Enzyme Kinetics
Part II. Enzyme Inhibition
III. Enzyme Inhibitors B. Non-competitive Inhibition.
A Non-Competitive Inhibitor does not compete with substrate and the [S] has no influence on the degree of inhibition of the enzyme's catalytic rate. For example, enzymes with a thiol ( -SH ) not at the active site can be inhibited:
Figure 5. Example of a heavy metal inhibiting an enzyme by binding to a thiol group not at the active site and inactivating the enzyme.
Non-Competitive Inhibition can be model using the standard model for the Michaelis-Menten enzyme where E + S form the ES complex which leads to formation of product P. In this case where the non-competitive inhibitor (Inc) reacts with the enzyme at a site other than the active site, both the free enzyme (E) and the enzyme-substrate complex (E-S) react with Inc. Clearly, in this case the reaction of the non-competitive inhibitor is irreversible and the substrate can not over come the inhibitors impact on the enzyme:
Figure 6. Model of the Non-Competitive Inhibitor (Inc). The equilibrium between enzyme and Inc now depends on the total concentration of enzyme in all forms present (ie. both the free E and the E-S complex) and defines the Ki.
A Vo versus [S] plot for the Non-competitive Inhibitor looks very different than that for a competitive inhibitor since increasing the [S] has no impact:
Figure 7. Vo versus [S] plot for enzyme in the absence and presence of Inc.
The double reciprocal plot for this same model shows that Inc decreases Vmax, as if some of the enzyme had been removed from the system. In classic example of pure non-competitive inhibition, the uninhibited reaction and the enzyme in the presence of Inc will yield the same Km value.
Figure 8A. Double Reciprocal plot for the Non-Competitive Inhibitor (Inc).
Non competitive inhibitors decrease Vmax but have no effect on Km.
The apparent Vmax' is smaller than the real Vmax and the Ki for the Non-Competitive Inhibitor can be calculated using the following equation and the known [I]:
Figure 8B. Equation showing the relationship between Vmax' (apparent Vmax) and real Vmax in the presence of a Non-Competitive Inhibitor. Use this equationfor calculating the Ki of the Non-Competitive Inhibitor at known [Inc].
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu