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Background  Conversion of industrial processes to more nature-friendly modes is a crucial subject for achieving sustainable development. Utilization of hydrogen-oxidation reactions by hydrogenase as a driving force of bioprocess reaction can be an environmentally ideal method because the reaction creates no pollutants. We expressed NAD-dependent alcohol dehydrogenase from  Kluyveromyces lactis  in a hydrogen-oxidizing bacterium:  Ralstonia eutropha . This is the first report of hydrogen-driven  in vivo  coupling reaction of the alcohol dehydrogenase and indigenous soluble NAD-reducing hydrogenase. Asymmetric reduction of hydroxyacetone to ( R )-1,2-propanediol, which is a commercial building block for antibacterial agents, was performed using the transformant as the microbial cell catalyst.    Results  The two enzymes coupled  in vitro  in vials without a marked decrease of reactivity during the 20 hr reaction because of the hydrogenase reaction, which generates no by-product that affects enzymes. Alcohol dehydrogenase was expressed functionally in  R. eutropha  in an activity level equivalent to that of indigenous NAD-reducing hydrogenase under the hydrogenase promoter. The hydrogen-driven  in vivo  coupling reaction proceeded only by the transformant cell without exogenous addition of a cofactor. The decrease of reaction velocity at higher concentration of hydroxyacetone was markedly reduced by application of an  in vivo  coupling system. Production of ( R )-1,2-propanediol (99.8% e.e.) reached 67.7 g/l in 76 hr with almost a constant rate using a jar fermenter. The reaction velocity under 10% P H2  was almost equivalent to that under 100% hydrogen, indicating the availability of crude hydrogen gas from various sources. The  in vivo  coupling system enabled cell-recycling as catalysts.    Conclusions  Asymmetric reduction of hydroxyacetone by a coupling reaction of the two enzymes continued in both  in vitro  and  in vivo  systems in the presence of hydrogen. The  in vivo  reaction system using  R. eutropha  transformant expressing heterologous alcohol dehydrogenase showed advantages for practical usage relative to the  in vitro  coupling system. The results suggest a hopeful perspective of the hydrogen-driven bioprocess as an environmentally outstanding method to achieve industrial green innovation. Hydrogen-oxidizing bacteria can be useful hosts for the development of hydrogen-driven microbial cell factories.

microbial cell factories

Hydrogen-driven asymmetric reduction of hydroxyacetone to (R)-1,2-propanediol by Ralstonia eutropha transformant expressing alcohol dehydrogenase from Kluyveromyces lactis