The enzymes are acting as catalysts due to the presence of certain specific regions on their surface, called active sites or catalytic sites. Two models have been proposed for enzyme action.
Lock and Key model
The active site of a given enzyme is so shaped that only its specific substrates fit into it. We can compare substrate or reactant molecule to the key and active site to the lock.
Induced-fit model
Modern X-ray crystallographic and spectroscopic methods show that in many cases the enzyme changes shape when the substrate lands at the active site. This induced fit model of enzyme action pictures the substrate inducing the active site to adopt a perfect fit, rather than a rigidly shaped lock and key.
We can explain the mechanism of enzyme action using the transition state theory or the intermediate compound formation theory. It involves the following steps.
1) The reactant molecules bind to a region on the surface of the enzyme called active site which results in the formation of an enzyme-substrate complex.
E + S ------> [ES]
2) The enzyme functions by lowering the activation energy of a particular reaction and forms enzyme product complex. [ES] ------> EP
3) The products are released from the enzyme-product complex and the enzyme is then free to bind a fresh molecule of substrate. EP ------> E + P
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