GraphPad Curve Fitting Guide

Key concepts: Enzyme inhibition

Key concepts: Enzyme inhibition

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Key concepts: Enzyme inhibition

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Many drugs work by inhibiting enzyme activity, either by preventing the substrate from binding to the enzyme, or by stabilizing the enzyme-substrate complex so as to slow formation of product.To distinguish between the models of enzyme inhibition and determine the Ki of the inhibitor, measure substrate-velocity curves in the presence of several concentrations of inhibitor (including one curve with no inhibitor).

Prism can fit your data to six models of enzyme inhibition:

A competitive inhibitor reversibly binds to the same site as the substrate, so its inhibition can be entirely overcome by using a very high concentration of substrate. The maximum velocity of the enzyme doesn't change (if you give it enough substrate), but it takes more substrate to get to half maximal activity. The substrate-velocity curve is shifted to the left but not down.

A noncompetitive inhibitor binds with equal affinity to the enzyme, and the enzyme-substrate complex. The inhibition is not surmountable by increasing substrate concentration. The substrate-velocity curve is shifted down but neither to the right or left.

An uncompetitive inhibitor reversibly binds to the enzyme-substrate complex, but not to the enzyme itself. This reduces both the effective Vmax and the effective Km. The substrate-velocity curve is shifted down and to the left.

The mixed model is a general model that includes competitive, noncompetitive and uncompetitive models as special cases. The model has one more parameter than the others, and the extra parameter (alpha) determines the degree to which the binding of inhibitor changes the affinity of the enzyme for substrate.

Substrate inhibition. In some cases, the substrate of an enzyme also inhibits the enzyme by binding to a second site on the enzyme.

Tight inhibition. This model accounts for tight binding, so it does not assume that the free concentration of inhibitor equals the total concentration.

Reference                                                                         

RA Copeland, Evaluation of Enzyme Inhibitors in Drug Discovery, Wiley 2005. IBSN:0471686964.