Electron bifurcation is considered as a third fundamental mode of energy conservation mechanism, in which endergonic and exergonic redox reactions are coupled. The newly discovered flavin based electron bifurcation in Electron transfer flavoproteins (ETFs) helps to reduce low potential ferredoxin, which provides electrons to drive biologically demanding reactions such as atmospheric dinitrogen fixation in diazotroph and methane production in methanogens. Current research demonstrates the capacity for electron bifurcation in the Rhodopseudomonas palustris ETF (RpalETF) system. RpalETF contains two chemically identical but functionally different FADs: ET-FAD is bound in highly mobile domain II, which sits in a stable base created by domains I and III. Bf-FAD is buried in between domain I and III. The two flavins execute contrasting, complementary electron transfer reactions. Whereas one mediates single electron transfer (ET-FAD), the other accepts electrons pairwise (Bf-FAD), yet both flavins’ sites include a conserved Arg sidechain. R273 favors the ASQ of ET-FAD, whereas R165 near the Bf-FAD appears not to, possibly due to neutralization of its positive charge by nearby C174. R273 forms a pi- pi stacking interaction with ET-FAD whereas R165 appears to form hydrogen bond interactions with Bf-FAD. To learn whether the active site arginine residues each have different effects on their respective neighboring flavins, we replaced each of the Args in turn with chemically conservative, and divergent substitutions. Our data shows, R273 plays a vital role in BfETF by stabilizing the ASQ of the ET-FAD, whereas R165 favors binding of the Bf-FAD that is essential for electron bifurcation in RpalETF. Along with the electron bifurcation studies, we report an irreversible, pH dependent, site selective, enzyme mediated, anaerobic chemical modification of ET-FAD to a pink amino FAD, which opens a new perspective with which to understand the 726 nm band formed in bifurcating ETF.
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