Thiamine Pyrophosphate-Catalyzed Enzymatic Decarboxylation of α-Oxo Acids

This chapter focuses on thiamine pyrophosphate-catalyzed enzymatic decarboxylation of α-oxo acids. The first enzymatic decarboxylation of an α-oxo acid was described as: a fermenting extract from brewer's yeast cleaves pyruvate into acetaldehyde and carbon dioxide. Later, it was found that besides the protein of the enzyme, a thermostable cofactor (“cocarboxylase”) is necessary for the enzymatic cleavage of pyruvate. For decarboxylation of other α-oxo acids to the corresponding aldehydes, for their oxidative decarboxylation leading to acyl coenzyme A derivatives, and for a number of other enzymatic reactions, the same coenzyme was found to be necessary. By comparison of thiamine catalysis with that of cyanide in the acyloin condensation, Breslow came to the conclusions that thiamine forms a 2-carbanion with a pK comparable to that of HCN; the upper limit found for the pK was 20. The exclusively kinetic measurements could not account for activation energy, and therefore the pK of thiamine is expected to be several units lower. In this chapter, mechanism of nonenzymatic thiamine (Thiazolium) catalysis are analyzed in detail. Isolation of 2-α-hydroxyalkyl derivatives of thiamine pyrophosphate from enzymatic incubation mixtures is described. Effects of substituents at the thiazolium ring are also discussed.