● We have already understood the idea of an `color{brown}("‘active site’.")`

● The chemical or `color{violet}("metabolic conversion")` refers to a reaction.

● The chemical which is converted into a product is called a `color{brown}("‘substrate’.")`

● Hence `color{violet}("enzymes,")` i.e. proteins with `color{violet}("three dimensional structures")` including an `color{violet}("‘active site’, convert")` a substrate (S) into a product (P).

● Symbolically, this can be depicted as: `color{brown}("S →P")`

● It is now understood that the substrate `‘S’` has to bind the enzyme at its `color{violet}("‘active site’")` within a given `color{brown}("cleft or pocket. ")`

● The substrate has to `color{brown}("diffuse")` towards the `color{violet}("‘active site’.")`

● There is thus, an obligatory formation of an `color{brown}("‘ES’ complex. ")`

● `color{violet}("E stands")` for enzyme.

● This complex formation is a `color{violet}("transient phenomenon. ")`

● During the state where substrate is bound to the `color{violet}("enzyme active site,")` a new structure of the substrate called `color{brown}("transition state structure")` is formed.

● Very soon, after the expected `color{brown}("bond breaking/making")` is completed, the product is released from the
`color{violet}("active site.")`

● In other words, the `color{violet}("structure of substrate")` gets transformed into the structure of `color{violet}("product(s).")`

● The `color{violet}("pathway of this transformation")` must go through the so-called `color{brown}("transition state structure.")`

● There could be many more `color{brown}("‘altered structural states’")` between the stable substrate and the product.

● Implicit in this statement is the fact that all other `color{violet}("intermediate structural states")` are unstable.

● `color{brown}("Stability")` is something related to energy status of the `color{violet}("molecule or the structure.")`

● Hence, when we look at this `color{violet}("pictorially")` through a graph it looks like something as in the Figure.

● The y-axis represents the `color{brown}("potential energy")` content.

● The x-axis represents the progression of the `color{brown}("structural transformation")` or states through the `color{violet}("‘transition state’.")`

● You would notice two things.

● The `color{brown}("energy level difference")` between `S` and `P`.

● If `‘P’` is at a lower level than `‘S’,` the reaction is an `color{brown}("exothermic")` reaction.

● One need not `color{violet}("supply energy (by heating)")` in order to form the product.

● However, whether it is an `color{violet}("exothermic or spontaneous reaction")` or an `color{violet}("endothermic or energy requiring reaction")` , the `‘S’` has to go through a much higher energy state or `color{violet}("transition state.")`

● The difference in `color{violet}("average energy")` content of `‘S’` from that of this `color{violet}("transition state")` is called `color{brown}("‘activation energy’.")`

● `color{violet}("Enzymes eventually bring")` down this energy barrier making the `color{violet}("transition")` of `‘S’` to `‘P’` more easy.