Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Types of Enzyme-Catalyzed Reactions
Enolic Intermediates in Isomerization Reactions
Internal Oxidation-Reduction Reactions Proceeding via Dehydration to Enol or Enol Phosphate

When a carboxylic acid contains hydroxyl groups at both the а- and β-positions, dehydration leads to The formation of an enol, which can then undergo tautomerization to yield the 2-keto-3-deoxy derivative of the original acid:

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Thus, phosphogluconate dehydratase catalyzes the formation of 2-keto-3-deoxyphosphogluconate1). This reaction initiates a unique pathway of sugar Catabolism (the Entner–Doudoroff pathway; Chapter 9, Section D,4) in certain organisms. 6-phosphogluconate is formed via The oxidation of the aldehyde group of glucose-6-phosphate. This metabolic route, involving the oxidation of a sugar to an aldonic acid followed by dehydration according to equation (7-59), is widespread in METABOLISM. A related reaction is the enolase-catalyzed dehydration of 2-phosphoglyceric acid [equation (7-46)]. The reaction product is phosphoenolpyruvic acid, which is the stabilized form of the enolic intermediate shown in equation (7-59).

1) When the reaction is carried out in 2H2O, the incorporated 2H is found to have a random configuration at the C-3 atom. This indicates that the enzyme catalyzes only the dehydration step, whereas the tautomeric Conversion of the enol to the ketone is a non-enzymatic process.

Methylglyoxal synthase catalyzes a more complex internal oxidation-reduction reaction [140]:

The enzyme does not act on free glyceraldehyde-3-phosphate; rather, it is enzyme-bound glyceraldehyde-3-phosphate [shown in parentheses in equation (7-60)] that undergoes enzymatic Cleavage to form the enol. The intermediately formed enol then rearranges into methylglyoxal.



Last update: 06/08/2026

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