Sometimes it has been found that when a series of reactions is catalysed by a number of enzymes in sequence, the accumulation of the final end-product may cause inhibition in the activity of the first enzyme of the series. This inhibition due to a compound (final end product) which is totally different in structure from the substrate of the enzyme is called as allosteric inhibition or feedback inhibition and such an enzyme is called as allosteric enzyme.
This type of inhibition takes place due to the presence of allosteric site (Greek allo = ‘other’; stereos = ‘space’ or ‘site’) on the surface of the allosteric enzyme away from the active site. The final end-product molecule fits in the allosteric site and in some way brings about a change in shape of the enzyme so that the active site of the enzyme becomes unfit for making complex with its substrate. The allosteric inhibition is reversible. When the concentration of the final end product in the cell falls, it leaves the allosteric site, and the activity of the allosteric enzyme is restored.
Allosteric inhibition is shown diagrammatically in Fig. 10.11.
One of the classical and first discovered examples of allosteric inhibition is furnished by the bacterial enzyme system of E. coli which catalyses the conversion of L-Threonine into L-Isoleucine involving 5 different enzymes in sequence viz., 1. Threonine dehydratase 2. Acetolactate synthase 3. Ketoacid reductoisomerase 4. Dihydroxy acid dehydratase and 5. Transaminase (See Fig. 10.12).
In this sequence, only the first enzyme i.e., threonine dehydratase is inhibited by Isoleucine which is the end-product of this sequence. The activity of this enzyme is neither inhibited by any other intermediate of the sequence, nor is any other enzyme of this sequence inhibited by Isoleucine.
The inhibition of the first enzyme threonie dehydratase is reversible. When the concentration of isoleucine in the cells increases the activity of this enzyme is decreased so that production of isoleucine falls. But, when isoleucine concentration decreases, the activity of threonine dehydratase increases and the production of isoleucine in the cells is restored’.