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The active site of an enzyme is the binding site where catalysis occurs. The structure and chemical properties of the active site allow the recognition and binding of the substrate.
The active site is usually a small pocket at the surface of the enzyme that contains residues responsible for the susbtrate 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 Pyridoxal, Thiamine or NAD.
The active site is often the site of inhibition of enzymes (see Enzyme inhibitor article).
Models
There are several models of how enzymes work: the lock-and-key model and the induced fit model. Substrates bind to the active site of the enzyme or a specificity pocket by hydrogen bonds, hydrophobic interactions, temporary covalent bond or a combination. Residues of the active site will act as donors or acceptors 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 regenerate the enzyme to its initial state.
"Active site"