CHARACTERIZATION OF A C-C BOND HYDROLASE FROM Sphingomonas wittichii RW1 WITH NOVEL SPECIFICITIES TOWARDS PCB METABOLITES

Department of Biochemistry, Université Laval, Québec City, Canada; Department of Microbiology and Immunology, Life Sciences Institute, University of British Columbia, Vancouver, Canada; Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada; Department of Microbiology, Immunology & Molecular Genetics, University of Kentucky, Lexington, KY 40536, USA

^* To whom correspondence should be addressed. Email: leltis{at}interchange.ubc.ca.

	Abstract

Sphingomonas wittichii RW1 degrades chlorinated dibenzofurans and dibenzo-p-dioxins via meta-cleavage. We used inverse PCR to amplify dxnB2, a gene encoding one of three meta-cleavage product (MCP) hydrolases identified in the organism that are homologues of BphD involved in biphenyl catabolism. Purified DxnB2 catalyzed the hydrolysis of 8-OH HOPDA 6-times faster than HOPDA at saturating substrate concentrations. Moreover, the specificity of DxnB2 for 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPDA; k_cat/K_m = 1.2 x 10⁷ M^-1s^-1) was about half that of the BphDs of Burkholderia xenovorans LB400 and Rhodococcus globerulus P6, two potent polychlorinated biphenyl (PCB)-degrading strains. Interestingly, DxnB2 transformed 3-Cl and 4-OH HOPDAs, compounds that inhibit the BphDs and limit PCB degradation. DxnB2 had higher specificity for 9-Cl HOPDA than for HOPDA, but lower specificity for 8-Cl HOPDA (k_cat/K_m = 1.7 x 10⁶ M^-1s^-1), the chlorinated analog of 8-OH HOPDA produced during dibenzofuran catabolism. Phylogenetic analyses based on structure-guided sequence alignment revealed that DxnB2 belongs to a previously unrecognized class of MCP hydrolases, evolutionarily divergent from the BphDs although the physiological substrates of both enzyme types are HOPDAs. However, both classes of enzymes have mainly small hydrophobic residues lining the sub-site that binds the C6 phenyl of HOPDA in contrast to the bulky hydrophobic residues (Phe106, Phe135, Trp150 and Phe197) found in the Class II enzymes that prefer substrates possessing a C6 alkyl. Thr196 and/or Asn203 appear to be important determinants of specificity for DxnB2, potentially forming hydrogen bonds with the 8-OH substituent. This study demonstrates that the substrate specificities of evolutionarily divergent hydrolases may be useful for degrading mixtures of pollutants such as PCBs.