Bioreduction of phenylglyoxylic acid to R-(−)-mandelic acid by Saccharomyces cerevisiae FD11b

Screening from 40 microorganisms belonging to different taxonomical groups (Saccharomyces cerevisiae, Lactoballius, Streptococcus faecalis and Candida albicans) was performed to select the strain with high production rate of R-(−)-mandelic acid (R-MA). A sp. strain S. cerevisiae FD11b with high yield and enantiomeric excess (e.e.) and little byproducts was obtained by means of UV-mutation breeding. In the optimal conditions of pH 6.5, 32 °C, anaerobic and glucose supply rate (Fglu) 0.0833 mmol gdw−1 h−1, the strain FD11b showed a high specific MA production rate (qp = 0.35 mmol gdw−1 h−1) and enantioselectivity (e.e. = 97.1%). The phenylglyoxylic acid (PGA) concentration showed a remarkable effect on the apparent activity of MA dehydrogenase. The specific production rate (qp) reached the maximum at 30-40 mM of PGA concentrations. At higher concentrations of PGA more than 40 mM, the qp decreased obviously in a linear way. However, the e.e. of R-MA kept about 95-96.5%, when the PGA concentration was below 60 mM, even at the concentration of PGA 80 mM, the e.e. was still higher than 90%. A modified Monod type equation based on a hypothesis of substrate inhibition could fit the experimental data well. The estimated maximum specific production rate (qpm) and saturation constant (Ks) were 0.431 mmol gdw−1 h−1 and 5.88 mM, respectively.