A stereoselective esterase from Bacillus megaterium: Purification, gene cloning, expression and catalytic properties

Esterases (EC 3.1.1.X) have been used as biocatalysts due to their good stability, high chemo-, regio- and stereoselectivity. In our previous studies, Bacillus megaterium WZ009 harboring esterase displayed the unique capability to convert (S)-4-Chloro-3-hydroxyethylbutyrate (CHBE) in the racemate to (S)-3-hydroxy-γ-butyrolactone (HL) through stereoselective hydrolysis, dechlorination, and lactonization. The remaining (R)-CHBE and formed (S)-HL could be obtained in a one-pot enzymatic reaction. An esterase from B. megaterium WZ009 was purified and was found to have 466 encoded amino acids and an apparent molecular mass of 55 kDa. The purified esterase exhibited maximal activity at a temperature of 25 °C and at a pH of 11.5 towards 100 mM CHBE. When the stereoselective biocatalysis of rac-CHBE was performed using the recombinant Escherichia coli BL21 (DH3) cells harboring the esterase, the catalytic activity increased by 20-fold compared with the original strain B. megaterium WZ009. With the addition of activated carbon (62 g/L) in the reaction system, the conversion was increased from 39% to 45% at a substrate concentration of 750 mM. Another remarkable advantage is that both of the obtained residual (R)-CHBE and the formed (S)-HL had high optical purities (e.e._s > 99.9%, e.e._p > 99.9%), thereby making this esterase a usable biocatalyst for industrial application.