All biochemical reactions that occur in the body are subject to specific control, which is carried out through an activating or inhibitory effect on regulatory enzymes. The latter are usually at the beginning of the chain of metabolic transformations and either trigger a multi-stage process or inhibit it. Some single reactions are also subject to regulation. Competitive inhibition is one of the main mechanisms for controlling the catalytic activity of enzymes.
What is inhibition?
The mechanism of enzymatic catalysis is based on the binding of the active center of the enzyme to the substrate molecule (ES complex), resulting in a chemical reaction with the formation and release of the product (E + S = ES = EP = E + P).
Inhibition of the enzyme is called a decrease in speed or a complete stop of the catalysis process. In a narrower sense, this term means a decrease in the affinity of the active center for the substrate, which is achieved by binding of enzyme molecules to inhibitor substances. The latter can act in various ways, on the basis of which they are divided into several types, which correspond to the same mechanisms of inhibition.
The main types of inhibition
According to the nature of the process, inhibition is of two types:
- Irreversible - causes persistent changes in the enzyme molecule, depriving it of functional activity (the latter cannot be restored). It can have both specific and non-specific character. The inhibitor firmly binds to the enzyme by covalent interaction.
- Reversible is the main type of negative regulation of enzymes. It is carried out due to the reversible specific attachment of the inhibitor to the enzyme protein by weak non-covalent bonds; it lends itself to a kinetic description according to the Michaelis-Menten equation (the exception is allosteric regulation).
There are two main types of reversible inhibition of enzymes: competitive (can be weakened by an increase in substrate concentration) and non-competitive. In the latter case, there is a decrease in the maximum possible rate of catalysis.
The main difference between competitive and non-competitive inhibition lies in the site of attachment of the regulatory substance to the enzyme. In the first case, the inhibitor binds directly to the active center, and in the second, to another site of the enzyme, or to the enzyme-substrate complex.
There is also a mixed type of inhibition in which binding to the inhibitor does not prevent the formation of ES, but inhibits catalysis. In this case, the regulatory substance is part of binary or ternary complexes (EI and EIS). With a non-competitive type, the enzyme binds only to ES.
Features of reversible competitive inhibition of enzymes
The competitive inhibition mechanism is based on the structural similarity of the regulatory substance to the substrate. As a result, an active center complex with an inhibitor is formed, conventionally referred to as EI.
Reversible competitive inhibition has the following features:
- binding to the inhibitor occurs in the active center;
- inactivation of the enzyme molecule is reversible;
- the inhibitory effect can be reduced by increasing the concentration of the substrate;
- the inhibitor does not affect the maximum rate of enzymatic catalysis;
- the EI complex can decompose, which is characterized by the corresponding dissociation constant.
With this type of regulation, the inhibitor and substrate, as it were, compete (compete) with each other for a place in the active center, from where the name of the process originated.
As a result, competitive inhibition can be defined as a reversible process of inhibition of enzymatic catalysis, based on the specific affinity of the active center for the inhibitor substance.
Mechanism of action
The binding of the inhibitor to the active site prevents the formation of an enzyme-substrate complex necessary for the implementation of catalysis. As a result, the enzyme molecule becomes inactive. Nevertheless, the catalytic center can contact not only with the inhibitor, but also with the substrate. The probability of formation of a particular complex depends on the ratio of concentrations. If there are significantly more substrate molecules, the enzyme will react with them more often than with an inhibitor.
Effect on the rate of a chemical reaction
The degree of inhibition of catalysis during competitive inhibition is determined by how much enzyme EI complexes will form. In this case, it is possible to increase the concentration of the substrate to such an extent that the role of the inhibitor is displaced, and the catalysis rate reaches the maximum possible value corresponding to the value of V max according to the Michaelis-Menten equation.
This phenomenon is explained by a strong dilution of the inhibitor. As a result, the probability of binding of the enzyme molecules to it is reduced to zero, and the active centers react only with the substrate.
Kinetic dependences of the enzymatic reaction with the participation of a competitive inhibitor
Competitive inhibition increases the Michaelis constant (K m ), which is equal to the concentration of substrate required to achieve ½ maximum catalysis rate at the start of the reaction. The amount of the enzyme that is hypothetically capable of contacting the substrate remains constant, and the number of actually formed ES complexes depends only on the concentration of the latter (EI complexes are not constant and can be replaced by the substrate).
Competitive inhibition of enzymes is easily determined by the kinetic dependence graphs plotted for different substrate concentrations. In this case, the value of K m will change, and V max will remain constant.
With non-competitive inhibition, the opposite is true: the inhibitor binds outside the active center and the presence of the substrate cannot affect this in any way. As a result, some of the enzyme molecules are “turned off” from catalysis, and the maximum possible rate is reduced. Nevertheless, active enzyme molecules can freely bind to the substrate both at low and high concentrations of the latter. Consequently, the Michaelis constant remains constant.
The graphs of competitive inhibition in the system of double inverse coordinates are several straight lines intersecting the ordinate axis at the point 1 / V max . Each line corresponds to a specific substrate concentration. Different points of intersection with the abscissa axis (1 / [S]) indicate a change in the Michaelis constant.
The effect of a competitive inhibitor on the example of malonate
A typical example of competitive inhibition is the process of reducing the activity of succinate dehydroginase, an enzyme that catalyzes the oxidation of succinic acid (succinate) to fumaric acid. Malonate, which has a structural similarity with succinate, acts as an inhibitor.
Adding an inhibitor to the medium causes the formation of complexes of malonate with succinate dehydrogenase. Such a connection does not cause damage to the active center, but blocks its accessibility to succinic acid. An increase in the concentration of succinate reduces the inhibitory effect.
Use in medicine
The mechanism of competitive inhibition is based on the action of many drugs, which are structural analogues of substrates of certain metabolic pathways, the inhibition of which is a necessary part of the treatment of diseases.
For example, to improve the conductivity of nerve impulses with muscular dystrophies, it is necessary to increase the level of acetylcholine. This is achieved by inhibiting the activity of its hydrolyzing acetylcholinesterase. Quaternary ammonium bases that are part of drugs (proresin, endrophonium, etc.) act as inhibitors.
Antimetabolites are identified in a special group, which, in addition to the inhibitory effect, exhibit the properties of a pseudosubstrate. In this case, the formation of the EI complex leads to the formation of a biologically inert abnormal product. Antimetabolites include sulfonamides (used in the treatment of bacterial infections), nucleotide analogs (used to stop cell growth of a cancerous tumor), etc.