Allosteric regulation is a mechanism based on a change in the spatial conformation of an enzyme as a result of its binding to a modulator ligand. This process is reversible and can have both an activation (positive) and an inhibitory (negative) effect. Enzymes with this type of regulation are called allosteric, and the effect exerted on them is called allosteria (from the Greek allos - “other” and stereo - “spatial”).
Allosteric enzymes
Allosteric enzymes are characterized by two key features:
- catalytic activity depends not only on the concentration of the substrate;
- can change the spatial structure when interacting with a ligand, which affects the affinity of the active center for the substrate (binding efficiency).
Most of the enzymes in this group are oligomeric proteins consisting of 2 or more subunits linked to each other by non-covalent interactions (quaternary structure).
Each subunit has 2 functional forms that can replace each other depending on the binding to a specific modulator:
- R-conformation (from the English. Relax - relaxation) has a high affinity for the substrate.
- T-conformation (from the English tennse - stress) is characterized by low affinity.
A change in the conformations of individual protomers leads to a change in the activity of the entire enzyme.
In addition to the active center, allosteric enzymes have one or more regulatory enzymes designed to bind to effectors. Most often, these 2 types of centers are spatially removed from each other, but in some proteins they coincide. In this case, one site is bifunctional (it can play both a catalytic and regulatory role). Allosteric centers have specificity for a specific modulator, which may be absolute or group in nature.
Allosteric Effectors
Ligands that modify the conformation of enzymes are called allosteric effectors (or modulators), which are divided into 2 groups:
- negative effectors (inhibitors) reduce enzymatic activity;
- positive effectors (activators) increase enzymatic activity.
Low molecular weight cellular metabolites, and sometimes the substrate itself, often act as modulators. An allosteric enzyme may contain several regulatory centers, some of which are for activators, and others for inhibitors.
Homotropic and heterotropic regulation
The ligand that affects the conformation of the protein is often different from the substrate (the substance with which the active center of the enzyme binds to catalyze the reaction). In this case, they speak of heterotropic regulation.
In some situations, one substance is both a ligand and a substrate. Such regulation is called homotropic and is much less common. It is characteristic of metabolic pathways, where fast processing of excess accumulated substrate is required. In homotropic enzymes, the active and regulatory centers coincide.
Regulation of enzyme activity by allosteric modification
The essence of alosteric modification is to change the affinity of the enzyme for the substrate by conformational rearrangement of the whole molecule. For example, in the process of inhibition, the binding of a negative effector to a regulatory site causes the protein to acquire a conformation that destroys the active center.
The positive effector acts in the opposite way. It causes spatial rearrangement, increasing the likelihood of binding of the catalytic center to the substrate.
Allosteric regulation has the following features:
- binding of the effector to the enzyme is non-covalent;
- the catalytic and regulatory centers are located on different protomers with the corresponding names;
- the work of allosteric enzymes does not correspond to the Michaelis – Menten kinetic model;
- regulation is reversible (when the ligand is disconnected, the enzyme restores the initial catalytic activity).
The condition for this mechanism to work is the conformational relationship between the catalytic and regulatory protomers. Therefore, the basis of allosteric regulation of oligomeric enzymes is the cooperative effect, which spreads the influence of the modulator on the whole molecule.
Cooperative effect
A change in the conformation of one of the subunits of the allosteric enzyme entails a spatial rearrangement of all others. This phenomenon is called the cooperative effect. The latter can be carried out in two ways: symmetric and sequential.
In the first case, almost all subunits are in the same conformation (R or T). For example, a two-subunit enzyme may consist of protomers of either RR or TT, but not RT. Upon binding to the substrate, all subunits change conformation simultaneously.
According to a sequential model, protomers of different conformations can be present in one molecule at the same time, and they change alternately, from one subunit to another.
Depending on how the work of the active center changes, co-op is negative or positive.
Interfacing and clutch mechanisms
The mechanisms of allosteric regulation of enzyme activity are based on the principles of conjugation and adhesion, which characterize the influence of the ligand-binding and active centers of the enzyme on each other.
The conjugation principle is valid when the substrate and the modulator have a positive affinity for the same conformation of the enzyme. In this case, protein binding to one of these ligands will increase the affinity for the other. As a result, the presence of a modulator increases the efficiency of the active center.
The negative mechanism of allosteric regulation works when the substrate and the modulator prefer different enzyme conformations for binding. Then the presence of one ligand prevents the attachment of another. As a result, the active center of the enzyme associated with the modulator loses affinity for the substrate. This mechanism is called clutch.
The difference between allosteric modification and non-competitive inhibition
In the biochemical processes of the cell, there are 2 mechanisms that, according to some signs, resemble allostaric regulation — noncompetitive and mixed inhibition, which are characterized by the attachment of a ligand to a site other than the active center. However, this does not always lead to conformational changes that cause the transition of the active form of the enzyme to inactive. In addition, this process has completely different kinetic characteristics, so non-competitive inhibitors and negative effectors are not identical concepts.
Another difference of allosteric regulation is that it can be not only negative, but also positive. The suppression of enzyme activity is often carried out by a feedback mechanism, which is also called retro-inhibition, and a common form of positive regulation is activation by a precursor (foractivation).
Retroinhibition and thoracic activation
A special case of allosteric regulation of enzymes is the inhibition of catalytic activity by the feedback mechanism (retroinhibition). In this case, the product of the multi-stage reaction manifests itself as a negative effector. This mechanism prevents the expenditure of energy on the excessive accumulation of the synthesized substance.
Foractivation is an enzyme activation process in which the primary metabolite of the reaction is a positive effector.
The role of the allosteric regulatory mechanism in cell metabolism
The biological value of allosteric enzymes in metabolism lies in their ability to respond to the slightest changes in the state of the cell and finely regulate biochemical reactions. The allosteric mechanism is useful in the following cases:
- anabolic reactions - retroinhibition and thoracic activation regulate product synthesis;
- catabolic processes - with a sufficient concentration of ATP, its synthesis is inhibited by the feedback mechanism;
- ensuring coordination and balance between anabolism and catabolism - ADP and ATP molecules act as allosteric effectors with the opposite effect;
- coordination between parallel metabolic pathways (e.g., synthesis of purine and pyrimidine bases).
Allosteric regulation of metabolism links the reaction chains of multi-stage processes. In this case, the product of the previous metabolic pathway is a positive effector for the following.