One-Carbon Metabolism in Methanogens: Evidence for Synthesis of a Two-Carbon Cellular Intermediate and Unification of Catabolism and Anabolism in Methanosarcina barkeri

One-carbon metabolic transformations associated with cell carbon synthesis and methanogenesis were analyzed by long- and short-term14 CH3 OH or14 CO2 incorporation studies during growth and by cell suspensions.14 CH3 OH and14 CO2 were equivalently incorporated into the major cellular components (i.e., lipids, proteins, and nucleic acids) during growth on H2 -CO2 -methanol.14 CH3 OH was selectively incorporated into the C-3 of alanine with decreased amounts fixed in the C-1 and C-2 positions, whereas14 CO2 was selectively incorporated into the C1 moiety with decreasing amounts assimilated into the C-2 and C-3 atoms. Notably,14 CH4 and [3-14 C]alanine synthesized from14 CH3 OH during growth shared a common specific activity distinct from that of CO2 or methanol. Cell suspensions synthesized acetate and alanine from14 CO2 . The addition of iodopropane inhibited acetate synthesis but did not decrease the amount of14 CH3 OH or14 CO2 fixed into one-carbon carriers (i.e., methyl coenzyme M or carboxydihydromethanopterin). Carboxydihydromethanopterin was only labeled from14 CH3 OH in the absence of hydrogen. Cell extracts catalyzed the synthesis of acetate from14 CO (∼1 nmol/min per mg of protein) and an isotopic exchange between CO2 or CO and the C-1 of pyruvate. Acetate synthesis from14 CO was stimulated by methyl B12 but not by methyl tetrahydrofolate or methyl coenzyme M. Methyl coenzyme M and coenzyme M were inhibitory to acetate synthesis. Cell extracts contained high levels of phosphotransacetylase (>6 μmol/min per mg of protein) and acetate kinase (>0.14 μmol/min per mg of protein). It was not possible to distinguish between acetate and acetyl coenzyme A as the immediate product of two-carbon synthesis with the methods employed.