Escherichia coli cells expressing Rhodococcus DK17 o-xylene dioxygenase genes were used for bioconversion of m-xylene. Gas chromatography-mass spectrometry analysis of the oxidation products detected 3-methylbenzylalcohol and 2,4-dimethylphenol in the ratio 9:1. Molecular modeling suggests that o-xylene dioxygenase can hold xylene isomers at a kink region between α6 and α7 helices of the active site and α9 helix covers the substrates. m-Xylene is unlikely to locate at the active site with a methyl group facing the kink region because this configuration would not fit within the substrate-binding pocket. The m-xylene molecule can flip horizontally to expose the meta-position methyl group to the catalytic motif. In this configuration, 3-methylbenzylalcohol could be formed, presumably due to the meta effect. Alternatively, the m-xylene molecule can rotate counterclockwise, allowing the catalytic motif to hydroxylate at C-4 yielding 2,4-dimethylphenol. Site-directed mutagenesis combined with structural and functional analyses suggests that the alanine-218 and the aspartic acid-262 in the α7 and the α9 helices play an important role in positioning m-xylene, respectively.
Bibliographical noteFunding Information:
Acknowledgment This work was supported by a grant from the Ministry of Education, Science and Technology, of the Republic of Korea through the 21C Frontier Microbial Genomics and Applications Center Program and also by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (2009-0079296). DK acknowledges the support of the Korea Polar Research Institute under project PE09050. KC is a recipient of the Brain Korea 21 scholarship.
All Science Journal Classification (ASJC) codes
- Applied Microbiology and Biotechnology