Ultrasound-assisted d-tartaric acid whole-cell bioconversion by recombinant Escherichia coli

d-Tartaric acid has wide range of application in the pharmaceutical industry and scarcely exists in nature. In this study, cis-epoxysuccinate hydrolase (CESH)-containing Escherichia coli was used to perform whole-cell bioconversion of cis-epoxysuccinate (CES) to D-tartaric acid and the catalytic efficiency was investigated by ultrasound treatment. The bioconversion rate of CES sodium reached 70.36% after 60 min treated after ultrasound, which is 3-fold higher than that in the control. The specific rate could be further improved by 2-fold after 5 repeated batches compared with the first one, however, the specific rate gradually decreased with the increase of repeat batches (>5 batches). The CESH from Bordetella sp. BK-52 was a typical Michaelis-Menten enzyme with V_max and K_m values of 28.17 mM/h/g WCW (wet of cell weight) and 30.18 mM, respectively. The process for the d-tartaric acid bioconversion, which consisted of 102.31 g/L CES sodium, 8.78 mg/mL whole cell and ultrasound power of 79.36 W, is further optimized using response surface methodology. The specific rate finally reached 194.79 ± 1.78 mM/h/g WCW under the optimal conditions. Furthermore, the permeability of inner and outer membrane was improved approximately 1.6 and 1.4-fold after ultrasound treatment, respectively, which may be a crucial factor for improvement of the bioconversion efficiency.