<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> <P> Once the substrate is bound and oriented in the active site, catalysis can begin . The residues of the catalytic site are typically very close to the binding site, and some residues can have dual - roles in both binding and catalysis . </P> <P> Catalytic residues of the site interact with the substrate to lower the activation energy of a reaction and so make it proceed faster . They do this by a number of different mechanisms . Firstly, they can act as donors or acceptors of protons or other groups on the substrate to facilitate the reaction . They can also form electrostatic interactions to stabilise charge buildup on the transition state or leaving group . They can also directly take part in covalent catalysis, forming an acyl - enzyme intermediate that is then resolved to regenerate the catalytic residue and release the product . In this way, the catalytic residues of the active site provide an alternative reaction mechanism with lower activation energy . </P> <P> Enzymes can use cofactors as' helper molecules' . Coenzymes are one example of cofactors . Coenzymes bind to the enzyme temporarily and are released after the reaction has occurred . Metal ions are another type of cofactor . </P>

The active site of an enzyme can only consist of protein