Formate-dependent h2 production by the mesophilic methanogen methanococcus maripaludis{triangledown}

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Methanococcus maripaludis, an H2- and formate-utilizing methanogen, produced H2 at high rates from formate. The rates and kinetics of H2 production depended upon the growth conditions, and H2 availability during growth was a major factor. Specific activities of resting cells grown with formate or H2 were 0.4 to 1.4 U·mg–1 (dry weight). H2 production in formate-grown cells followed Michaelis-Menten kinetics, and the concentration of formate required for half-maximal activity (Kf) was 3.6 mM. In contrast, in H2-grown cells this process followed sigmoidal kinetics, and the Kf was 9 mM. A key enzyme for formate-dependent H2 production was formate dehydrogenase, Fdh. H2 production and growth were severely reduced in a mutant containing a deletion of the gene encoding the Fdh1 isozyme, indicating that it was the primary Fdh. In contrast, a mutant containing a deletion of the gene encoding the Fdh2 isozyme possessed near-wild-type activities, indicating that this isozyme did not play a major role. H2 production by a mutant containing a deletion of the coenzyme F420-reducing hydrogenase Fru was also severely reduced, suggesting that the major pathway of H2 production comprised Fdh1 and Fru. Because a {Delta}fru-{Delta}frc mutant retained 10% of the wild-type activity, an additional pathway is present. Mutants possessing deletions of the gene encoding the F420-dependent methylene-H4MTP dehydrogenase (Mtd) or the H2-forming methylene-H4MTP dehydrogenase (Hmd) also possessed reduced activity, which suggested that this second pathway was comprised of Fdh1-Mtd-Hmd. In contrast to H2 production, the cellular rates of methanogenesis were unaffected in these mutants, which suggested that the observed H2 production was not a direct intermediate of methanogenesis. In conclusion, high rates of formate-dependent H2 production demonstrated the potential of M. maripaludis for the microbial production of H2 from formate.

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