X-ray crystallographic snapshots of reaction intermediates in pyruvate oxidase and transketolase illustrate common themes in thiamin catalysis

Thiamin diphosphate (ThDP)-dependent enzymes play pivotal roles in intermediary metabolism of virtually all organisms. Although extensive mechanistic work on cofactor models and various enzymes has served as a guide to understand general principles of catalysis, high-resolution structural information of reaction intermediates along the catalytic pathway was scarcely available until recently. Here, we review cryocrystallographic studies on the prototypical ThDP enzymes pyruvate oxidase and transketolase, which provided exciting insights into the chemical nature and structural features of several key intermediates and into the stereochemical course of substrate processing. The structures revealed a conserved (S)-configuration at the C2alpha stereocenter of the initially formed tetrahedral intermediate in the different enzymes with the scissile C2alpha–C2beta bond being directed perpendicular to the aromatic ring plane of the thiazolium portion of ThDP confirming the proposed maximum overlap mechanism. Elimination of the respective leaving groups (carbon dioxide, sugar phosphates) appears to be driven – amongst other factors such as stereoelectronic control – by strain relief as the C2–C2alpha bond, which connects C2 of ThDP with the carbonyl of the substrate, substantially deviates from planarity and relaxes to an in-plane conformation only after bond fission to give an enamine-type intermediate with considerable delocalization of the free electron pair onto the thiazolium ring. Except for the apparent flexibility of the cofactor itself, no major structural rearrangements are detectable indicating that the enzyme active centers are poised for catalysis. The structures also provide the basis for understanding the origins of substrate and reaction specificity.