In the model, MP donates electrons to the heterodisulfide reductase HdrDE accompanied by translocation of protons which further contributes to ATP synthesis. An electron 10058-F4 cost transport chain has been hypothesized for the marine
isolate Methanosarcina acetivorans, the only non-H2-metabolizing acetotrophic methanogen for which the genome is sequenced. Although encoding Cdh, the genome does not encode Ech hydrogenase [10, 11]. Furthermore, in contrast to all H2-utilizing aceticlastic Methanosarcina species investigated [12], acetate-grown M. acetivorans synthesizes a six-subunit complex (Ma-Rnf) [13] encoded within a co-transcribed eight-gene (MA0658-0665) cluster with high identity https://www.selleckchem.com/products/AG-014699.html to membrane-bound Rnf (R hodobacter nitrogen fixation) complexes from the domain Bacteria. It is hypothesized that the Ma-Rnf complex plays an essential role in the electron transport chain, generating a sodium gradient that is exchanged for a proton gradient driving ATP synthesis [13]. Consistent with this idea, it was recently shown that the six-subunit Rnf complex from Acetobacterium woodii of the domain Bacteria couples electron transport from reduced ferredoxin to NAD+ with the generation of a sodium gradient [14]. Remarkably, the Ma-Rnf complex of M. acetivorans is co-transcribed with a gene (MA0658) encoding a multi-heme cytochrome c, and another
flanking gene (MA0665) encoding a hypothetical membrane integral Alvocidib research buy protein with unknown function [13]. Indeed, the cytochrome c was shown to be synthesized in high levels of acetate-grown cells where it completely dominates the UV-visible spectrum of the purified membranes http://www.selleck.co.jp/products/pci-32765.html and is distinguishable from b-type cytochromes [13]. Furthermore, it was recently reported (A. M. Guss and W. W. Metcalf, unpublished results) that a six-subunit Ma-Rnf/cytochrome c (ΔMA0658-0665) deletion mutant of M. acetivorans fails
to grow with acetate [15]. However, biochemical evidence necessary to support the hypothesized role of cytochrome c has not been forthcoming. The only other report of cytochromes c in methanogens is for the H2-metabolizing species Methanosarcina mazei (f. Methanosarcina strain Gö1) grown with methanol [16]. The freshwater isolate Methanosarcina thermophila is the only non-H2-metabolizing acetotrophic methanogen for which electron transport components have been investigated biochemically [17]. Like H2-metabolizing Methanosarcina species, ferredoxin mediates electron transfer between Cdh and the membrane-bound electron transport chain in which a cytochrome b participates and dominates the UV-visible absorbance spectrum of membranes. It is also reported that MP is the electron donor to HdrDE [18]. Electron carriers other than cytochrome b that participate between ferredoxin and MP were not identified.