An improved stereoselective reduction of a bicyclic diketone by Saccharomyces cerevisiae combining process optimization and strain engineering

Abstract.

The stereoselective reduction of the bicyclic diketone bicyclo[2.2.2]octane-2,6-dione, to the ketoalcohol (1R,4S,6S)-6-hydroxybicyclo[2.2.2]octane-2-one, was used as a model reduction to optimize parameters involved in NADPH-dependent reductions in Saccharomyces cerevisiae with glucose as co-substrate. The co-substrate yield (ketoalcohol formed/glucose consumed) was affected by the initial concentration of bicyclic diketone, the ratio of yeast to glucose, the medium composition, and the pH. The reduction of 5 g l^–1 bicyclic diketone was completed in less than 20 h in complex medium (pH 5.5) under oxygen limitation with an initial concentration of 200 g l^–1 glucose and 5 g l^–1 yeast. The co-substrate yield was further enhanced by genetically engineered strains with reduced phosphoglucose isomerase activity and with the gene encoding alcohol dehydrogenase deleted. Co-substrate yields were increased 2.3-fold and 2.4-fold, respectively, in these strains.