(ii) In the second step, once the E–S complex forms, the enzyme is able to catalyse the formation of product
(P), which is then released from the enzyme surface.
Eg: Action of sucrase as shown in Fig.
Hydrogen bonding and other electrostatic interactions hold the enzyme and substrate together in the
complex. The structural features or functional groups on the enzyme that participate in these interactions
are located in a cleft or pocket on the enzyme surface. This pocket, where the enzyme combines with the
substrate and transforms the substrate to product is called the active site of the enzyme.
The substrate is a molecule upon which an enzyme acts.
The active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction.
The product is a molecule obtained from the conversion of substrate by enzyme action.
Eg: Action of sucrase on sucrose :
Sucrose binds to the active site on sucrase, and this puts stress on the bond between the 2 sugars that make
up sucrose. The bond breaks, releasing glucose and fructose.
There are two view points by which enzymes are supposed to bring about chemical reaction.
i. Lock and Key Hypothesis:
- It was put forward by Emil Fischer in 1894. According to this hypothesis, both enzyme and substrate
molecules have specific geometrical shapes. - The contact is such that the substrate molecules or reactants come together causing the chemical change.
It is similar to the system or lock and key. Just as a lock can be opened by its specific key, a substrate
molecule can be acted upon by a particular enzyme. This also explains the specificity of enzyme action.
Features:
(1) Structure or conformation of the enzyme is rigid.
(2) Active site is rigid and pre-shaped template.
(a) Because the substrate and the active site of the enzyme have complementary
structures and bonding groups, they fit together as a key fits a lock.