Electron transfer reactions in methanogens

Abstract

Methanogenic bacteria comprise a specialized group of obligately anaerobic microorganisms able to reduce a limited number of substrates to CH4. The intermediates involved in this reduction process remain bound to a series of typical C1‐carriers. Reducing equivalents are either obtained from the oxidation of H2 or from oxidation of carbon substrates to CO2. Electron transfer reactions thus constitute the very essence of the process of methanogenesis. In recent years much progress has been made in the elucidation of the special metabolic pathways and the nature of the C1‐carriers involved in methanogenic bacteria. The energy generated at the oxidoreduction reactions, notably at the methylreductase step, is conserved by ATP synthesis. The energy is used for cell carbon synthesis and, in catalytic amounts, for the reductive activation of some methanogenic enzymes. Before the condensing reaction resulting in the formation of acetyl‐CoA takes place, 2 C1‐units are reduced or oxidized depending on the substrate to a carbonyl and a ‐CH3 group. Formation of the latter proceeds via the methanogenic route. Intermediary cell carbon synthesis starting from acetyl‐CoA involves reductive carboxylations and oxidoreductions by the participation of the enzymes of the tricarboxylic acid cycle.

Methanogens and the diversity of archaebacteria.

“…A comparison of the free energies of hydrolysis of ATP (-31.8 kJ/mol) and those of methane formation from the substrates hydrogen and carbon dioxide (-135.6 kJ/mol of CH4), formate (-130.1 kJ), methanol (-104.9 kJ), methylamines (about -74 kJ), carbon monoxide (-196.7 kJ), and acetate (-31.0 kJ) leads to the conclusion that only small amounts of energy are available to these organisms (63,464). The bioenergetics of methanogenesis have been thoroughly reviewed (63,226). The brief discussion which follows points out major findings that have begun to clarify how the anabolic needs of these organisms are met.…”

Changes in concentrations of coenzyme F420 analogs during batch growth of Methanosarcina barkeri and Methanosarcina mazei

“…In its oxidized state, coenzyme F420 exhibits a specific blue-green fluorescence which has been used for the identification of methanogenic bacteria by fluorescence microscopy (8,24) and permits the selective determination of coenzyme F420 in cell extracts. Coenzyme F420 principally acts as an electron carrier in metabolism (2,16), although secondary roles have also been proposed (19,20). Initial methods of coenzyme F420 quantification involved a simple extraction with direct fluorescence measurement.…”

Methyl‐Coenzyme M Reductase and its Nickel Corphin Coenzyme F 430 in Methanogenic Archaea

“…∆G o for the MCR-catalyzed reaction is obtained from the difference in the free energy changes associated with several reactions [4,138]. One of these reactions is the reduction of CoM-S-S-CoB with H 2 .…”

“…As a result, ∆G o for methyl-coenzyme M reduction with coenzyme B decreased from Ϫ45 kJ/mol to Ϫ33 kJ/mol. This value also has some uncertainty since it is in part based on ∆G o ϭ Ϫ28 kJ/mol associated with methyl-coenzyme M formation from methanol and coenzyme M, which was calculated from differences in bond energies which neglects differences in solvation energies [138]. ∆G o for methyl-coenzyme M reduction is probably best given as being Ϫ30 ± 10 kJ/mol.…”