Coupled oxidation–reduction of butanol–hexanal by resting Rhodococcus erythropolis NCIMB 13064 cells in liquid and gas phases

Rhodococcus erythropolis is a promising Gram-positive bacterium capable of numerous bioconversions including those involving alcohol dehydrogenases (ADHs). In this work, we compared and optimized the redox biocatalytic performances of 1-butanol-grown R. erythropolis NCIMB 13064 cells in aqueous and in non-conventional gas phase using the 1-butanol–hexanal oxidation–reduction as model reaction. Oxidation of 1-butanol to butanal is tightly coupled to the reduction of hexanal to 1-hexanol at the level of a nicotinoprotein–ADH-like enzyme. Cell viability is dispensable for reaction. In aqueous batch conditions, fresh and lyophilized cells are efficient redox catalysts (oxidation–reduction rate = 765 μmol min−1 g cell dry mass−1) being also reactive towards benzyl alcohol, (S)-2-pentanol, and geraniol as reductants. However, butanol–hexanal oxidation–reduction is strongly limited by product accumulation and by hexanal toxicity that is a major factor influencing cell behavior and performance. Reaction rate is maximal at 40 °C-pH 7.0 in aqueous phase and at 60 °C-pH 7.0–9.0 in gas phase. Importantly, lyophilized cells also showed to be promising redox catalysts in the gas phase (at least 65 μmol min−1 g cell dry mass−1). The system is notably stable for several days at moderate thermodynamic activities of hexanal (0.06–0.12), 1-butanol (0.12) and water (0.7).