초록
<P><B>Significance</B></P><P>A flavodoxin from the domain <I>Archaea</I> has been characterized. It is the first of any flavodoxin shown to stabilize an anionic semiquinone, providing a platform for understanding how the protein environment modulates the reduction potentials of flavins. The unusual flavodoxin plays an electron transport role in the pathway of acetate conversion to methane in <I>Methanosarcina acetivorans</I>, a model methanogen for investigating the process by which two-thirds of the 1 billion metric tons of methane are produced annually in Earth’s anaerobic biospheres with a substantial contribution to global warming effecting climate change. Homologs of the gene encoding the flavodoxin are uniformly distributed in diverse acetotrophic methanogens consistent with a wider range of electron transport functions awaiting discovery.</P><P>Flavodoxins, electron transfer proteins essential for diverse metabolisms in microbes from the domain <I>Bacteria</I>, are extensively characterized. Remarkably, although genomic annotations of flavodoxins are widespread in microbes from the domain <I>Archaea</I>, none have been isolated and characterized. Herein is described the structural, biochemical, and physiological characterization of an unusual flavodoxin (FldA) from <I>Methanosarcina acetivorans</I>, an acetate-utilizing methane-producing microbe of the domain <I>Archaea</I>. In contrast to all flavodoxins, FldA is homodimeric, markedly less acidic, and stabilizes an anionic semiquinone. The crystal structure reveals an flavin mononucleotide (FMN) binding site unique from all other flavodoxins that provides a rationale for stabilization of the anionic semiquinone and a remarkably low reduction potentials for both the oxidized/semiquinone (−301 mV) and semiquinone/hydroquinone couples (−464 mV). FldA is up-regulated in acetate-grown versus methanol-grown cells and shown here to substitute for ferredoxin in mediating the transfer of low potential electrons from the carbonyl of acetate to the membrane-bound electron transport chain that generates ion gradients driving ATP synthesis. FldA offers potential advantages over ferredoxin by (<I>i</I>) sparing iron for abundant iron-sulfur proteins essential for acetotrophic growth and (<I>ii</I>) resilience to oxidative damage.</P>