8] Enzymology of enol intermediates

Abstract

Publisher Summary This chapter discusses reactions most commonly classified as stepwise processes with enol intermediates. The term “enol” is not meant to exclude the enolate as the functioning species except when the question is specifically addressed. There are many examples of multifunctional enzymes that are thought to use enols as intermediates: ribulose-1,5-P 2 carboxylase, glyoxalase, oxidativeβ-ketodecarboxylases, ketosteroid isomerase, and thymidylate synthetase. Examples of enzymes that use imine intermediates are known for all the reactions shown above except those that specifically call for an oxygen atom, such as the usual aldose-ketose isomerases, dehydration, and enol-kinase. Enols that are produced catalytically as normal products of enzymes may be used as a source to test as an intermediate of another enzyme. A more general approach, also biosynthetic, is to use the bound intermediate of an enzymic reaction as a source of the free form to be tested with another enzyme. This approach is promising when the source enzyme is available in adequate amount and requires that the intermediate be enriched on the enzyme above its expected unfavorable concentration in solution equilibrium. The enols that have been mentioned already, hydroxy maleate, p-OH-phenylenolpyruvate, and enolpyruvate, have sufficient stability to survive the operations of denaturation and neutralization, which at the fastest would require 5 msec.