Molecular basis of the flavin‐based electron‐bifurcating caffeyl‐CoA reductase reaction.

Flavin‐based electron bifurcation (FBEB) is a recently discovered mode of energy coupling in anaerobic microorganisms. The electron‐bifurcating caffeyl‐CoA reductase (CarCDE) catalyzes the reduction of caffeyl‐CoA and ferredoxin by oxidizing NADH. The 3.5 Å structure of the heterododecameric Car(CDE)₄ complex of <italic>Acetobacterium woodii</italic>, presented here, reveals compared to other electron‐transferring flavoprotein/acyl dehydrogenase family members an additional ferredoxin‐like domain with two [4Fe–4S] clusters N‐terminally fused to CarE. It might serve, <italic>in vivo</italic>, as specific adaptor for the physiological electron acceptor. Kinetic analysis of a CarCDE(∆Fd) complex indicates the bypassing of the ferredoxin‐like domain by artificial electron acceptors. Site‐directed mutagenesis studies substantiated the crucial role of the C‐terminal arm of CarD and of ArgE203, hydrogen‐bonded to the bifurcating FAD, for FBEB. [ABSTRACT FROM AUTHOR]