What are examples of competitive inhibitors

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Non-competitive inhibition

In the case of non-competitive inhibition, the rate of formation or disintegration of the enzyme-substrate complex changes through modification of the enzyme. A special case of non-competitive inhibition is allosteric inhibition: It is based on the change in the affinity of the allosteric center of the enzyme for an effector, e.g. as a result of treatment with heavy metals, urea or proteolytic enzymes.

In the case of non-competitive inhibition, a non-competitive inhibitor binds to a site that is not identical to the substrate binding site. The binding of the inhibitor therefore does not lead to the blocking of the substrate binding, but to a conformational change of the enzyme, which is thereby inactivated. The binding of the inhibitor and the inactivation of the enzyme take place regardless of the presence of the substrate.

If the inhibitor and enzyme are present in a solution, in the case of non-competitive inhibition the concentration of the active enzyme and thus - according to the equation - the value is reduced, but changes only slightly.

To characterize this type of enzyme inhibition, conversion rates of several series of measurements with varying substrate concentrations are determined. In a control series of measurements, no inhibitor is used and so and so determined. The direct plot (Fig. 4) shows hyperbolic curves.

For the representation of the non-competitive inhibition, the linear representation forms (Fig. 5) are favored. The various straight lines in each series of measurements are arranged in patterns that are characteristic of the type of inhibition and the respective linearization method. In the double-reciprocal representation, all straight lines meet at a common point of intersection to the left of the ordinate. The relationship between the two inhibition constants can be read from the position of the intersection.

application

The non-competitive inhibition is particularly important for the regulation of the cell metabolism, since here it is possible to influence the activity by metabolites without direct substrate analogy. Examples of this are end product inhibition and allostery. In allostery, non-hyperbolic saturation curves, which can be linearized by simple methods, are often shown due to overlapping due to cooperative effects. In such a case, the non-competitive mechanism can be recognized by extrapolation to infinite substrate concentrations. In the case of multi-substrate reactions, a non-competitive inhibition often also occurs in the form of a product inhibition. In the course of the substrate turnover, products increasingly remain bound in the active center and inhibit the enzyme reaction, including on the one hand the binding of the substrate and on the other hand the binding of the cosubstrate.