Function of coenzyme F-420-dependent NADP reductase in methanogenic archaea containing an NADP-dependent alcohol dehydrogenase

Methanogenic archaea growing on ethanol or isopropanol as the electron donor for CO2 reduction to CH4 contain either an NADP-dependent or a coenzyme F-420-dependent alcohol dehydrogenase. We report here that in both groups of methanogens, the N-5,N-10-methylenetetrahydromethanopterin dehydrogenase and the N-5,N-10-methylenetetrahydromethanopterin reductase, two enzymes involved in CO2 reduction to CH4, are specific for F-420. This raised the question how F420H2 is regenerated in the methanogens with an NADP-dependent alcohol dehydrogenase. We found that these organisms contain catabolic activities of an enzyme catalyzing the reduction of F-420 with NADPH. The F-420-dependent NADP reductase from Methanogenium organophilum was purified and characterized. The N-terminal amino acid sequence showed 42% sequence identity to a putative gene product in Methanococcus jannaschii, the total genome of which has recently been sequenced.