All biochemical reactions occurring 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 located at the beginning of chains of metabolic transformations and either start a multi-stage process or slow it down. 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 site 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 an enzyme is a reduction in the rate or a complete stop of the catalysis process. In a narrowersense, this term means a decrease in the affinity of the active center for the substrate, which is achieved by binding 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 inhibition mechanisms of the same name.
Main types of inhibition
By the nature of the process, inhibition can be of two types:
- Irreversible - causes persistent changes in the enzyme molecule, depriving it of functional activity (the latter cannot be restored). It can be either specific or non-specific. The inhibitor binds strongly to the enzyme through covalent interaction.
- Reversible - 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, amenable to kinetic description according to the Michaelis-Menten equation (with the exception of allosteric regulation).
There are two main types of reversible enzyme inhibition: competitive (may be attenuated by increasing substrate concentration) and non-competitive. In the latter case, the maximum possible rate of catalysis decreases.
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 site, and in the second case, to another site of the enzyme, or to the enzyme-substrate complex.
There is also a mixed type of inhibition, in which binding to an inhibitor does not prevent the formation of ES, but slows down catalysis. In this case, the regulator substance is in the composition of double or triple complexes (EI and EIS). In the uncompetitive type, the enzyme only binds to ES.
Features of reversible competitive inhibition of enzymes
The competitive mechanism of inhibition is based on the structural similarity of the regulatory substance with the substrate. As a result, a complex of the active site with the inhibitor is formed, conventionally designated as EI.
Reversible competitive inhibition has the following features:
- binding to the inhibitor occurs at the active site;
- inactivation of the enzyme molecule is reversible;
- the inhibitory effect can be reduced by increasing the concentration of the substrate;
- 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 the substrate seem to compete (compete) with each other for a place in the active center, hence the name of the process.
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 site for the inhibitor substance.
Mechanism of action
Tetheringan inhibitor with an active site prevents the formation of an enzyme-substrate complex necessary for catalysis. As a result, the enzyme molecule becomes inactive. Nevertheless, the catalytic center can bind not only to the inhibitor, but also to the substrate. The probability of formation of one or another complex depends on the ratio of concentrations. If there are significantly more substrate molecules, then the enzyme will react with them more often than with the inhibitor.
Influence on the rate of a chemical reaction
The degree of inhibition of catalysis during competitive inhibition is determined by how much of the enzyme will form EI-complexes. In this case, it is possible to increase the concentration of the substrate to such an extent that the role of the inhibitor will be replaced, and the catalysis rate will reach the maximum possible value corresponding to the value Vmax according to the Michaelis-Menten equation.
This phenomenon is due to the strong dilution of the inhibitor. As a result, the probability of enzyme molecules binding to it is reduced to zero, and active centers react only with the substrate.
Kinetic dependences of an enzymatic reaction involving a competitive inhibitor
Competitive inhibition increases the Michaelis constant (Km), which is equal to the substrate concentration required to achieve ½ the maximum rate of catalysis at the start of the reaction. The amount of the enzyme hypothetically capable of binding to the substrate remains constant, while the number of ES-complexes depends only on the concentration of the latter (EI complexes are not constant and can be displaced by the substrate).
Competitive inhibition of enzymes is easy to determine from the graphs of the kinetic dependence built for different concentrations of the substrate. In this case, the value of Km will change, while Vmax 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 decreases. Nevertheless, active enzyme molecules can easily bind to the substrate both at low and at high concentrations of the latter. Therefore, the Michaelis constant remains constant.
Graphs of competitive inhibition in the system of double inverse coordinates are several straight lines intersecting the y-axis at the point 1/Vmax. Each straight line corresponds to a certain concentration of the substrate. Different points of intersection with the abscissa axis (1/[S]) indicate a change in the Michaelis constant.
The action of a competitive inhibitor on the example of malonate
A typical example of competitive inhibition is the process of reducing the activity of succinate dehydrogenase, an enzyme that catalyzes the oxidation of succinic acid (succinate) to fumaric acid. Here as an inhibitormalonate acts, having a structural similarity to succinate.
Adding an inhibitor to the medium causes the formation of complexes of malonate with succinate dehydrogenase. Such a bond does not cause damage to the active site, but blocks its accessibility to succinic acid. Increasing the concentration of succinate reduces the inhibitory effect.
Medical use
The action of many drugs, which are structural analogues of the substrates of some metabolic pathways, the inhibition of which is a necessary part of the treatment of diseases, is based on the mechanism of competitive inhibition.
For example, to improve the conduction of nerve impulses in muscular dystrophies, it is required to increase the level of acetylcholine. This is achieved by inhibiting the activity of its hydrolyzing acetylcholinesterase. The inhibitors are quaternary ammonium bases that are part of drugs (proresin, endrophonium, etc.).
Antimetabolites are distinguished into 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 anomalous product. Antimetabolites include sulfonamides (used in the treatment of bacterial infections), nucleotide analogs (used to stop the cell growth of a cancerous tumor), etc.
