Enzyme Inhibitors

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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];

  1. Irreversible Inhibitors
  2. Reversible Inhibitors


Irreversible 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.

Reversible Inhibitors

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;

Reversible-competitive 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, e.g. If our body has an inflammatory response to a certain stimuli, then Ibroprofen can be used as it is a competitive inhibitor of the enzymes involved in the signaling pathways of the inflammatory response. Competitive Inhibitors cause KM to increase and Vmax to remain the same.

Reversible-Non-competitive Inhibitor

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 1 second i.e the turnover number (Kcat). Non-competitive inhibitors have no effect on KM but Vmax is lowered.


These substances binds only to an already existing enzyme-substrate complex. Once bound to the enzyme-substrate complex the enzyme-substrate-inhibitor complex, ESI complex, will take a very long time to produce any product. Uncompetitive inhibition essentially decreases the concentration of functional enzymes.Like Non-competitive inhibition,uncompetitive inhibition cannot be overcome by increasing the substrate concentration. Uncompetitive inhibitors decrease both KM and Vmax.


  1. Berg J.,Tymoczko J. and Stryer L.(2011) Biochemistry,7th edition,New York:W.H Freeman
 Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: W.H. Freeman and Company, pg227.

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