Glucose dehydrogenase, action mechanism

Figure 4.9. Structural evidence favoring a hydride-transfer mechanism for the action of the PQQ cofactor in the soluble glucose dehydrogenase of Acinetobacter calcoaceticus (structure and concept of Oubrie et al. [71] ...

The C-2-exchange of azolium salts via an ylide mechanism has already been discussed (section 21.1.2.1). Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and in these, its mode of action also depends on the intermediacy of a 2-deprotonated species. For example, in the later stages of alcoholic fermentation, which converts glucose into ethanol and carbon dioxide, the enzyme pyruvate decarboxylase converts pyruvate into ethanal and carbon dioxide, the former then being converted into ethanol by the enzyme, alcohol dehydrogenase. It is believed, that in the operation of the former enzyme, the coenzyme, thiamin pyrophosphate, adds as its ylide to the ketonic carbonyl group of pyruvate this is followed by loss of carbon dioxide then the release of ethanal by expulsion of the original ylide. 

The mechanism of action of the coenzymes was discovered by Warburg and his co-workers in 1935. They studied the oxidation of glucose-6-phosphate in yeast extracts. This oxidation is caused by the combination of two enzymes, glucose-6-phosphate dehydrogenase (Zwischenferment) and the old yellow enzyme. 

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