Catalytic domain

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). 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 its residues are not altered at the end of the reaction (they may change during the reaction, but are regenerated by the end).

Usually, an enzyme molecule has only one active site, and the active site fits with one specific type of substrate. An active site contains a binding site that binds the substrate and orients it for catalysis. Residues in the binding site form hydrogen bonds, hydrophobic interactions, or temporary covalent interactions (van der Waals) with the substrate to make an enzyme-substrate complex. In order to function, the active site needs to be in a specific conformation and so denaturation of the protein by high temperatures or extreme pH values will destroy its catalytic activity. A tighter fit between an active site and the substrate molecule is believed to increase efficiency of a reaction. Most enzymes have deeply buried active sites, which can be accessed by a substrate via access channels.

There are two proposed models of how enzymes fit to their specific substrate: the lock and key model and the induced fit model.

Emil Fischer's 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 occurs.

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