Biology WORKING OF ENZYMES

### KEY TOPICS

star Transformation of substrate to product
star Transition State

### TRANSFORMATION OF SUBSTRATE TO PRODUCT

● 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).")

### TRANSITION STATE

● 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.