The substrate .S. has to bind the enzyme of its active site with in a given cleft. The substrate has to diffuse towards the active site leads to the formation of ES complex. This is called transition state structure. Very Soon, after the expected bond breaking/making is completed, the product is released from the active site. In other words, the structure of substrate gets transformed into the structure of products.
When we represent this pictorially through a graph by taking potential energy on y is and Activation energy progress of reaction on x-axis, we notice the energy level difference between .S. and .P.. If .P. is at lower level than S, it is an exothermic reaction. One need not supply energy to form the product. However, whether it is an exothermic or an endothermic reaction (Energy requiring reaction) the .s. has to go through a much higher energy state or transition state. The difference between average energy content of .S. and that of this transition state is called Concept of activation energy activation energy.
Each enzyme [E] has a substrate [S] binding site in its molecule so that a highly reactive enzyme substrate complex (ES) is produced. This complex is short lived and dissociates into its products [P] and the unchanged enzyme with an intermediate formation of the enzyme product complex [EP].
`E +S to ES to EP to E+P`
Formation of ES complex has been explained with lock and key hypothesis by Fisher and much later with induced fit hypothesis by Daniel E. Koshland.
The catalytic cycle of an enzyme action is
1) The substrate binds to the active site of the enzyme.
2) The binding of the substrate induces the enzyme to alter its shape, fitting more tightly around the substrate.
3). The active site of the enzyme breaks the chemical bonds of the substrate and the new enzyme product complex is formed.
4). The enzyme relases the products of the reaction and the free enzymes is ready to bind to another molecule of the substrate.
