Electron transfer in acetohydroxy acid synthase as a side reaction of catalysis. Implications for the reactivity and partitioning of the carbanion/enamine form of (α-hydroxyethyl)thiamin diphosphate in a "nonredox" flavoenzyme

Kai Tittmann, Kathrin Schröder, Ralph Golbik, Jennifer McCourt, Alexander Kaplun, Ronald G. Duggleby, Ze'Ev Barak, David M. Chipman, Gerhard Hübner

Research output: Contribution to journalArticlepeer-review

38 Scopus citations

Abstract

Acetohydroxy acid synthases (AHAS) are thiamin diphosphate- (ThDP-) and FAD-dependent enzymes that catalyze the first common step of branched-chain amino acid biosynthesis in plants, bacteria, and fungi. Although the flavin cofactor is not chemically involved in the physiological reaction of AHAS, it has been shown to be essential for the structural integrity and activity of the enzyme. Here, we report that the enzyme-bound FAD in AHAS is reduced in the course of catalysis in a side reaction. The reduction of the enzyme-bound flavin during turnover of different substrates under aerobic and anaerobic conditions was characterized by stopped-flow kinetics using the intrinsic FAD absorbance. Reduction of enzyme-bound FAD proceeds with a net rate constant of k′ = 0.2 s-1 in the presence of oxygen and approximately 1 s-1 under anaerobic conditions. No transient flavin radicals are detectable during the reduction process while time-resolved absorbance spectra are recorded. Reconstitution of the binary enzyme-FAD complex with the chemically synthesized intermediate 2-(hydroxyethyl)-ThDP also results in a reduction of the flavin. These data provide evidence for the first time that the key catalytic intermediate 2-(hydroxyethyl)-ThDP in the carbanionic/enamine form is not only subject to covalent addition of 2-keto acids and an oxygenase side reaction but also transfers electrons to the adjacent FAD in an intramolecular redox reaction yielding 2-acetyl-ThDP and reduced FAD. The detection of the electron transfer supports the idea of a common ancestor of acetohydroxy acid synthase and pyruvate oxidase, a homologous ThDP- and FAD-dependent enzyme that, in contrast to AHASs, catalyzes a reaction that relies on intercofactor electron transfer.

Original languageEnglish
Pages (from-to)8652-8661
Number of pages10
JournalBiochemistry
Volume43
Issue number27
DOIs
StatePublished - 13 Jul 2004

Fingerprint

Dive into the research topics of 'Electron transfer in acetohydroxy acid synthase as a side reaction of catalysis. Implications for the reactivity and partitioning of the carbanion/enamine form of (α-hydroxyethyl)thiamin diphosphate in a "nonredox" flavoenzyme'. Together they form a unique fingerprint.

Cite this