Improved assay of coenzyme F420 analogues from methanogenic bacteria

Metabolism of methanogens

Methanogenic archaea convert a few simple compounds such as H2 + CO2, formate, methanol, methylamines, and acetate to methane. Methanogenesis from all these substrates requires a number of unique coenzymes, some of which are exclusively found in methanogens. H2-dependent CO2 reduction proceeds via carrier-bound C1 intermediates which become stepwise reduced to methane. Methane formation from methanol and methylamines involves the disproportionation of the methyl groups. Part of the methyl groups are oxidized to CO2, and the reducing equivalents thereby gained are subsequently used to reduce other methyl groups to methane. This process involves the same C1 intermediates that are formed during methanogenesis from CO2. Conversion of acetate to methane and carbon dioxide is preceded by its activation to acetyl-CoA. Cleavage of the latter compound yields a coenzyme-bound methyl moiety and an enzyme-bound carbonyl group. The reducing equivalents gained by oxidation of the carbonyl group to carbon dioxide are subsequently used to reduce the methyl moiety to methane. All these processes lead to the generation of transmembrane ion gradients which fuel ATP synthesis via one or two types of ATP synthases. The synthesis of cellular building blocks starts with the central anabolic intermediate acetyl-CoA which, in autotrophic methanogens, is synthesized from two molecules of CO2 in a linear pathway.

Methyl-coenzyme M reductase and other enzymes involved in methanogenesis from CO2 and H2 in the extreme thermophile Methanopyrus kandleri

Methanopyrus kandleri belongs to a novel group of abyssal methanogenic archaebacteria that can grow at 110 degrees C on H2 and CO2 and that shows no close phylogenetic relationship to any methanogen known so far. Methyl-coenzyme M reductase, the enzyme catalyzing the methane forming step in the energy metabolism of methanogens, was purified from this hyperthermophile. The yellow protein with an absorption maximum at 425 nm was found to be similar to the methyl-coenzyme M reductase from other methanogenic bacteria in that it was composed each of two alpha-, beta- and gamma-subunits and that it contained the nickel porphinoid coenzyme F430 as prosthetic group. The purified reductase was inactive. The N-terminal amino acid sequence of the gamma-subunit was determined. A comparison with the N-terminal sequences of the gamma-subunit of methyl-coenzyme M reductases from other methanogenic bacteria revealed a high degree of similarity. Besides methyl-coenzyme M reductase cell extracts of M. kandleri were shown to contain the following enzyme activities involved in methanogenesis from CO2 (apparent Vmax at 65 degrees C): formylmethanofuran dehydrogenase, 0.3 U/mg protein; formyl-methanofuran:tetrahydro-methanopterin formyltransferase, 13 U/mg; N5,N10-methylenetetrahydromethanopterin cyclohydrolase, 14U/mg; N5,N10-methenyltetrahydromethanopterin dehydrogenase (H2-forming), 33 U/mg; N5,N10-methylenetetrahydromethanopterin reductase (coenzyme F420 dependent), 4 U/mg; heterodisulfide reductase, 2 U/mg; coenzyme F420-reducing hydrogenase, 0.01 U/mg; and methylviologen-reducing hydrogenase, 2.5 U/mg. Apparent Km values for these enzymes and the effect of salts on their activities were determined. The coenzyme F420 present in M. kandleri was identified as coenzyme F420-2 with 2-gamma-glutamyl residues.

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

Coenzyme F420 has been assayed by high-performance liquid chromatography with fluorimetric detection; this permits quantification of individual coenzyme F420 analogs whilst avoiding the inclusion of interfering material. The total intracellular coenzyme F420 content of Methanosarcina barkeri MS cultivated on methanol and on H2-CO2 and of Methanosarcina mazei S-6 cultured on methanol remained relatively constant during batch growth. The most abundant analogs in M. barkeri were coenzymes F420-2 and F420-4, whilst in M. mazei coenzymes F420-2 and F420-3 predominated. Significant changes in the relative proportions of the coenzyme F420 analogs were noted during batch growth, with coenzymes F420-2 and F420-4 showing opposite responses to each other and the same being also true for coenzymes F420-3 and F420-5. This suggests that an enzyme responsible for transferring pairs of glutamic acid residues may be active. The degradation fragment FO was also detected in cells in late exponential and stationary phase. Coenzyme F420 analogs were present in the culture supernatant of both methanogens, in similar proportions to that in the cells, except for FO which was principally located in the supernatant.