Enzymatic resolution of racemic phenyloxirane by a novel epoxide hydrolase from Aspergillus niger SQ-6 and its fed-batch fermentation
2006
Liu, Yanbin | Sha, Qian | Wu, Sheng | Wang, Jianjun | Yang, Liu | Sun, Wanru
A microorganism with the ability to catalyze the resolution of racemic phenyloxirane was isolated and identified as Aspergillus niger SQ-6. Chiral capillary electrophoresis was successfully applied to separate both phenyloxirane and phenylethanediol. The epoxide hydrolase (EH) involved in this resolution process was (R)-stereospecific and constitutively expressed. When whole cells were used during the biotransformation process, the optimum temperature and pH for stereospecific vicinal diol production were 35°C and 7.0, respectively. After a 24-h conversion, the enantiomer excess of (R)-phenylethanediol produced was found to be >99%, with a conversion rate of 56%. In fed-batch fermentations at 30°C for 44 h, glycerol (20 g L-¹) and corn steep liquor (CSL) (30 g L-¹) were chosen as the best initial carbon and nitrogen sources, and EH production was markedly improved by pulsed feeding of sucrose (2 g L-¹ h-¹) and continuous feeding of CSL (1 g L-¹ h-¹) at a fermentation time of 28 h. After optimization, the maximum dry cell weight achieved was 24.5±0.8 g L-¹; maximum EH production was 351.2±13.1 U L-¹ with a specific activity of 14.3±0.5 U g-¹. Partially purified EH exhibited a temperature optimum at 37°C and pH optimum at 7.5 in 0.1 M phosphate buffer. This study presents the first evidence for the existence of a predicted epoxide racemase, which might be important in the synthesis of epoxide intermediates.
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