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In biochemistry, allosteric regulation is the regulation of an enzyme or protein by binding an Effector (biology) molecule at the protein's allosteric site (that is, a site other than the protein's active site). Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are called allosteric inhibitors. The term allostery comes from the Greek language allos, "other," and stereos, "space," referring to the regulatory site of an allosteric protein's being separate from its active site. Allosteric regulation is a natural example of feedback control.

Models of allosteric regulation Most allosteric effects can be explained by the concerted MWC model put forth by Jacques Monod, Wyman, and Jean-Pierre Changeux,J. Monod, J. Wyman, J.P. Changeux. (1965). On the nature of allosteric transitions:A plausible model. J. Mol. Biol., May;12:88-118. or by the sequential model described by Koshland, Nemethy, and Filmer.D.E. Jr Koshland, G. Némethy, D. Filmer (1966) Comparison of experimental binding data and theoretical models in proteins containing subunits. Biochemistry. Jan;5(1):365-8 Both postulate that enzyme subunits exist in one of two conformations, tensed (T) or relaxed (R), and that relaxed subunits bind substrate more readily than those in the tense state. The two models differ most in their assumptions about subunit interaction and the preexistence of both states.

Concerted model The concerted model of allostery, also referred to as the symmetry model or MWC- model, postulates that enzyme subunits are connected in such a way that a conformational change in one subunit is necessarily conferred to all other subunits. Thus all subunits must exist in the same conformation. The model further holds that in the absence of any ligand (substrate or otherwise), the equilibrium favors one of the conformational states, T or R. The equilibrium can be shifted to the R or T state through the binding of one ligand (biochemistry) (the allosteric effector or ligand) to a site that is different from the active site (the allosteric site).

Sequential model The sequential model of allosteric regulation holds that subunits are not connected in such a way that a conformational change in one induces a similar change in the others. Thus, all enzyme subunits do not necessitate the same conformation. Moreover, the sequential model dictates that molecules of substrate bind via an induced fit protocol. In general, when a subunit randomly collides with a molecule of substrate, the active site essentially forms a glove around its substrate. While such an induced fit converts a subunit from the tensed state to relaxed state, it does not propagate the conformational change to adjacent subunits. Instead, substrate-binding at one subunit only slightly alters the structure of other subunits so that their binding sites are more receptive to substrate. To summarize:

• subunits need not exist in the same conformation
• molecules of substrate bind via induced-fit protocol
• conformational changes are not propagated to all subunits
• substrate-binding causes increased substrate affinity in adjacent subunits

Allosteric activation and inhibition Activation Allosteric activation, such as the binding of oxygen molecules to hemoglobin, occurs when the binding of one ligand enhances the attraction between substrate molecules and other binding sites. With respect to hemoglobin, oxygen is effectively both the substrate (biochemistry) and the effector. The allosteric, or "other," site is the catalytic site of an adjoining protein subunit. The binding of oxygen to one subunit induces a conformational change in that subunit that interacts with the remaining active sites to enhance their oxygen affinity.

Inhibition Allosteric inhibition occurs when the binding of one ligand decreases the affinity for substrate at other active sites. For example, when 2,3-bisphosphoglycerate binds to an allosteric site on hemoglobin, the affinity for oxygen of all subunits decreases.

Another good example is strychnine, a Seizure poison, acting as an allosteric inhibitor of glycine. Glycine is a major post-synapse inhibitory neurotransmitter in mammalian spinal cord and brain stem. Strychnine acts at a separate binding site on the glycine receptor in an allosteric manner; i.e. its binding lowers the affinity of the glycine receptor for glycine. Strychnine thus inhibits the action of an inhibitory transmitter, causing convulsions.

Types of effectors Many allosteric proteins are regulated by their substrate; such a substrate is considered a homotropic allosteric modulator, and is typically an activator. Non-substrate regulatory molecules are called heterotropic allosteric modulators and can be either activators or inhibitors.

Some allosteric proteins can be regulated by their substrates and by other molecules, as well. Such proteins are capable of both homotropic and heterotropic interactions.

Pharmacology Allosteric modulation of a receptor results from the binding of allosteric modulators at a different site (regulatory site) other then of the endogenous ligand (orthosteric ligand) and enhances or inhibits the effects of the endogenous ligand. Normally acts by causing conformational change in receptor molecule which results in a change in the binding Dissociation constant#Protein-Ligand binding of the ligand. In this way allosteric ligands “modulates” its activation by a primary “ligand” and can be thought to act like a dimmer switch in an electrical circuit, adjusting the intensity of the receptor’s activation.

The anti-anxiety drugs Valium, Xanax, Librium and Ativan, for example, “Synergy” or turn up the activity of the benzodiazepine receptor when it binds to its primary ligand, the neurotransmitter gamma-aminobutyric acid (GABA).

Allosteric sites as drug targets Allosteric sites may represent a novel Drug discovery#Targets: New and Established.There are a number of advantages in using allosteric modulators as preferred therapeutic agents over classic orthosteric ligands. For example, GPCR allosteric binding sites have not faced the same evolutionary pressure as orthosteric sites to accommodate an endogenous ligand so are more diverse.A. Christopoulos, L.T. May, V.A. Avlani and P.M. Sexton (2004) G-protein-coupled receptor allosterism:the promise and the problem(s). Biochemical Society Transactions Volume 32, part 5 Therefore greater G protein coupled receptor selectivity may be obtained by targeting allosteric sites.This is particularly useful for GPCRs where selective orthosteric therapy has been difficult because of sequence conservation of the orthosteric site across receptor subtypes. L.T. May , K. Leach, P.M. Sexton, and A. Christopoulos. (2007). Allosteric Modulation of GProtein–Coupled Receptors Annu. Rev. Pharmacol. Toxicol. 47:1–51 Also these modulators is a decreased potential for toxic effects, since modulators with limited co-operativity will have a ceiling level to their effect, irrespective of the administered dose. Another type of pharmacological selectivity that is unique to allosteric modulators is based on cooperativity. An allosteric modulator may display neutral cooperativity with an orthosteric ligand at all subtypes of a given receptor except the subtype of interest, which is termed absolute subtype selectivity. If an allosteric modulator does not possess appreciable efficacy, it can provide another powerful therapeutic advantage over orthosteric ligands, namely, the ability to selectively tune up or down tissue responses only when the endogenous agonist is present.

See also

• Cooperative binding
• Enzyme kinetics

References

In biochemistry, allosteric regulation is the regulation of an enzyme or protein by binding an Effector (biology) molecule at the protein's allosteric site (that is, a site other than the protein's active site). Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are called allosteric inhibitors. The term allostery comes from the Greek language allos, "other," and stereos, "space," referring to the regulatory site of an allosteric protein's being separate from its active site. Allosteric regulation is a natural example of feedback control.

Models of allosteric regulation Most allosteric effects can be explained by the concerted MWC model put forth by Jacques Monod, Wyman, and Jean-Pierre Changeux,J. Monod, J. Wyman, J.P. Changeux. (1965). On the nature of allosteric transitions:A plausible model. J. Mol. Biol., May;12:88-118. or by the sequential model described by Koshland, Nemethy, and Filmer.D.E. Jr Koshland, G. Némethy, D. Filmer (1966) Comparison of experimental binding data and theoretical models in proteins containing subunits. Biochemistry. Jan;5(1):365-8 Both postulate that enzyme subunits exist in one of two conformations, tensed (T) or relaxed (R), and that relaxed subunits bind substrate more readily than those in the tense state. The two models differ most in their assumptions about subunit interaction and the preexistence of both states.

Concerted model The concerted model of allostery, also referred to as the symmetry model or MWC- model, postulates that enzyme subunits are connected in such a way that a conformational change in one subunit is necessarily conferred to all other subunits. Thus all subunits must exist in the same conformation. The model further holds that in the absence of any ligand (substrate or otherwise), the equilibrium favors one of the conformational states, T or R. The equilibrium can be shifted to the R or T state through the binding of one ligand (biochemistry) (the allosteric effector or ligand) to a site that is different from the active site (the allosteric site).

Sequential model The sequential model of allosteric regulation holds that subunits are not connected in such a way that a conformational change in one induces a similar change in the others. Thus, all enzyme subunits do not necessitate the same conformation. Moreover, the sequential model dictates that molecules of substrate bind via an induced fit protocol. In general, when a subunit randomly collides with a molecule of substrate, the active site essentially forms a glove around its substrate. While such an induced fit converts a subunit from the tensed state to relaxed state, it does not propagate the conformational change to adjacent subunits. Instead, substrate-binding at one subunit only slightly alters the structure of other subunits so that their binding sites are more receptive to substrate. To summarize:

• subunits need not exist in the same conformation
• molecules of substrate bind via induced-fit protocol
• conformational changes are not propagated to all subunits
• substrate-binding causes increased substrate affinity in adjacent subunits

Allosteric activation and inhibition Activation Allosteric activation, such as the binding of oxygen molecules to hemoglobin, occurs when the binding of one ligand enhances the attraction between substrate molecules and other binding sites. With respect to hemoglobin, oxygen is effectively both the substrate (biochemistry) and the effector. The allosteric, or "other," site is the catalytic site of an adjoining protein subunit. The binding of oxygen to one subunit induces a conformational change in that subunit that interacts with the remaining active sites to enhance their oxygen affinity.

Inhibition Allosteric inhibition occurs when the binding of one ligand decreases the affinity for substrate at other active sites. For example, when 2,3-bisphosphoglycerate binds to an allosteric site on hemoglobin, the affinity for oxygen of all subunits decreases.

Another good example is strychnine, a Seizure poison, acting as an allosteric inhibitor of glycine. Glycine is a major post-synapse inhibitory neurotransmitter in mammalian spinal cord and brain stem. Strychnine acts at a separate binding site on the glycine receptor in an allosteric manner; i.e. its binding lowers the affinity of the glycine receptor for glycine. Strychnine thus inhibits the action of an inhibitory transmitter, causing convulsions.

Types of effectors Many allosteric proteins are regulated by their substrate; such a substrate is considered a homotropic allosteric modulator, and is typically an activator. Non-substrate regulatory molecules are called heterotropic allosteric modulators and can be either activators or inhibitors.

Some allosteric proteins can be regulated by their substrates and by other molecules, as well. Such proteins are capable of both homotropic and heterotropic interactions.

Pharmacology Allosteric modulation of a receptor results from the binding of allosteric modulators at a different site (regulatory site) other then of the endogenous ligand (orthosteric ligand) and enhances or inhibits the effects of the endogenous ligand. Normally acts by causing conformational change in receptor molecule which results in a change in the binding Dissociation constant#Protein-Ligand binding of the ligand. In this way allosteric ligands “modulates” its activation by a primary “ligand” and can be thought to act like a dimmer switch in an electrical circuit, adjusting the intensity of the receptor’s activation.

The anti-anxiety drugs Valium, Xanax, Librium and Ativan, for example, “Synergy” or turn up the activity of the benzodiazepine receptor when it binds to its primary ligand, the neurotransmitter gamma-aminobutyric acid (GABA).

Allosteric sites as drug targets Allosteric sites may represent a novel Drug discovery#Targets: New and Established.There are a number of advantages in using allosteric modulators as preferred therapeutic agents over classic orthosteric ligands. For example, GPCR allosteric binding sites have not faced the same evolutionary pressure as orthosteric sites to accommodate an endogenous ligand so are more diverse.A. Christopoulos, L.T. May, V.A. Avlani and P.M. Sexton (2004) G-protein-coupled receptor allosterism:the promise and the problem(s). Biochemical Society Transactions Volume 32, part 5 Therefore greater G protein coupled receptor selectivity may be obtained by targeting allosteric sites.This is particularly useful for GPCRs where selective orthosteric therapy has been difficult because of sequence conservation of the orthosteric site across receptor subtypes. L.T. May , K. Leach, P.M. Sexton, and A. Christopoulos. (2007). Allosteric Modulation of GProtein–Coupled Receptors Annu. Rev. Pharmacol. Toxicol. 47:1–51 Also these modulators is a decreased potential for toxic effects, since modulators with limited co-operativity will have a ceiling level to their effect, irrespective of the administered dose. Another type of pharmacological selectivity that is unique to allosteric modulators is based on cooperativity. An allosteric modulator may display neutral cooperativity with an orthosteric ligand at all subtypes of a given receptor except the subtype of interest, which is termed absolute subtype selectivity. If an allosteric modulator does not possess appreciable efficacy, it can provide another powerful therapeutic advantage over orthosteric ligands, namely, the ability to selectively tune up or down tissue responses only when the endogenous agonist is present.

See also

• Cooperative binding
• Enzyme kinetics

References

Allosteric regulation - Wikipedia, the free encyclopedia
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site (that is, a site other than ...

Institute of Cancer Research Repository - Allosteric regulation of ...
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allosteric control. Inhibition or activation of an enzyme by a small regulatory molecule that interacts with the enzyme at a site (allosteric site) other than the active site (at ...

Tutorial 6.2 Allosteric Regulation of Enzymesl
Textbook Reference - Figure 6.19, p. 121

Allosteric regulation - definition of Allosteric regulation in the ...
pertaining to an effect on the biological function of a protein, produced by a compound not directly involved in that function (an allosteric effector) or to regulation of an ...

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Research article Integrated allosteric regulation in the S. cerevisiae carbamylphosphate synthetase – aspartate transcarbamylase multifunctional protein

A to H
Allosteric regulation is the regulation of the activity of allosteric enzymes. (See also Allosteric binding sites; Allosteric enzymes). Analog. An analog is a drug whose structure is ...

NIMR, London :: Division of Physical Biochemistry :: Birdsall group
Birdsall group :: My research concentrates on elucidating the mechanisms of activation, inhibition and allosteric regulation of G protein-coupled receptors by ...

Glasgow ePrints Service - CXCR2 chemokine receptor antagonism enhances ...
CXCR2 chemokine receptor antagonism enhances DOP opioid receptor function via allosteric regulation of the CXCR2-DOP receptor hetero-dimer. Parenty, G. and Applebe, S. and Milligan, G. ...

Directory of open access journals
Find articles: Allosteric regulation of small-molecule binding to aptamers Author: MILAN N. STOJANOVIC Journal: Journal of the Serbian Chemical Society Year: 2004 Vol: 69 Issue: 11 ...