Flavodoxin hydroquinone provides electrons for the ATP‐dependent reactivation of protein‐bound corrinoid cofactors

Corrinoid‐dependent enzyme systems rely on the super‐reduced state of the protein‐bound corrinoid cofactor to be functional, for example, in methyl transfer reactions. Due to the low redox potential of the [Co II ]/[Co I ] couple, autoxidation of the corrinoid cofactor occurs and leads to the formation of the inactive [Co II ]‐state. For the reactivation, which is an energy‐demanding process, electrons have to be transferred from a physiological donor to the corrinoid cofactor by the help of a reductive activator protein. In this study, we identified reduced flavodoxin as electron donor for the ATP‐dependent reduction of protein‐bound corrinoid cofactors of bacterial O ‐demethylase enzyme systems. Reduced flavodoxin was generated enzymatically using pyruvate:ferredoxin/flavodoxin oxidoreductase rather than hydrogenase. Two of the four flavodoxins identified in Acetobacterium dehalogenans and Desulfitobacterium hafniense DCB‐2 were functional in supplying electrons for corrinoid reduction. They exhibited a midpoint potential of about −400 mV ( E SHE , pH 7.5) for the semiquinone/hydroquinone transition. Reduced flavodoxin could be replaced by reduced clostridial ferredoxin. It was shown that the low‐potential electrons of reduced flavodoxin are first transferred to the iron‐sulfur cluster of the reductive activator and finally to the protein‐bound corrinoid cofactor. This study further highlights the importance of reduced flavodoxin, which allows maintaining a variety of enzymatic reaction cycles by delivering low‐potential electrons.