To understand this we should study enzymes a little more. We have already understood the idea of an ‘active site’. The chemical or metabolic conversion refers to a reaction. The chemical which is converted into a product is called a ‘substrate’.
Hence enzymes, i.e. proteins with three dimensional structures including an ‘active site’, convert a substrate (S) into a product (P). Symbolically, this can be depicted as: S → P It is now understood that the substrate ‘S’ has to bind the enzyme at its ‘active site’ within a given cleft or pocket.
The substrate has to diffuse towards the ‘active site’. There is thus, an obligatory formation of an ‘ES’ complex. E stands for enzyme. This complex formation is a transient phenomenon. During the state where substrate is bound to the enzyme active site, a new structure of the substrate called transition state structure is formed. 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 product(s). The pathway of this transformation must go through the so-called transition state structure. There could be many more ‘altered structural states’ between the stable substrate and the product. Implicit in this statement is the fact that all other intermediate structural states are unstable. Stability is something related to energy status of the molecule or the structure.
Hence, when we look at this pictorially through a graph it looks like something as in Figure 9.6. The y-axis represents the potential energy content. The x-axis represents the progression of the structural transformation or states through the ‘transition state’. You would notice two things. The energy level difference between S and P. If ‘P’ is at a lower level than ‘S’, the reaction is an exothermic reaction. One need not supply energy (by heating) in order to form the product.
However, whether it is an exothermic or spontaneous reaction or an endothermic or energy requiring reaction, the ‘S’ has to go through a much higher energy state or transition state. The difference in average energy content of ‘S’ from that of this transition state is called ‘activation energy’. Enzymes eventually bring down this energy barrier making the transition of ‘S’ to ‘P’ more easy.