Allosteric Enzyme: Define Definition, Example, Regulation, Kinetics

Some Important Topics about Allosteric Enzyme

Allosteric enzyme

Allosteric enzymes or enzyme are called, the action of which “by definition” is associated with a change in shape (alios – different, different; stereos – form). The activity of such enzymes is regulated by substances that act like non-competitive inhibitors.

Allosteric enzyme example

Examples of allosteric enzymes – delta-aminolevulinate synthetase, carbamoyl phosphate synthetase II. Another example of an allosteric enzyme is phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate to form fructose-1,6-bisphosphate. This reaction takes place during glycolysis, which is one of the stages of the respiration process.

allosteric enzyme regulation

Allosteric regulation is one of the types of regulation of enzyme activity. This type of regulation is an effect observed in cases where small molecules (effectors), by binding to an enzyme outside the active site, change the rate of the reaction.

allosteric enzyme definition

Allosteric enzyme [greek. allos – other and stereos – spatial; lat. fermentum – ferment] – an enzyme with a quaternary structure (consists of several polypeptide chains), which, in addition to the active center, has separate ” allosteric ” centers.

allosteric enzyme inhibition

Allosteric inhibition Allosteric inhibitors bind to individual sites of the enzyme outside the active site. This binding entails conformational changes in the enzyme molecule, which lead to a decrease in its activity.

allosteric enzyme regulation is usually associated with

Allosteric regulation –The enzyme alters activity with the aid of an effector non-covalently bound to it. Binding occurs at a site spatially distant from the active (catalytic) center. This binding causes conformational changes in the protein molecule, leading to a change in the specific geometry of the catalytic center.

how do allosteric enzymes work

Allosteric enzymes are called enzymes, the activity of which is regulated not by their substrates, but by other substances that attach to enzymes in special areas remote from their active center.

where is the allosteric enzyme located?

Allosteric (regulatory) center – portion in the molecule of the enzyme, located outside the active center. Various substances can be attached to the allosteric center, which differs in structure from the substrate molecules – regulators ( allosteric effectors).

can allosteric enzymes

Allosteric enzymes are called enzymes, the activity of which is regulated not by their substrates, but by other substances that attach to enzymes in special areas remote from their active center.

 

 

Allosteric enzymes are enzymes whose activity is regulated not only by the number of substrate molecules but also by other substances called effectors. The effectors involved in allosteric regulation are cell metabolites, often of the very pathway which they regulate.

In structure, such enzymes differ from ordinary enzymes. Allosteric enzymes are usually built from two or more subunits. Along with active and substrate centers, they have an allosteric center. This center can reversibly bind certain metabolites that inhibit or activate the enzyme. These metabolites are called effectors. When the effector is attached to the allosteric center, the conformation of the protein as a whole change, and, consequently, the conformation of the active center. As a result, the activity of the enzyme changes.

One of the most common ways to regulate metabolic pathways is through allosteric enzymes. Allosteric enzymes are called, the action of which “by definition” is associated with a change in shape (alios – different, different; stereos – form).

The activity of such enzymes is regulated by substances that act like non-competitive inhibitors. These substances bind to enzymes at special sites distant from the active center and change the activity of the enzyme, causing a reversible change in the structure of the active center.

As a result, the ability of the substrate to bind to the enzyme also changes (which makes this phenomenon different from noncompetitive inhibition ). The substances acting in this way are called allosteric inhibitors. The figure explains the mechanism of allosteric inhibition.

An example of this phenomenon is the reaction that occurs during glycolysis, which is one of the stages of the process of cellular respiration. Cellular respiration serves as a source of ATP. If the concentration of ATP is high, then ATP, acting as an allosteric inhibitor, inhibits the activity of one of the glycolysis enzymes. If cellular metabolism increases, and consequently, ATP is consumed and its total concentration decreases, then after the inhibitor is removed, this metabolic pathway re-enters into action. This can also serve as an example of inhibition by the end product

The role of allosteric enzymes in cell metabolism. Allosteric enzymes play an important role in metabolism, as they react extremely quickly to the slightest changes in the internal state of the cell. Allosteric regulation is of great importance in the following situations:

  • in anabolic processes. Inhibition by the final product of the metabolic pathway and activation by initial metabolites allow the regulation of the synthesis of these compounds;
  • during catabolic processes. In the case of accumulation of ATP in the cell, inhibition of metabolic pathways that provide energy synthesis occurs. In this case, the substrates are consumed in the reactions of storing reserve nutrients;
  • to coordinate anabolic and catabolic pathways. ATP and ADP are allosteric effectors that act as antagonists;
  • for the coordination of parallel and interconnected metabolic pathways (for example, the synthesis of purine and pyrimidine nucleotides used for the synthesis of nucleic acids). Thus, the end products of one metabolic pathway can be allosteric effectors of another pathway.

Allosteric effectors – An effector that causes a decrease (inhibition) in the activity of an enzyme is called a negative effector, or inhibitor. An effector that causes an increase (activation) of enzyme activity is called a positive effector, or activator.

Various metabolites are often allosteric effectors. The end products of the metabolic pathway are often inhibitors of allosteric enzymes, and the starting materials are activators. This is the so-called heterotropic regulation. This type of allosteric regulation is very common in biological systems.

A more rare case of allosteric regulation, when the substrate itself can act as a positive effector. Such regulation is called homotropic (effector and substrate are one and the same substance). These enzymes have several binding sites for the substrate, which can serve a dual function: catalytic and regulatory. Allosteric enzymes of this type are used in situations where the substrate accumulates in excess and must be rapidly converted into a product.

It is possible to identify enzymes with allosteric regulation by studying the kinetics of these enzymes. These enzymes do not obey the Michaelis-Menten laws; they have a characteristic S-shaped curve of the dependence of the reaction rate on the substrate concentration.

 

Features of the structure and functioning of allosteric enzymes:

  • usually, these are oligomeric proteins consisting of several protomers or having a domain structure;
  • they have an allosteric center spatially distant from the catalytic active center;
  • effectors bind to the enzyme non-covalently in allosteric (regulatory) centers;
  • allosteric centers, as well as catalytic, can exhibit different
  • specificity in relation to ligands: it can be absolute and group. Some enzymes have several allosteric centers, some of which are specific to activators, others to inhibitors.
  • the protomer, on which the allosteric center is located, – the regulatory protomer, in contrast to the catalytic protomer, which contains the active center, in which a chemical reaction takes place;
  • allosteric enzymes have the property of cooperativity: the interaction of the allosteric effector with the allosteric center causes a sequential cooperative change in the conformation of all subunits, leading to a change in the conformation of the active center and a change in the enzyme’s affinity for the substrate, which reduces or increases the catalytic activity of the enzyme (Fig. 2-30);
  • regulation of allosteric enzymes is reversible: disconnection of the effector from the regulatory subunit restores the original catalytic activity of the enzyme;
  • β-allosteric enzymes catalyze key reactions of this metabolic pathway.

 

Localization of allosteric enzymes in the metabolic pathway. The rate of metabolic processes depends on the concentration of substances used and formed in this chain of reactions. Such regulation seems logical, since with the accumulation of the final product, it (the final product) can act as an allosteric inhibitor of the enzyme, which most often catalyzes the initial stage of this metabolic pathway:

 

In addition to catalytic centers that recognize and bind substrates, regulatory enzymes also have other stereospecific regions, the so-called allosteric centers. These are the binding sites for the effects that change the enzyme’s affinity for the substrate. There are specific sites for the binding of positive effectors (activators) and for negative effectors (inhibitors). The shape of the saturation curve changes under the influence of the effects.

A - the action of a negative effector (inhibitor); B - the action of a positive effector (activator).

Fig. 2-30. Diagram explaining the work of the allosteric enzyme. A – the action of a negative effector (inhibitor); B – the action of a positive effector (activator).