Some molecules are capable of lowering the activity of enzymes by binding with them; such molecules are known as Enzyme Inhibitors. Inhibitors can be used as poisons as well as medicines. Two major types of inhibitors are Competitive and Non-Competitive.
In competitive inhibition the inhibitor and the substrate compete for placement in the active site of the enzyme. These inhibitors are said to have the same shape to that of the substrate and prevents the enzyme/substrate complex from forming. This is done when the inhibitor enters the active site thus preventing the substrate from entering. Therefore the rate of reaction is decreased due to less substrate molecules binding to the active site. This type of inhibition can be overcome by increasing the substrate concentration.
In this form of inhibition the inhibitor does not compete with substrate for a place in the active site but however does reduces enzyme activity by binding to another site on the enzyme known as the Allosteric site. Unlike the competitive inhibition, non-competitive inhibition prevents the formation of products from substrates (enzyme/product complex). Another thing to note about these inhibitors is that they are unaffected by substrate concentration thus most of them are permanent.
Examples of inhibition:
The inhibitor poison malonate can prevent respiration by binding to the enzyme Succinate Dehydrogenase thus preventing succinate from entering the active site by competing with it.