<P> In biology, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction . The active site consists of residues that form temporary bonds with the substrate (binding site) and residues that catalyse a reaction of that substrate (catalytic site). Although the active site is small relative to the whole volume of the enzyme (it only occupies 10 ~ 20% of the total volume), it is the most important part of the enzyme as it directly catalyzes the chemical reaction . It usually consists of three to four amino acids, while other amino acids within the protein are required to maintain the protein tertiary structure of the enzyme . </P> <P> Each active site is specially designed in response to their substrates, as a result, most enzymes have specificity and can only react with particular substrates . This specificity is determined by the arrangement of amino acids within the active site and the structure of the substrates . Sometimes enzymes also need to bind with some cofactors to fulfil their function . The active site is usually a groove or pocket of the enzyme which can be located in a deep tunnel within the enzyme, or between the interfaces of multimeric enzymes . An active site can catalyse a reaction repeatedly as residues are not altered at the end of the reaction (they may change during the reaction, but are regenerated by the end). This process is achieved by lowering the activation energy of the reaction, so more substrates have enough energy to undergo reaction . </P>

Where does the enzyme attach to the substrate