In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. 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. The active site consists of amino acid residues that form a specific three-dimensional structure, which is uniquely suited to bind to a particular target, the enzymes substrate or substrates, and help them undergo a chemical reaction. The set of amino acids found in the active site, along with their positions in 3D space, give the active site a very specific size, shape, and chemical behavior. The active site can catalyze a reaction repeatedly as residues are not altered at the end of the reaction, and this process is achieved by lowering the activation energy of the reaction, so more substrates have enough energy to undergo reaction. Active sites can be mapped to aid the design of new drugs such as enzyme inhibitors.