Rapid kinetics reveal surprising flavin chemistry in 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 β-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.