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Proteins are used as catalysts for most of the chemical reactions that occur in the cell (some RNA molecules also act as catalysts). Due to their consistent three dimensional structure, proteins can form specialised local chemical environments in precise spatial arrangements on the surface which favour the conversion of substrate into product. The diversity of reactions catalysed by enzymes is enormous, from the modification of small organic molecules such as in the synthesis of amino acids, to the degradation of large proteins. Many enzymatic reactions require the exclusion of water, and so the catalytic surface, or active site, is often recessed in a cleft formed from hydrophobic side chains. Active site residues are usually well conserved in the sequences of related proteins with similar functions. Interestingly, some possibly unrelated proteins with similar reaction mechanisms show spatial conservation in three dimensional relationships of active site residues[Wallace et al., 1996].