Reaction Mechanisms of Thiamin Diphosphate Enzymes: Redox Reactions

Amongst a wide variety of different biochemical reactions in cellular carbon metabolism, thiamin diphosphate-dependent enzymes catalyze the oxidative decarboxylation of 2-keto acids. This type of reaction typically involves redox coupled acyl transfer to CoA or phosphate and is mediated by additional cofactors, such as flavins, iron-sulfur clusters or lipoamide swinging arms, which transmit the reducing equivalents that arise during keto acid oxidation to a final electron acceptor. EPR spectroscopic and kinetic studies have implicated the intermediacy of radical cofactor intermediates in pyruvate:ferredoxin oxidoreductase and an acetyl phosphate-producing pyruvate oxidase, whereas the occurrence of transient on-pathway radicals in other enzymes is less clear. The structures of pyruvate:ferredoxin oxidoreductase and pyruvate oxidase with different enzymic reaction intermediates along the pathway including a radical intermediate were determined by cryo-crystallography and used to infer electron tunneling pathways and the potential roles of CoA and phosphate for an intimate coupling of electron and acyl group transfer. Viable mechanisms of reductive acetylation in pyruvate dehydrogenase multi-enzyme complex, and of electron transfer in the peripheral membrane enzyme pyruvate oxidase from Escherichia coli, are also discussed.