Inhibitors interfere with enzyme function. Competitive inhibitors are similar in shape to the enzyme's substrate and bind to the active site of the enzyme. A competitive inhibitor physically competes with the substrate for access to the enzyme. Non-competitive inhibitors bind to the control site and partially block the active site or alter the shape of the enzyme. Other inhibitors may attach to the enzyme for briefer periods of time and simply block temporarily the enzyme's activity.
Many enzymes are allosteric enzymes.
The presence of an inhibitor will change the shape of the enzyme, rendering it nonfunctional. When the inhibitor is removed, the once inactivated enzyme can resume its function. Often allosteric enzymes are located at metabolic branch points and are called branch point enzymes. These end product inhibitors can 'turn off' a whole metabolic pathway by preventing substrate molecules from entering the first reaction of a pathway. End product inhibition is an important mechanism for negative feedback control of metabolism.