| Abstrakt: |
In most methanogenic archaea, two hydrogenase systems that can catalyze the reduction of coenzyme F420 (F420) with H2 are present: (1) the F420-reducing hydrogenase, which is a nickel iron-sulfur flavoprotein composed of three different subunits, and (2) the N 5, N10-methylenetetrahydromethanopterin dehydrogenase system, which is composed of H2-forming methylenetetrahydromethanopterin dehydrogenase and F420-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 F420-reducing Ni/Fe-hydrogenase was essentially zero, whereas that of the H2-forming methylenetetrahydromethanopterin dehydrogenase was six times higher, and that of the F420-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 F420 with H2 is catalyzed by the metal-free methylenetetrahydromethanopterin dehydrogenase system. [ABSTRACT FROM AUTHOR] |
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