Optimisation of the kinetics of the stereoselective reduction of geraniol to citronellol in a two liquid phase system

Kinetics of the stereoselective reduction of geraniol to citronellol by baker's yeast (Saccharomyces cerevisiae) were studied by examining the influence of system parameters on the reaction rate in an aqueous/organic two phase system. This system was used due to the hydrophobic nature of the reactant and product, and enabled simple and accurate measurement of the kinetics, and hence optimisation. This reaction system provided 'pseudo steady state' aqueous conditions in batch experiments over short time periods, and therefore allowed direct determination of the biotransformation rate. The maximum biotransformation activity was determined to be 300 nmol min^-1 g biomass^-1. Of the parameters tested those which most significantly affected the biocatalytic activity were: biomedium pH, reactant concentration, solvent type, and aeration. The optimum pH was found to be 9.5, and this was associated with increased intracellular NADPH concentrations. Slight substrate inhibition with geraniol occurred at concentrations above 0.3 g/l in the aqueous phase. The effect of the solvent phase on biocatalytic activity was in general accordance with previous work: solvents with high log P resulted in greatest activity, and the transition value occurred between 4.5 and 5.6. Finally, maximum biotransformation activity was only exhibited when the bioreactor was aerated, however, the reason for this was not established.