhibition is where either the substrate or product of an enzyme reaction inhibit the enzyme's actvity. This inhibition may follow the competitive, uncompetitive or mixed patterns. With substrate inhibition, there is a progressive decrease in activity at high substrate concentrations. This may indicate the existence of two substrate-binding sites in the enzyme. At low substrate, the high-affinity site is occupied and normal kinetics are followed. However, at higher concentrations, the second inhibitory site becomes occupied, inhibiting the enzymeDixon, M. Webb, E.C., Thorne, C.J.R. and Tipton K.F., Enzymes (3rd edition) Longman, London (1979) See p. 126. Product inhibition is often a regulatory feature in metabolism and can be a form of negative feedback.

Irreversible inhibitors

Irreversible inhibitors usually covalently modify an enzyme and inhibition can therefore not be reversed. Suicide inhibition is a specialised type of irreversible inhibition where the enzyme activates the inhibitor into a reactive form in its active site.

In figure 2, the irreversible inhibitor forms a reversible non-covalent complex with the enzyme (EI or ESI) and this then reacts to form the "dead-end complex" EI*. The rate at which EI* is formed is called the inactivation rate or k_inact.

In figure 3 diisopropylfluorophosphate (DFP) is shown as an example of an irreversible protease inhibitor. It alkylates active-site serine residues in serine proteases such as trypsinJ. A. Cohen , R. A. Oosterbaan and F. Berends Organophosphorus compounds Meth. Enzymol. (1967) 11, 686. However, since DFP also reacts with the active site of acetylcholine esterase this chemical is a potent neurotoxin.

Some reversible inhibitors bind so tightly to their target enzyme that they are essentially irreversible. These tight-binding inhibitors may show kinetics similar to covalent irreversible inhibitors since some of these inhibitors rapidly bind to the enzyme in a low-affinity EI complex and this then undergoes a slower rearrangement to a very tightly-bound EI* complex. This kinetic behaviour is called slow-binding.Szedlacsek, S.E. and Duggleby, R.G. . Kinetics of slow and tight-binding inhibitors. Meth. Enzymol., (1995) 249: 144-180.

See also

Enzyme | Enzyme kinetics

• Web tutorial on enzyme inhibition, Tutorial by Dr Peter Birch of the University of Paisley, containing very clear animations.
• Symbolism and Terminology in Enzyme Kinetics, Recommendations of the Nomenclature Committee of the International Union of Biochemistry (NC-IUB) on enzyme inhibition terminology.

References