Chapter 19, Problem 19.35QP

Interpretation Introduction

Interpretation:

The term rate-limiting step has to be defined.

Concept Introduction:

In an enzyme-catalyzed reaction first the enzyme and substrate approaches, form an enzyme-substrate complex that undergoes some transformation to form enzyme product and finally the enzyme is removed from the substrate.

The key steps of an enzyme-catalyzed reaction can be illustrated as follows:

  $\begin{array}{l}\text{E}+\text{S }\stackrel{\text{Step }1}{\rightleftarrows }\text{ ES}\\ \left(\text{Enzyme-substrate}\right)\end{array}$

This enzyme-substrate thus formed undergoes a transformation to form a transition state as follows:

  $\begin{array}{l}\text{ ES }\stackrel{\text{Step 2}}{\rightleftarrows }{\text{ ES}}^{*}\\ \left[\text{Enzyme-substrate}\right]\text{ Transition state}\end{array}$

${\text{ES}}^{*}$ has characteristics of both the enzyme and the substrate molecule. This can be considered as the intermediate form of the enzyme-substrate complex that enables that transformation of substrate to the desired product as indicated in step 3 below:

  $\begin{array}{l}{\text{ ES}}^{*}\stackrel{\text{Step 3}}{\rightleftarrows }\text{ EP}\\ \text{Transition state }\left[\text{Enzyme-product}\right]\end{array}$

Finally, the enzyme-product complex decomposes to give product and the enzyme is liberated as illustrated below:

  $\text{EP}\stackrel{\text{Step 4}}{\rightleftarrows }\text{E}+\text{P}$