High productivity biotransformation process for the production of 3-(S)-hydroxybutanoic acid esters

Abstract

A continuous process for the reduction of acetoacetic acid esters to the 3-(S)-hydroxybutanoic acid esters by the yeast Saccharomyces cerevisiae was to be developed with the objective to maximize both productivity and product quality. The optical purity of 3-(S)-hydroxybutanoic acid esters depended on pH; maximal optical purity of > 95% was obtained at pH 2.2 with cells grown on ethanol. These conditions allowed only low specific growth rates (μ = 0.05 h −1) in the single stage, steady state production system. Implementation of cell recycling resulted in uncoupling of the dilution rate from growth rate and, consequently, in increased volumetric productivity: the fully automated cell recycling bioreactor operated in steady state yielded a productivity of 3-(S)-hydroxybutanoic acid esters of up to 5.2 g 1 −1 h −1; this is more than 20 times greater than in the single stage production system without cell recycling. The same product quality (ee > 95%) could be maintained. The continuous cell recycling process was found to be very stable and the biological system was highly reproducible (constant catalyst properties and constant, excellent product quality).