Regular articleImprovement of the yeast based (R)-phenylacetylcarbinol production process via reduction of by-product formation

- Decreasing NADPH availability led to reduced benzyl alcohol (BZOH) formation.
- Benzoic acid could be recycled by expression of a carboxylic acid reductase.
- Optimised PAC production strain and feeding strategy were developed.
- (R)-phenylacetylcarbinol(PAC)/BZOH ratio could be increased more than four times.

(R)-phenylacetylcarbinol (PAC) is a precursor of the pharmaceutical alkaloid ephedrine. It is produced in biotransformations with the yeast Saccharomyces cerevisiae via condensation of benzaldehyde and fermentation-derived acetaldehyde mediated by pyruvate decarboxylase (Pdc). To improve PAC formation with S. cerevisiae various Pdc isoforms isolated from an industrial PAC producing yeast strain and mutant versions thereof were characterized. However, none of them could improve PAC production indicating that carboligase activity might not be limiting. In industrial processes the maximal yield of PAC is reduced due to the conversion of benzaldehyde to by-products benzyl alcohol and benzoic acid. Expression of a heterologous carboxylic acid reductase could efficiently re-cycle benzoic acid back to benzaldehyde. By reducing NADPH synthesis the formation of benzyl alcohol was significantly diminished. With these modifications the PAC/benzyl alcohol ratio could be increased more than four times compared to the reference strain.