Radical reactions of thiamin pyrophosphate in 2-oxoacid oxidoreductases

Thiamin pyrophosphate (TPP) is essential in carbohydrate metabolism in all forms of life. TPP-dependent decarboxylation reactions of 2-oxo-acid substrates result in enamine adducts between the thiazolium moiety of the coenzyme and decarboxylated substrate. These central enamine intermediates experience different fates from protonation in pyruvate decarboxylase to oxidation by the 2-oxoacid dehydrogenase complexes, the pyruvate oxidases, and 2-oxoacid oxidoreductases. Virtually all of the TPP-dependent enzymes, including pyruvate decarboxylase, can be assayed by 1-electron redox reactions linked to ferricyanide. Oxidation of the enamines is thought to occur via a 2-electron process in the 2-oxoacid dehydrogenase complexes, wherein acyl group transfer is associated with reduction of the disulfide of the lipoamide moiety. However, discrete 1-electron steps occur in the oxidoreductases, where one or more [4Fe-4S] clusters mediate the electron transfer reactions to external electron acceptors. These radical intermediates can be detected in the absence of the acyl-group acceptor, coenzyme A (CoASH). The π-electron system of the thiazolium ring stabilizes the radical. The extensively delocalized character of the radical is evidenced by quantitative analysis of nuclear hyperfine splitting tensors as detected by electron paramagnetic resonance (EPR) spectroscopy and by electronic structure calculations. The second electron transfer step is markedly accelerated by the presence of CoASH. While details of the second electron transfer step and its facilitation by CoASH remain elusive, expected redox properties of potential intermediates limit possible scenarios. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.