What is the allosteric inhibitor? Explain with an example

Many enzymes have an allosteric regulation mechanism. In allosteric regulation, effector (inhibitor or activator) binds to a site other than the active site to bring about conformational changes and thereby affecting the activity of the enzyme.

Allosteric enzymes

Allosteric enzymes are enzymes that have an additional binding site for effector molecules other than the active site. The binding brings about conformational changes, thereby changing its catalytic properties. The effector molecule can be an inhibitor or activator.

Allosteric Inhibitor

An allosteric inhibitor by binding to allosteric site alters the protein conformation in the active site of enzyme which consequently changes the shape of the active site. Thus enzyme no longer remains able to bind to its specific substrate. Hence enzyme is unable to perform it’s catalytic activity i.e enzyme is now inactive. This process is called allosteric inhibition.

Mechanism

There are two models proposed for the mechanism of regulation of allosteric enzymes:

Simple Sequential Model

  • It was given by Koshland. In this model, the binding of substrate induces a change in the conformation of the enzyme from T (tensed) to R (relaxed).
  • The substrate binds according to the induced fit theory. A conformational change in one unit stimulates similar changes in other subunits.
  • This explains the cooperative binding.
  • The same way inhibitors and activators bind, the T form is favoured, when the inhibitor binds and R form is favoured, when the activator binds. The binding at one subunit affects the conformation of other subunits.
  • The sequential model explains the negative cooperativity in enzymes, e.g. tyrosyl tRNA synthetase, where the binding of substrate inhibits the binding of another substrate.

Concerted or Symmetry Model

  • According to this model, there is a simultaneous change in all the subunits of an enzyme.
  • All the subunits are either present in R form (active form) or T form (inactive form), having less affinity to a substrate.
  • An inhibitor shifts the equilibrium of T ⇄ R, towards T, and activator shifts the equilibrium towards R form and favours the binding.
  • It explains the cooperative regulation of activators as well as inhibitors.

Example:

1. Hexokinase (I) acts as an activator to extract the glucose in the glycolysis pathway.

2. Glucokinase is an enzyme activator that combines with enzyme released by pancreatic cell used to the treatment of diabetes.

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