Promote the expression and corrected folding of an extremely stable N-demethylase by promoter reconstruction, native environment simulation and surface design

Abstract

Thermomicrobium roseum sarcosine oxidase (TrSOX) was a N-demethylase with specific substrate chiral selectivity, outstanding thermostability and environmental resistance. To promote the expression of TrSOX in Bacillus subtilis W600, the HpaII promoter of pMA5 plasmid was replaced by constitutive or inducible promoters. Through orthogonal experiment, the expression process was optimized, B. subtilis W600 cells containing pMA5-Pxyl-trSOX plasmid were cultivated until OD600nm reached 2.0 and were then induced with 1.6% xylose at 37 °C for 2 h, and the native environment of T. roseum was simulated by heating at 80 °C, with the productivity of TrSOX increased from ~8.3 to ~66.7 μg/g wet cells; and the simulated high temperature was the key switch for the final folding. To reduce the surface hydrophobicity, a S320R mutant was built to form a hydrophilic lid around the entrance of the substrate pocket, and the yield of TrSOX (S320R) was ~163.0 μg/g wet cells, approximately 20 folds as that in the initial expression system. This mutant revealed the similar secondary structure, stability, resistance, chiral substrate selectivity and optimal reaction environment with wild type TrSOX; however, the N-demethylation activities for amino acid derivative substrates were dramatically increased, while those for hydrophobic non-amino acid compounds were repressed.