Electron transport phosphorylation in rumen butyrivibrios: unprecedented ATP yield for glucose fermentation to butyrate

Abstract

From a genomic analysis of rumen butyrivibrios (Butyrivibrio and Pseudobutyrivibrio sp.), we have re-evaluated the contribution of electron transport phosphorylation (ETP) to ATP formation in this group. This group is unique in that most (76%) genomes were predicted to possess genes for both Ech and Rnf transmembrane ion pumps. These pumps act in concert with the NifJ and Bcd-Etf to form a electrochemical potential (ΔμH+ and ΔμNa+), which drives ATP synthesis by ETP. Of the 62 total butyrivibrio genomes currently available from the Hungate 1000 project, all 62 were predicted to possess NifJ, which reduces oxidized ferredoxin (Fdox) during pyruvate conversion to acetyl-CoA. All 62 possessed all subunits of Bcd-Etf, which reduces Fdox and oxidizes reduced NAD during crotonyl-CoA reduction. Additionally, 61 genomes possessed all subunits of the Rnf, which generates ΔμH+ or ΔμNa+ from oxidation of reduced Fd (Fdred) and reduction of oxidized NAD. Further, 47 genomes possessed all six subunits of the Ech, which generates ΔμH+ from oxidation of Fdred. For glucose fermentation to butyrate and H2, the electrochemical potential established should drive synthesis of ∼1.5 ATP by the F0F1-ATP synthase (possessed by all 62 genomes). The total yield is ∼4.5 ATP/glucose after accounting for three ATP formed by classic substrate-level phosphorylation, and it is one the highest yields for any glucose fermentation. The yield was the same when unsaturated fatty acid bonds, not H+, served as the electron acceptor (as during biohydrogenation). Possession of both Ech and Rnf had been previously documented in only a few sulfate-reducers, was rare in other rumen prokaryotic genomes in our analysis, and may confer an energetic advantage to rumen butyrivibrios. This unique energy conservation system might enhance the butyrivibrios’ ability to overcome growth inhibition by unsaturated fatty acids, as postulated herein.

Genomes of rumen bacteria encode atypical pathways for fermenting hexoses to short‐chain fatty acids

“…S2A). However, as in a previous study (Hackmann and Firkins, ), we found that some Butyrivibrio and Pseudobutyrivibrio ( n = 3) encode an atypical pathway for forming butyrate (Supporting Information Fig. S2B).…”

“…A previous study from our group found an atypical pathway for butyrate formation in most butyrivibrios ( Butyrivibrio and Pseudobutyrivibrio spp.) (Hackmann and Firkins, ). Specifically, the pathway involves the ion pump Ech hydrogenase, which together with Rnf generates an electrochemical potential and conserves energy.…”

Rumen microbiota and dietary fat: a mutual shaping

“…Among bacteria genera or species that were studied in several experiments, Fibrobacter and Ruminococcus were negatively affected in most cases, but the effects on Butyrivibrio and Prevotella were highly variable. These latter genera comprise many species with somewhat different functions, different metabolic pathways (Hackmann and Firkins ) and different sensitivities to LA in cultures (Maia et al . ).…”

“…A specific effect of the cis double‐bonds could explain why most known biohydrogenating bacteria produce trans‐ C18:1, but do not further reduce it to stearic acid (see below). Hackmann and Firkins () reported that B. fibrisolvens could be less sensitive to LA than B. hungatei and Butyrivibrio proteoclasticus because its membrane has a lower fluidity due to more palmitic acid and less branched‐chain FA.…”

Microbial Community Structure and Functionality in Ruminants Fed the Probiotic Bacillus amyloliquefaciens H57

“…Through aerobic oxidation, this stored energy can be made readily available to the host for cellular growth and maintenance. During anaerobic fermentation, the coenzyme NAD + is reduced to NADH/H + through a process of substrate linked phosphorylation (Hackmann and Firkins, 2015). For fermentation to continue, NAD + needs to be regenerated (oxidised) by NAD-linked hydrogenases, which are only thermodynamically favourable at low partial pressures of hydrogen.…”