Figure 1.

Conceptual schematic of M. maripaludis and D. vulgaris syntrophic interaction, highlighting the central energy-generating and -consuming the pathways of the methanogen. Relative changes (Table 2) in transcript abundance during syntrophic growth are indicated by red (increase) and green (decrease) coloration. Blue coloration indicates no statistically significant change as specified in the Materials and methods section. Hydrogen-limited monocultures served as the control growth condition. Oxidation of formate to CO₂ and H₂ coupled with coenzyme F₄₂₀-reduction (not depicted) is catalyzed by two alternative formate dehydrogenases, Fdh1 and Fdh2. One of two membrane-bound energy-conserving hydrogenases (Eha and Ehb) couple the chemiosmotic energy of ion gradients to H₂ oxidation and ferredoxin reduction. Of these, Ehb generates the low potential electron carrier used for anabolism, whereas Eha is hypothesized to function primarily in the energy-generating methanogenesis pathway, generating low potential-reducing equivalents for the reduction of CO₂ to formylmethanofuran (Major et al., 2010). Two different formylmethanofuran dehydrogenases catalyze this first step in methanogenesis, tungsten (Fwd) and molybdenum (Fmd) forms. Transfer of the formyl group from methanofuran to methanopterin by Ftr and subsequent elimination of H₂O by Mch yields methenyl-H₄-methanopterin. Two different enzymes can then reduce methenyl-H₄-MPT to methylene-H₄MPT, one (Mtd) using H₂ as reductant and the other (Hmd) using reduced coenzyme F₄₂₀. M. maripaludis has an Hmd paralog of unknown function (Mmp1716, Hmd_II) that may also function in this step (Hendrickson et al., 2004). Reduction of methylene-H₄MPT by another F₄₂₀-dependent reductase (Mer) yields methyl-H₄MPT. The reduced coenzyme F₄₂₀ required for the formation of methyl-H₄MPT by these two steps is generated by one of two alternative F₄₂₀-reducting hydrogenase (Fru and Frc). The final steps to methane production are catalyzed by a methyl transferase (Mtr) and a reductase (Mcr) coupled to two forms of a F₄₂₀-nonreducing hydrogenase (Vhu and Vhc). The mixed disulfide (CoM-CoB) produced by reduction of methyl coenzyme M is then reduced by one of two forms of the heterodisulfide reductase determined by the composition of the HdrA subunit (HdrA_U or HdrA_V). Fdh/ Hdr/Vhu/ Fwd are reported to have protein–protein interactions (Costa et al., 2010). (Note: The interaction with Fwd could not be depicted here without compromising the clarity of the figure. Vhc is not part of this interaction). Other reactions include the transport of alanine (AlsT), and subsequent coversion to pyruvate via an alanine racemase (Alr) and dehydrogenase (Ald). Additional M. maripaludis proteins shown: pyruvate oxidoreductase, acetyl-CoA decarbonylase/synthase. The D. vulgaris metabolic pathway is based upon results as described in Walker et al., 2009 and is updated to include an unspecified sodium/alanine transporter (Na⁺/ala sym). Other D. vulgaris proteins shown: lactate permease (lac per), lactate deydrogenase (ldh), membrane-bound Coo hydrogenase (Coo), high-molecular weight cytochrome (Hmc), periplasmic hydrogenases (Hyd and Hyn), cytochrome c3 (Cyt c3) and oxidized and reduced ferredoxin (Fd).