On oxygen limitation in a whole cell biocatalytic Baeyer-Villiger oxidation process

In this article, a recombinant cyclohexanone monooxygenase (CHMO), overexpressed in Escherichia coli has been used to study the oxidation of bicyclo[3.2.0]hept-2-en-6-one to its two corresponding lactones at very high enantiomeric excess. The reaction is a useful model for the study of biocatalytic oxidations to create optically pure molecules. The major limitations to a highly productive biocatalytic oxidation in this case are oxygen supply, product inhibition, and biocatalyst stability. In this article, we investigate the effects of whole cell biocatalyst concentration on the rate of reaction at a range of scales from shake flasks to 75 L bioreactors. At low cell concentrations (<2 g(dcw)/L) the maximum specific rate (0.65 g/g(dcw).h) is observed. However, at higher cell concentrations (> 2 g(dcw)/L), the reaction becomes oxygen limited and both the specific rate and absolute rate decrease with further increases in cell concentration. The role of oxygen limitation in reducing the rate of reaction with scale was investigated by increasing the maximum oxygen transfer rate in the reactor at a high cell concentration and observing the increase in product formation rate. We propose a qualitative model demonstrating the relationship between oxygen limitation, biocatalyst concentration, and the rate of reaction. This conceptual model will be a useful guide in the industrial scale-up of whole cell mediated Baeyer-Villiger biocatalysis.