It is a molecule that binds to an enzyme but not directly to the active site of such enzyme. This other site where it binds is called an allosteric site. Such molecules are inhibitors in that where they bind with the enzyme there is a shift in the dimensional structure of the enzyme that prevents substrate from binding to the active site. These components or molecules are also called allosteric group. The concentration of the substrate has no effect on the rate of reaction since the inhibitor is not competing for an active site of the enzyme. Examples of this type of inhibitors are: Arsenic and Mercury which combine permanently with sulphohydril (SH) groups on either the active site or elsewhere thereby changing the structure of the enzyme thus making it unable to act as a catalyst again.