How to determine types of enzyme inhibition?


Inhibition are caused by effects of molecules that are a substrate and decrease rates of enzymes. Inhibition of enzymes is normal and organisms will use inhibitor molecules to regulate enzymes. Inhibition can be caused by several mechanisms. It is possible to determine what type of inhibition is caused by certain inhibitor molecules. Using Michael-Menton kinetics and by mixing the enzyme with the substrate and without any inhibitor we can identify a standard velocity curve for the enzyme. After we know the rate of an enzyme without inhibition we can add the inhibitor and measure the change in rate with different concentrations of inhibitors. The easiest way to determine the type of inhibitor would be to use a double reciprocal or Lineweaver Burk Plot. Because this plot is a double reciprocal the closer to the origin the greater the velocity and substrate concentration. From here you would continue to measure the rate of the enzyme now with a specific concentration of inhibitor and titer in substrate. There would be a total of four different types of plots that could be recognized to determine the type of inhibition.

Competitive InhibitionCompetitive Inhibition – Inhibitor competes for binding to the active site of the enzyme, has no effect on the Vmax as substrate concentration increases, only effects the K’m equal to the aKm. a is determined from the concentration of inhibitor over the equilibrium constant of the inhibitor, Ki. K is equal to the enzyme concentration [E] multiplied by the [I] concentration divided by the combined EI complex concentration. Looking at a lineweaver-Burk equation the Y-intercept is conserved indicating the Vmax is conserved. These inhibitors are important for developing drugs to block the activity of negative enzymes as well as analyzing enzyme          mechanisms.

Uncompetitive Inhibition – directly effects both the maximum velocity, Vmax of the enzyme and the half max velocity, Km. An uncompetitive inhibitor would directly bind the intermediate formed between enzyme, E, and substrate, S. E+S àES+IàESI Ultimately the product can be formed when the inhibitor falls of the intermediate, thus effects Vmax and Km by equal proportion, a. a is equal to one plus the concentration of inhibitor divided by the equilibrium constant of the inhibitor and enzyme intermediate. a=(1+[I])/KI

Mixed inhibition – these bind either the enzyme or the enzyme substrate intermediate at a site separate from the active site. The major difference from the mixed and uncompetitive is that the lines from this plot intersect. This can be understood because the inhibitor can bind both the enzyme away from the active site as well as the enzyme substrate intermediate. E + I à EI +Sà ESI àES à ED Thus there are two equilibrium constants effecting the rate of the enzyme thus generating to values proportional to rate a and a’. Thus the new half max and max velocity would be proportional to a/a’. Km’ = (Km)(a)/a’ and Vmax’ = (Vmax)(a)/(a’)

How to determine types of enzyme inhibition?

The final type if reversible inhibitor is actually just a special case of mixed inhibitor called non-competitive. This is due to the a being equal to a’ from mixed inhibitors.


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