<P> Usually, an enzyme molecule has only two active sites, and the active sites fit 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 non-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 . </P> <P> There are two proposed models of how enzymes fit to their specific substrate: the lock and key model and the induced fit model . </P> <P> 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 . </P> <P> Daniel Koshland's theory of enzyme - substrate binding is that the active site and the binding portion of the substrate are not exactly complementary . The induced fit model is a development of the lock - and - key model and assumes that an active site is flexible and it changes shape until the substrate is completely bound . The substrate is thought to induce a change in the shape of the active site . The hypothesis also predicts that the presence of certain residues (amino acids) in the active site will encourage the enzyme to locate the correct substrate . Conformational changes may then occur as the substrate is bound . After the products of the reaction move away from the enzyme, the active site returns to its initial shape . </P>

The active site is a large part of an enzyme