Modulations of the reduction potentials of flavin-based electron bifurcation complexes and semiquinone stabilities are key to control directional electron flow

Abstract

The flavin-based electron bifurcation (FBEB) system from Acidaminococcus fermentans is composed of the electron transfer flavoprotein (EtfAB) and butyryl-CoA dehydrogenase (Bcd). alpha-FAD binds to domain II of the A-subunit of EtfAB, beta-FAD to the B-subunit of EtfAB and delta-FAD to Bcd. NADH reduces beta-FAD to beta-FADH(-), which bifurcates one electron to the high potential alpha-FAD(center dot-) semiquinone followed by the other to the low potential ferredoxin (Fd). As deduced from crystal structures, upon interaction of EtfAB with Bcd, the formed alpha-FADH(-) approaches delta-FAD by rotation of domain II, yielding delta-FAD(center dot-). Repetition of this process leads to a second reduced ferredoxin (Fd(-)) and delta-FADH(-), which reduces crotonyl-CoA to butyryl-CoA. In this study, we measured the redox properties of the components EtfAB, EtfaB (Etf without alpha-FAD), Bcd, and Fd, as well as of the complexes EtfaB:Bcd, EtfAB:Bcd, EtfaB:Fd, and EftAB:Fd. In agreement with the structural studies, we have shown for the first time that the interaction of EtfAB with Bcd drastically decreases the midpoint reduction potential of alpha-FAD to be within the same range of that of beta-FAD and to destabilize the semiquinone of alpha-FAD. This finding clearly explains that these interactions facilitate the passing of electrons from beta-FADH(-) via alpha-FAD(center dot-) to the final electron acceptor delta-FAD(center dot-) on Bcd. The interactions modulate the semiquinone stability of delta-FAD in an opposite way by having a greater semiquinone stability than in free Bcd.