Intracellular regulation of enzyme activity

As discussed in section 2.3.3, the rate at which an enzyme catalyses a reaction increases with increasing concentration of substrate, until the enzyme is more or less saturated. This means that an enzyme that has a high relative to the usual concentration of its substrate will be sensitive to changes in substrate availability. By contrast, an enzyme that has a low relative to the usual concentration of its substrate will act at 

Allosteric regulation of enzyme activity is due to reversible, non-covalent, binding of effectors to regulatory sites, leading to a change in the conformation of the active site. This may result in either increased catalytic activity (allosteric activation) or decreased catalytic activity (allosteric inhibition). Enzymes that are subject to allosteric regulation are usually multiple subunit proteins. 

Many enzymes that are subject to allosteric regulation have two interconvertible conformations  

A relaxed (R) form, which binds substrates well, and therefore has high catalytic activity. Allosteric activators bind to, and stabilize, the active R form of the enzyme. A tense (T) form, which binds substrates poorly and therefore has low catalytic activity. Allosteric inhibitors bind to, and stabilize, the less active T form of the enzyme. 

Compounds that act as allosteric inhibitors are commonly end-products of the pathway, and this type of inhibition is known as end-product inhibition. The decreased rate of enzyme activity results in a lower rate of formation of a product that is present is adequate amounts. 

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