Enhancing the oxygen supply to whole-cell oxygenase bioconversions

The aim of this work was to investigate the effect of oxygen limitation on whole­ cell oxygenases, and to determine how the physiochemical properties of oils affect their ability to enhance the oxygen transfer rate. Whole-cell oxygenase biocatalysts require oxygen as a substrate for the reaction and for the electron transport chain. The productivity of these bioconversions is therefore influenced by the maximum oxygen transfer rate of the fermenter. Organic solvents are commonly used in oxygenase bioconversions to alleviate substrate or product limitation, and they can also increase the oxygen transfer rate to the aqueous phase. The model system was the bioconversion of bicycloheptenone to oxabicylooctenone using a recombinant E.coli biocatalyst overexpressing cyclohexanone monooxygenase (CHMO). Above a critical biomass concentration the oxygen transfer rate determined the maximum activity. When oxygen was not limited, the electron transport chain used twice as much oxygen as the CHMO. When it was limited, the CHMO and the electron transport chain competed for the oxygen the CHMO used approximately 20% whatever the severity of the limitation. The oxygen transfer rate to the aqueous phase increased up to 2.5 fold depending on the physical properties and the volume fraction of the oil phase. The oxygen transfer rate only increased if the oil drops were small enough the magnitude was determined by the oxygen solubility of the oil. There was no correlation between the spreading coefficient and the oxygen transfer rate. A model that predicted the enhancement in the oxygen transfer rate caused by the addition of oil was in good agreement with the experimental results. The specific activity of an oxygen limited CHMO bioconversion was increased up to 2.25 fold using perfluorotributylamine.