Biocatalytic synthesis of ethyl (R)-2-hydroxy-4-phenylbutyrate with a newly isolated Rhodotorula mucilaginosa CCZU-G5inanaqueous/organic biphasic system

Abstract

Background: Optically active ethyl (R)-2-hydroxy-4-phenylbutyrate [(R)-HPBE] is an important chiral building block for the synthesis of angiotensin-converting enzyme (ACE) inhibitors. It is reported that microbial or enzymatic reduction of ethyl 2-oxo-4-phenyl-butyrate (OPBE) is an attractive way to produce optically active (R)-HPBE. Results: The asymmetric reduction of OPBE to synthesize optically active (R)-HPBE with a newly isolated Rhodotorula mucilaginosa CCZU-G5 as catalyst was investigated in an aqueous/organic solvent biphasic system. R. mucilaginosa CCZU-G5 showed a good tolerance (the metabolic activity retention >80%) in the biphasic system composed of aqueous buffer and organic solvent with a log P value over 4.6. Isooctane was found to be the most suitable organic phase solvent. In the biphasic system, the volumetric phase ratio, OPBE concentration, cell concentration, reaction temperature, and buffer pH were optimized. Under the optimum conditions (volumetric phase ratio: 1/1, OPBE concentration: 100 mM, cell concentration: 0.075 g/mL, pH 7.5, 35°C), the final yield and the optical purity of (R)-HPBE reached 98.3% and >99.0% enantiomeric excess (ee), respectively, after 12 h of reaction. Conclusions: All the results suggested that the OPBE-reducing enzymes in a newly isolated R. mucilaginosa cells possess highly stable and excellent stereoselectivity by establishing an aqueous/organic biphasic system.