Inhibition of enzymes
Enzymes may be inhibited intentionally or unintentionally. Feedback inhibition is an efficient way for the cell to conserve resources. In this process, the product of a chemical pathway stops the action of the first pathway enzyme by blocking the active site. With the first enzyme of the pathway no longer functional, the product cannot be made. When the product declines in number, the intended substrate can now enter the pathway, leading to an increase in the concentration of the product.
There are a few ways in which an enzyme can be inhibited or regulated.
1. Competitive inhibition: In this case, the active site is blocked by a molecular mimic. The intended substrate cannot reach the active site and no reaction occurs. Many drugs and poisons work in this manner.
2. Allosteric regulation: Some enzymes have more than one active site and are able to alternate between active and inactive conformations. These are allosteric enzymes. A region remote from the active site can regulate the status of the enzyme. An inhibitor binds temporarily to the allosteric site and stabilizes the inactive form. An activator will bind temporarily to the allosteric site to stablilize the active conformation. The level of activity of the enzymes is dependent upon relative concentrations of activator and inhibitor molecules.
3. Cooperativity: In an allosteric enzyme, the binding of substrate to one active site makes it easier for the next substrate to bind. This also helps to stabilize the enzyme in its active form.
see also competitive inhibition animation
allosteric animation
