Examples of non-competitive inhibitors



  1. Chymotrypsin has a catalytic site which includes the amino acids Histidine57, Serine195 and Asparagine102 (the numbers refer to their positioning on the primary chain). These amino acids allow the substrate molecule to be cleaved by forming a charged relay network whereby hydrogens are passed from one amino acid to another. Increasing the hydrogen ion concentration (i.e. by lowering the pH) causes inhibition of the enzyme as the amino acids in the active site can no longer accept a hydrogen from the substrate because Asp102 becomes protonated (hydrogens added) at a low pH. Thus hydrogen ions have behaved as non-competitive inhibitors by preventing catalysis but they do not prevent the substrate from binding to the active site.
  2. Poisons like cyanide, heavy metal ions and some insecticides are all non-competitive inhibitors.
  3. The membrane-bound Na+/K+ ATPase is inhibited by oubain, a toxic glycoside (esters containing a sugar component (glycol) and a nonsugar (aglycone) component attached via an oxygen or a nitrogen bond; hydrolysis of a glycoside yields one or more sugars).
  4. Lead forms covalent bonds with the sulphydryl side chains of cysteine in proteins. The binding of the heavy metal shows non-competitive inhibition because the substrate still has access. For example, lead inhibits the enzyme Ferrochelatase which catalyses the insertion of Fe2+ into protoporphyrin.