Allosteric regulation

In biology, allosteric regulation is a term that refers to enzyme regulation or other protein by binding an effector molecule at the allosteric protein site (that is a site that is different from the active site of the protein). Allosteric site makes it possible for effectors to bind to the protein and often results in conformational change or in other cases, change in effectiveness of the enzyme.

Effectors with the ability of enhancing the activity of the protein are known as allosteric activators while t hose that decrease the protein activity are known as allosteric inhibitors. Allostery is a term that comes from the Greek word allos “other” and steros “solid (object)”. It is in reference to regulatory site of allosteric protein which is physically distinctive from the active site.

Allosteric regulations are also a natural example of control loops like feedback from feed forward or downstream products from upstream substrates. Long-range allostery is important especially in cell signaling.

The concerted MWC model that was put forward by Wyman, Changeux and Monod is often used to explain allosteric effects and so is the sequential model described by Filmer, Koshland and Nemethy. Both postulate that there are enzyme subunits that exist in one of two conformations, tensed (T) or relaxed (R).

They also hold that the relaxed subunits bind substrate readily than those in tense state. It is however essential to note these two models differ greatly in their assumption about subunit interaction as well as preexistence of both states.

There are different types of allosteric regulation and they include:

• Homotropic-Homotropic allosteric modulator is a substrate for its target enzyme and a regulatory molecule of enzyme activity. Typically, it is an activator of  the enzyme for instance oxygen is a homotropic modulator of hemoglobin
• Heterotropic- this is a regulatory molecule that is not the enzyme’s substrate it can either be an inhibitor or activator of the enzyme. For instance H⁺, CO2 and 2, 3 bisphosphoglycerate are modulators of hemoglobin.

There are some allosteric proteins that can be regulated by other molecules and both their substrates. Such proteins have the capability of both heterotrophic and homotrophic interactions. A non-regulatory allosteric site is a term that refers to non-regulatory component of an enzyme that is not by itself an amino acid. For instance, there are several enzymes that need sodium binding for purposes of ensuring they function properly.

While this is the case, the sodium doesn’t necessarily act as a regulatory subunit, sodium is always present and there are no known biological processes that can be used to add/remove sodium in order to regulate enzyme activity.

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