Figure 6.

Upper part: putative incorporation of H₂ in the conversion of acetate to butyrate in a ruminal Butyrivibrio spp. cell; lower part: putative mechanism of electron transfer from H₂ to NAD⁺ as described by Buckel and Thauer (2013) for non-ruminal organisms: Fd_ox + NAD⁺ + 2 H₂ → Fd${}_{\text{red}}^{2-}+$ NADH + 3 H⁺. Ech, Escherichia coli hydrogenase-3-type hydrogenase transmembrane ion pump; Rnf, Rhodobacter nitrogen fixation transmembrane ion pump; Fd_ox, oxidized ferredoxin; Fd_red, reduced ferredoxin; Solid arrows represent known or demonstrated pathways. Dashed arrows represent putative H₂ incorporation and electron transfer reactions. For simplicity, reactions are not balanced for H⁺, and extruded cations, water and HS-CoA are omitted. Adapted from Hackmann and Firkins (2015).