Flavin redox tuning and ferredoxin-dependent kinetics in the electron-bifurcating EtfABCX complex from Thermotoga maritima

Flavin-based electron bifurcation couples the reduction of high- and low-potential substrates, the latter of which supports essential processes such as carbon and nitrogen fixation in a variety of obligate anaerobes. A key element of the process is that the quinone/semiquinone and semiquinone/hydroquinone half-potentials of the flavin at the site of electron bifurcation are highly crossed, resulting in a very thermodynamically unstable and strongly reducing semiquinone oxidation state. A major class of electron-bifurcating systems consists of enzymes containing an electron-transferring flavoprotein (ETF) component. The structurally characterized menaquinone-coupled NADH: ferredoxin oxidoreductase EtfABCX from Thermotoga maritima is a representative member of this class of electron-bifurcating system. Here, we examine the kinetic and spectroscopic properties of the isolated EtfAB component-both replete and depleted in the second, electron-transferring flavin- and the intact EtfABCX complex. The results contribute to previous findings of other bifurcating systems indicating that under certain circumstances the half-potentials of the bifurcating flavin become uncrossed, and also elucidate the effect of ferredoxin on the kinetics of the reaction with NADH. These features shared by several homologous systems highlight shared strategies employed by ETF-containing systems that catalyze electron bifurcation.