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ACTIVATION

An activator is a substance, other than the catalyst itself or one of the substrates, that increases the rate of an enzyme-catalysed reaction, and it is said to bring about activation. If a reaction in the absence of an activator occurs with rate v _o, the degree of activation is given by

........ (23)

This equation is analogous to the definition of degree of inhihition (Eqn 20) and is, like , dimensionless. In general, the equations for activation closely resemble those for inhibition: wherever a term of the form (1 + [I]/ K _i) appears in an inhibition equation, a corresponding term with (I + K /[Q]) can be written for activation of the same kind by an activator Q.

Activation has been studied much less than inhibition. and at present no system of terminology enjoys wide currency. Nonetheless, the rather unsatisfactory term competitive activation is occasionally used (see below), and the following classification is offered in the hope of providing a more appropriate terminology. The fundamental division is between linear activation which can be fully defined by terms in the denominator of the rate expression that are linear in the reciprocal concentration of activator, i.e. terms of the form (1 + K /[Q]), and non-linear activation. which cannot. An alternative division is between essential activation. in which the rate is zero in the absence of activator, and non-essential activation, in which it is finite. These two classifications overlap to a large extent but are not identical: linear activation must be essential. and non-essential activation must be non-linear, but essential activation can in principle be non-linear. Linear activation can, like linear inhibition, be usefully classified further according to whether the activation affects the apparent value of the specificity constant, in which case it is called specific activation, the apparent value of the catalytic constant, in which case it is called catalytic activation, the apparent value of the Michaelis constant, in which case it is called binding activation, or some combination of these, in which case it is called mixed activation. There is no sense in which specific activation can be regarded as resulting from competition between activator and substrate; consequently the term competitive activation is meaningless and to be avoided: the terms uncompetitive and non-competitive are also best avoided in the context of activation.

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