Special Topic Enzymes and Reaction Rates

It is a simple matter to turn this all around and examine the other side of the Hammond postulate, The more exothermic a reaction, the more the transition state resembles starting material. Rgure 8.34 will do it for you if you simply read it backward, from right to left. See what happens to the transition state as the right-to-left reaction becomes less and less exothermic. 

The take-home lesson of all this discussion is that although one cannot take the correspondence between thermodynamics and kinetics as a given—there will be counterexamples—in general, the statement Y is more stable and thus formed faster will be true. There is more to it than it appears at first. This statement is tricky indeed, and it is worth stopping and examining it every time we make it. 

Nature can t overcome activation barriers to reactions important to us by adding catalysts such as sulfuric acid, and turning up the heat in our bodies to provide energy becomes uncomfortable very quickly indeed. So, Nature does it another way, and it is most successful. Nature uses enzymes, which are polyamino acid chains with catalytic activity, to lower the barriers to chemical transformations. Aconitase, an enzyme of molecular weight 89,000 ( ), catalyzes the conversion of citrate into cis-aconitate, an elimination reaction. Aconitate is then hydrated to give isocitrate. So, this enzyme makes possible one of the key reactions of Chapter 7, elimination, and then reverses the process, but putting the OH on the other alkene carbon, to give the isocitrate (Fig. 8.35). 

The vast molecular architecture of the enormous molecule aconitase serves to bind the substrates precisely, holding them in perfect position for the side chains of the amino acids to act as catalysts, speeding the elimination and addition reactions. Spectacular rate accelerations can be achieved by enzymes, allowing all sorts of reactions to take place at body temperature, 37 °C, that could not occur otherwise. 

Sometimes, our well-being depends on interfering with an enzyme-mediated rate acceleration. Cholesterol (p. 121) is formed in the body by a lengthy process in which the thioester A is reduced to mevalonic add, which is then converted in a series of reactions into cholesterol. 

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