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The active site is usually a big pocket or cleft surrounded by amino acid- and other side chains at the surface of the enzyme that contains residues responsible for the substrate specificity (charge, hydrophobicity, steric hindrance) and catalytic residues which often act as proton donors or acceptors or are responsible for binding a cofactor such as PLP, TPP or NAD. The active site is also the site of inhibition of enzymes (see Enzyme inhibitor article). The active site of an enzyme contains the catalytic and binding sites. The structure and chemical properties of the active site allow the recognition and binding of the substrate.



There are two proposed models of how enzymes work: the lock-and-key model and the induced fit model. The lock-and-key model assumes that the active site is a perfect fit for a specific substrate and that once the substrate binds to the enzyme no further modification is necessary; this is simplistic. The induced fit model is a development of the lock-and-key model and instead assumes that an active site is more flexible and that the presence of certain residues (amino acids) in the active site will encourage the enzyme to locate the correct substrate, after which conformational changes may occur as the substrate is bound.

Substrates bind to the active site of the enzyme or a specificity pocket through hydrogen bonds, hydrophobic interactions, temporary covalent interactions (van der Waals) or a combination of all of these to form the enzyme-substrate complex. Residues of the active site will act as donors or acceptors of protons or other groups on the substrate to facilitate the reaction. In other words, the active site modifies the reaction mechanism in order to decrease the activation energy of the reaction. The product is usually unstable in the active site due to steric hindrances that force it to be released and return the enzyme to its initial unbound state.

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