Oxidation–reduction of methanol, formaldehyde, serine, and formate in Methanosphaera stadtmanae using 14C short- and long-term labelling

Methanosphaera stadtmanae derives its energy from the reduction by H2 of CH3OH, but not CO2, indicating there is a block in the CO2 methanogenesis pathway. Both 14CH4 and 14CO2 production were detected in whole cells using [14C]formaldehyde or [14C]serine as substrate. 14CO2 was also observed from [14C]formate in both whole cells and cofactor-depleted cell-free extracts, and NADP-dependent formate dehydrogenase activity was detected. Both formate and serine blocked the formation of 14CO2 from formaldehyde in whole cells. The results confirmed that enzymes involved in the reduction of carbon from the level of methylene-tetrahydromethanopterin in a common methanogenic pathway and a tetrahydromethanopterin-dependent serine hydroxymethyltransferase were present in this organism. However, the production of 14CH4 could not be observed from [14C]formate or 14CO2 plus H2. [14C]Formate was incorporated specifically into histidine and RNA. [14C]Methanol was also found to label rRNA and cytoplasmic proteins, especially corrinoid proteins.Key words: methanogenesis, formate dehydrogenase, formaldehyde oxidation, C1 intermediates.