Rapid kinetics reveal surprising flavin chemistry in the bifurcating electron transfer flavoprotein from Acidaminococcus fermentans.

Electron bifurcation uses free energy from exergonic redox reactions to power endergonic reactions. β-FAD of the electron transfer flavoprotein (EtfAB) from the anaerobic bacterium Acidaminococcus fermentans bifurcates the electrons of NADH, sending one to the low potential ferredoxin and the other to the high potential α-FAD semiquinone (α-FAD·-). The resultant α-FAD hydroquinone (α-FADH-) transfers one electron further to butyryl-CoA dehydrogenase (Bcd); two such transfers enable Bcd to reduce crotonyl-CoA to butyryl-CoA. To get insight into the mechanism of these intricate reactions, we constructed an artificial reaction only with EtfAB containing α-FAD or α-FAD·- to monitor formation of α-FAD·- or α-FADH-, respectively, using stopped flow kinetic measurements. In the presence of α-FAD, we observed that NADH transferred a hydride to β-FAD at a rate of 920 s-1, yielding the charge transfer complex NAD+:β-FADH- with an absorbance maximum at 650 nm. β-FADH- bifurcated one electron to α-FAD and the other electron to α-FAD of a second EtfAB molecule, forming two stable α-FAD·-. With α-FAD·-, the reduction of b-FAD with NADH was 1500-times slower. Reduction of β-FAD in the presence of α-FAD displayed a normal kinetic isotope effect (KIE) of 2.1, whereas the KIE was inverted in the presence of α-FAD·-. These data indicate that a nearby radical (14 Å apart) slows the rate of a hydride transfer and inverts the KIE. This unanticipated flavin chemistry is not restricted to Etf-Bcd but certainly occurs in other bifurcating Etfs found in anaerobic bacteria and archaea.