Function of H2-forming methylenetetrahydromethanopterin dehydrogenase from Methanobacterium thermoautotrophicum in coenzyme F420 reduction with H2

In most methanogenic archaea, two hydrogenase systems that can catalyze the reduction of coenzyme F-420 (F-420) with H-2 are present: (1) the F-420-reducing hydrogenase, which is a nickel iron-sulfur flavoprotein composed of three different subunits, and (2) the N-5,N-10-methylenetetrahydromethanopterin dehydrogenase system, which is composed of H-2-forming methylenetetrahydromethanopterin dehydrogenase and F-420-dependent methylenetetrahydromethanopterin dehydrogenase, both metal-free proteins without an apparent prosthetic group. We report here that in nickel-limited chemostat cultures of Methanobacterium thermoautotrophicum, the specific activity of the F-420-reducing Ni/Fe-hydrogenase was essentially zero, whereas that of the H-2-forming methylenete trahydromethanopterin dehydrogenase was six times higher, and that of the F-420-dependent methylenetetrahydromethanopterin dehydrogenase was four times higher than in cells grown under non-nickel-limited conditions. This evidence supports the hypothesis that when M. thermoautotrophicum grows under conditions of nickel limitation, the reduction of F-420 with H-2 is catalyzed by the metal-free methylenetetrahydromethanopterin dehydrogenase system.