Cooperative bindings Allosteric enzymes, unlike simpler enzymes, do not obey Michaelis-Menten kinetics. Often, one active site of an allosteric enzyme molecule can positively affect another active site in the same molecule. This leads to cooperativity and sigmoidal enzyme kinetics in a plot of versus . The terms competitive inhibition and noncompetitive inhibition apply to Michaelis-Menten kinetics and not to allosteric enzymes. Allosteric Regulation i. Allosteric enzyme has one catalytic site where the substrate binds and another separate allosteric site where the modifier binds (allo= other) ii. Allosteric and substrate binding sites may or may not be physically adjacent. iii. The binding of the regulatory molecule can either enhance the activity of the enzyme (allosteric activation), or inhibit the activity of the enzyme (allosteric inhibition). iv. In the former case, the regulatory molecule is known as the positive modifier and in the latter case as the negative modifier. v. The binding of substrate to one of the subunits of the enzyme may enhance substrate binding by other subunits. This effect is said to be positive co-operativity. If the binding of substrate to one of the subunits decreases the avidity of substrate binding by other sites, the effect is called negative co-operativity. vi. In most cases, a combination is observed, resulting in a sigmoid shaped curve. vii.Km is usually increased. viii. Vmax is reduced. ix. The effect of allosteric modifier is maximum at or near substrate concentration equivalent to Km . xi.When an inhibitor binds to the allosteric site, the configuration of catalytic site is modified such that substrate cannot bind properly. xii. Most allosteric enzymes possess quaternary structure. They are made up of subunits, e.g. Aspaate transcarbamoylase has 6 subunits and pyruvate kinase has 4 subunits.