A thermostable mannitol-1-phosphate dehydrogenase is required in mannitol metabolism of the thermophilic acetogenic bacterium Thermoanaerobacter kivui |
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Authors: | Jimyung Moon Laura Henke Nadine Merz Mirko Basen |
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Affiliation: | Department of Molecular Microbiology & Bioenergetics, Institute of Molecular Biosciences, Johann Wolfgang Goethe University, Max-von-Laue Str. 9, D-60438 Frankfurt/Main, Germany |
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Abstract: | Acetogenic bacteria recently attracted attention because they reduce carbon dioxide (CO2) with hydrogen (H2) to acetate or to other products such as ethanol. Besides gases, acetogens use a broad range of substrates, but conversion of the sugar alcohol mannitol has rarely been reported. We found that the thermophilic acetogenic bacterium Thermoanaerobacter kivui grew on mannitol with a specific growth rate of 0.33 h−1 to a final optical density (OD600) of 2.2. Acetate was the major product formed. A lag phase was observed only in cultures pre-grown on glucose, not in those pre-grown on mannitol, indicating that mannitol metabolism is regulated. Mannitol-1-phosphate dehydrogenase (MtlD) activity was observed in cell-free extracts of cells grown on mannitol only. A gene cluster (TKV_c02830–TKV_c02860) for mannitol uptake and conversion was identified in the T. kivui genome, and its involvement was confirmed by deleting the mtlD gene (TKV_c02860) encoding the key enzyme MtlD. Finally, we overexpressed mtlD, and the recombinant MtlD carried out the reduction of fructose-6-phosphate with NADH, at a high VMAX of 1235 U mg−1 at 65°C. The enzyme was thermostable for 40 min at 75°C, thereby representing the first characterized MtlD from a thermophile. |
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