CO2 reduction pathways

CH4 formation from H2/CO2 is dependent on Na ions [109,110]. Since CH4 formation from H2/CH3OH was found not to be dependent on Na ions, a role for this cation in the terminal part of the CO2 reduction pathway, namely CH3-S-C0M reduction to CH4, and in ATP synthesis could be excluded [111]. CH4 formation from H2/CH3OH involves three steps (i) binding of CH3OH to coenzymeM via two methyltransferases (see ref [13]) ... 

The findings clearly indicate that the last step of the CO2 reduction pathway, the reduction of CoM-S-S-HTP by H2, is a coupling site for ATP synthesis. It is concluded that CoM-S-S-HTP is the terminal electron acceptor ( = —200 mV) of a membrane-bound electron transport chain, with molecular H2 ( ° = -414mV) being the electron donor in hydrogenotrophic methanogens [115]. The physiological electron donor for CoM-S-S-HTP reduction is not known (Fig. 5). 

In order to explain the Na stimulation of ATP synthesis driven by a diffusion potential the presence of a Na /H antiporter was proposed [175]. In this artificial system the acidification of the cytoplasm, which occurs in response to electrogenic potassium efflux, could be prevented by the antiporter. Subsequently, Na /H antiporter activity has been demonstrated in both Methanobacterium thermoautotrophicum [176] and in Methanosarcina harden [108]. An important result of these studies was that the Na /H antiporter could be inhibited by amiloride and harmaline, which have been described as inhibitors of eucaryotic Na" /H" antiporters [177]. Using these inhibitors it has been shown that an active antiporter is essential for methanogenesis from H2/CO2 [176,178]. The antiporter also accepts Li instead of Na, since Li stimulates CH4 formation from H2/CO2 in the absence of Na [176]. In subsequent studies the use of amiloride and the more potent derivative ethyl-isopropylamiloride permitted the discrimination of primary and secondary Na potentials generated in partial reactions of the CO2 reduction pathway. 

Figure 2 Is a plot of 6 D-H2O versus 6 D-CH for samples obtained from littoral zone sediments of several freshwater lakes and from several shallow (1 m or less water depth) areas of the Tampa Bay estuary. Also shown on Figure 2 are lines that describe predicted 6D-H2O/6D-CH Isotopic pairs resulting from varying the relative contributions to methane production of the acetate dissimilation and CO2 reduction pathways. This model was originally proposed by Woltemate et al. (22) and used In that study to estimate that methyl group transfer (from acetate or other methyl group donors such as methanol) was responsible for about 76% of total methane production In the sediments of Wurmsee, a shallow lake near Hannover, FRG.

In constructing this model, Woltemate et al. (22) assumed that for the CO2 reduction pathway all four methane hydrogens are... 

Figure 9. One- and two-electron pathways for CO2 reduction. A = an oxide ion acceptor.

Reduction of CO2 to CH4 is a multi-step process involving intermediates at formyl-(methenyl-), methylene-, and methyl-stages of oxidation (Fig. 1)[2,184]. Formyl-MF is the first stable intermediate in the C02-reduction pathway [88]. Purified formyl-MF dehydrogenase can oxidize the formyl group to CO2, using methylviologen as electron acceptor [95]. It has been hypothesized that the first step traps CO2 as a carbamate, MF-COO (Reaction 9) [184]. However, formation of MF-COO" has not been demonstrated. In water, furfuralamine carbamate is spontaneously decarboxylated (Reaction 10) a similar situation is expected with MF-COO. ... 

In photosynthesis, CO2 generally enters the leaves or stems of biomass through the stoma, the small intercellular openings in the epidermis. These openings provide the main route for both photosynthetic gas exchange and for water vapor loss in transpiration. At least three different biochemical pathways can occur during CO2 reduction to sugars (Rabinovitch, 1956 Loomis cf al, 1971 Osmond, 1978). 

Scheme 1 Illustration of hypothetical reduction pathway of CO2 on photocatalyst surface. M metal ion site on the surface.

Stable hydrogen and carbon Isotopic compositions of biogenic methane produced In the sediments of several freshwater and estuarine environments have been measured and Interpreted using a previously published model. The results Infer that acetate dissimilation Is the dominant methanogenlc pathway In these sediments, accounting for about 50 to 80% of the total methane production, with CO2 reduction responsible for the remainder. In general, 6D-CH and are Inversely correlated. Implying... 

Figure 18.2. AfG° in kJ/mol per carbon atom of TCA cycle intermediates from CO2(aq) and H2(aq), together with reference molecules on the reduction pathway to CH4(aq). For the reaction XCO2 + yH2-Y + ZH2O, reducing potential per carbon taken from the environment to form species X is plotted as ylx on the abscissa. AfG° values for cycle intermediates from [66] and for other organic compounds from [67].

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