Many drugs exert their action by inhibition of an enzyme activity in the body. If the activity of an enzyme is vital to the cell or organism, then inhibition may lead to death of the cell or organism. It is now possible to design new drugs which are enzyme inhibitors once a target enzyme has been identified.
Types of Inhibitors
A) Reversible Inhibitors: The effect of the inhibitor is instantaneous, and it can be removed from the enzyme by dialysis so that the enzyme activity is returned to normal. Such inhibitors interact with the enzyme by weak non-covalent bonds to form an enzyme inhibitor complex. E + I ⇌ EI B) Irreversible Inhibitors: These inhibitors bind very tightly to the enzyme, sometimes by formation of covalent bonds to form an enzyme inhibitor compound rather than a loose complex. The effect is therefore progressive with time reaching a maximum when all of the enzyme has reacted. This is not easily reversed by simple physical treatments such as dialysis. E + I → EI Reversible Inhibition of Enzymes
There are three types of reversible enzyme inhibition; competitive, non-competitive (also called mixed) and uncompetitive. Competitive- molecules which closely resemble the substrate in size, shape and charge distribution may also slip into the active site. This may result in reaction i.e. the second molecule is another substrate for the enzyme, or it may result in inhibition because the active site is blocked. The inhibitor has a separate equilibrium with the enzyme. The binding of substrate and inhibitor is mutually exclusive. E + S ⇌ ES → E + P, E + I ⇌ EI Each of these equilibria is characterised by a dissociation constant. The first by Km (the Michaelis constant) and the second by Ki which characterises the binding between enzyme and inhibitor.
If sufficient [S] is present then eventually the inhibition by I will be overcome. This is the diagnostic test for this type of inhibition. Both I and S compete for the available enzyme. The activity of an enzyme is described by the following equation: (Michaelis- Menton equation)
In the presence of a competitive reversible inhibitor, this equation becomes;
So the Michaelis constant (which is a reciprocal measure of affinity of E and S) is changed by the factor 1 + [I]/Ki where [I] is the inhibitor concentration and Ki is the dissociation constant for the equilibrium between E and I. Most importantly, Vmax is unchanged - this is diagnostic for this type of inhibition. Ki is best defined as the concentration of inhibitor required to slow the reaction to half the rate it shows in the absence of inhibitor. It is a reciprocal measure of the affinity of E and I. Lineweaver-Burk Plot for Competitive Reversible Inhibition
The intercept on the y axis represents 1/Vmax. The slope is altered by the factor 1 + [I]/Ki, but the easiest way to calculate Ki is from the ratio of the intercepts on the x axis. Without inhibitor the intercept is -1//Km, with inhibitor it is -1/Km(1+[I]/Ki), so the ratio (bigger over smaller so it is greater than 1) is 1 + [I]/Ki. Easiest way to calculate Ki is from the ratio of the intercepts on the x axis. Equation:
Other Types of Reversible Inhibition
Uncompetitive- This type of reversible inhibition is said to occur when the inhibitor binds with the enzyme-substrate complex rather than the enzyme. Substrate and inhibitor bind dependently.
Noncompetitive (Mixed)- This type occurs when the inhibitor binds to both the enzyme and enzyme-substrate complex. Substrate and inhibitor bind independently. Irreversible Inhibition of Enzymes
Reversible means that the timescale of the inhibition is similar to that of the enzyme action, usually measured over a few minutes. Irreversible means that the enzyme activity is inhibited for times...