Unexpected tautomeric equilibria of the carbanion-enamine intermediate in pyruvate oxidase highlight unrecognized chemical versatility of thiamin.

ABSTRACT: Thiamin diphosphate, the vitamin B1 coenzyme, plays critical roles in fundamental metabolic pathways that require acyl carbanion equivalents. Studies on chemical models and enzymes had suggested that these carbanions are resonance-stabilized as enamines. A crystal structure of this intermediate in pyruvate oxidase at 1.1 Å resolution now challenges this paradigm by revealing that the enamine does not accumulate. Instead, the intermediate samples between the ketone and the carbanion both interlocked in a tautomeric equilibrium. Formation of the keto tautomer is associated with a loss of aromaticity of the cofactor. The alternate confinement of electrons to neighboring atoms rather than π-conjugation seems to be of importance for the enzyme-catalyzed, redox-coupled acyl transfer to phosphate

SUBMITTER: Meyer D 

PROVIDER: S-EPMC3390845 | biostudies-literature | 2012 Jul

REPOSITORIES: biostudies-literature