Degree of inhibition

The rate of an enzymic reaction is often affected by substances other than the reactant(s). Inhibitors slow the rate and activators increase it. In dealing with inhibitors it is important to distinguish between the effects that are observed e-perimentally on the various analytical plots mentioned above, and the molecular mechanism (or models) proposed to explain the effects. The purpose behind studies of enzyme inhibition is often to enhance an understanding of the mechanism of the enzyme by interpreting the changes in apparent values of Vma- and Km in terms of various possible models of the mechanism (see Chapter 3).

There are three basic types of inhibition of an enzyme and these are defined in terms of the degree of inhibition, which is defined as v0 - v0

where v0 and v0 are the initial velocities of the reaction measured in the absence and presence of the inhibitor, respectively.

1. Pure noncompetitive inhibition is said to exist if i is unaffected by the concentration of the substrate.

2. Competitive inhibition exists if i decreases as the substrate concentration is increased.

3. Anti- or uncompetitive inhibitions exists if i increases as the substrate concentration is increased.

In addition to these canonical forms of inhibition is mixed inhibition, in which i increases or decreases as the substrate concentration increases, but not to the same extent as for the pure competitive or anticompetitive cases. Mechanistically, noncompetitive inhibition is a special case of mixed inhibition, but operationally, as defined in Eqn [2.5], mixed inhibition is a combination of two of the above canonical types of inhibition.

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