Enzyme Inhibitors are substances that reduce the rate of enzyme activity in an enzyme catalysed reaction.These substances may be in the form of molecules or ions that mimic the actual substrates in order to bind to the active site of the enzyme to form an enzyme-inhibitor(EI) complex.Enzyme inhibitors are of two types;
1) Irreversible Inhibitors
2) Reversible Inhibitors
These substances binds permanently to the active site of an enzyme to form an EI complex.These interraction usually occurs via a strong covalent bond between the enzyme and the inhibitor.It may also occur via a strong non-covalent linkage.In Irreversible inhibition, the separation of the inhibitor from the active site is usually very slow because of the strong covalent linkage that exist between the inhibitor and the enzyme.Irreversible inhibitors are used in Pharmaceuticals for the synthesis of drugs.An example is the inhibition of acetylcholinesterase by Sarin gas which reacts with the hydroxyl group of serine residue in the enzyme to form an ester.This prevents the breakdown of acetylcholines by acetylcholinesterase.
These substances bind to the active of an enzyme to form an EI complex.These bond is not as strong as that of irreversible inhibitors and this causes a separation of the EI complex.Reversible inhibitors are of three types;
a. Reversible-competitive Inhibitor
b. Reversible-Non-competitive Inhibitor
c. Reversible-Uncompetitive Inhibitor
In this type of inhibition,the formation of the EI complex prevents the binding of a substrate.The inhibitor compete with the substrate for the active site and therefore lowers the rate of enzyme catalysed reaction by reducing the number of active sites available to bind other substrate molecules.This can be overcome by increasing the substrate concentration until the inhibitor dissociates from the enzyme active site.They are mostly used as therapeutic agents.
These molecules binds to the active site of an enzyme and does not prevent the binding of a substrate to the same enzyme molecule.It can also bind to an already existing enzyme-substrate(ES) complex.This type of inhibition cannot be overcome by increasing the substrate concentration like reversible-competitive inhibition.Non-competitive inhibitor reduces the number of substrate molecules that can be converted to products by one enzyme molecule in 1second i.e the turnover number (Kcat).
These substances binds only to an already existing enzyme-substrate complex.Like Non-competitive inhibition,uncompetitive inhibition cannot be overcome by increasing the substrate concentration.
Berg J.,Tymoczko J. and Stryer L.(2011) Biochemistry,7th edition,New York:W.H Freeman