Sites of energy coupling

The small free energy change (—36kJ/mol) associated with the reaction is sufficient to allow the synthesis of about 0.5 mol ATP/mol CH4. In accordance, the growth yields Tch4 of Methanosarcina and Methanothrix on acetate as sole carbon and energy source are low, of the order of 2—4 g cells/mol CH4 [35,227-230]. 

After uptake of acetate into the cells (for a proposed transport mechanism see [231]) acetate is activated in Methanosarcina species to acetyl-CoA via acetyl-phosphate by stoichiometric consumption of 1 ATP in the acetate kinase reaction. This indicates that during acetyl-CoA conversion to CH4 more than 1 mol ATP (about 1.5) has to be formed. 

Three sites of energy coupling have been proposed for CH4 formation from acetate (Fig. 11) (1) CH3-S-C0M reduction to CH4, (2) oxidation of enzyme-bound CO to CO2 and H2, and (3) CH3-H4MPT H-S-CoM methyltransferase reaction. 

CH3-S-C0M is reduced to methane via the heterodisulfide of H-S-CoM and H-S-HTP. The reduction of the heterodisulfide has been shown to be coupled with ATP synthesis according to a chemiosmotic mechanism (see above). The electrons required for the reduction are derived from the oxidation of enzyme-bound CO ([CO]) which is oxidized to CO2 via CO-DH. It is assumed that electron transport from [CO] to the heterodisulfide is coupled with the generation of an electrochemical proton potential which then drives ATP synthesis. Possible eleetron transport components, a cytochrome b and a membrane-bound hydrogenase, have been identified [232]. Probably two H -translocating sites are present in electron transport from CO to the heterodisulfide the oxidation of CO to CO2 and H2, and the reduction of the heterodisulfide (or methyl-CoM) by H2. Both H2 and 

CO have been shown to reduce the heterodisulfide in cell extracts of Methanosarcina barkeri [133] (see also ref. [233]). 

---

Generally, the assimilatory nitrate and nitrite reductases are soluble enzymes that utilize reduced pyridine nucleotides or reduced ferrodoxin. In contrast, the dissimilatory nitrate reductases are membrane-bound terminal electron acceptors that are tightly linked to cytochrome by pigments. Such complexes allow one or more sites of energy conservation (ATP generation) coupled with electron transport. 

Production of H2 fuel from water via solar energy is of exceedingly high interest.115 Catalysis may involve H2 complexes at least as intermediates, and H2 complexes had previously been implicated in solar energy conversion schemes based on photoreduction of water.116 Industrially important water-gas shift and related H2-producing reactions undoubtedly proceed via transient H2 complexes.117 Biomimetic H2 production, particularly solar driven (via photocatalysis), is also a challenge and may take a cue from models of the active site of hydrogenase coupled with models of nature s photosystems.91-93 Here, the formation of H-H bonds from protons and... 

Yagi, T., Inhibition by capsaicin of NADH-quinone oxidoreductases is correlated with the presence of energy-coupling site 1 in various organisms. Arch. Biochem. Biophys., 281, 305, 1990. 

In Eqs. (4) a. designates the creation operator of an electron in an orbital of energy e. at site i, and b that of a normal mode of vibration of energy coupled to the electronic state labelled... 

Next, we estimate the magnitude of the attraction between virtual transition and the direct, lowest energy transitions on different sites. The corresponding coupling term— /,y 2,2y)(3,ly +H.C.—leads to the following contribution to the free energy in the lowest order ... 

Fig. 2.5. Possible applications of a coupling parameter, A, in free energy calculations, (a) and (b) correspond, respectively, to simple and coupled modifications of torsional degrees of freedom, involved in the study of conformational equilibria (c) represents an intramolecular, end-to-end reaction coordinate that may be used, for instance, to model the folding of a short peptide (d) symbolizes the alteration of selected nonbonded interactions to estimate relative free energies, in the spirit of site-directed mutagenesis experiments (e) is a simple distance separating chemical species that can be employed in potential of mean force (PMF) calculations and (f) corresponds to the annihilation of selected nonbonded interactions for the estimation of e.g., free energies of solvation. In the examples (a), (b), and (e), the coupling parameter, A, is not independent of the Cartesian coordinates, x. Appropriate metric tensor correction should be considered through a relevant transformation into generalized coordinates...

The reorganization free energy /.R represents the electronic-vibrational coupling, ( and y are fractions of the overpotential r] and of the bias voltage bias at the site of the redox center, e is the elementary charge, kB the Boltzmann constant, and coeff a characteristic nuclear vibration frequency, k and p represent, respectively, the microscopic transmission coefficient and the density of electronic levels in the metal leads, which are assumed to be identical for both the reduction and the oxidation of the intermediate redox group. Tmax and r max are the current and the overvoltage at the maximum. 

---