Identification of a serine hydrolase as a key determinant in the microbial degradation of polychlorinated biphenyls.
The ability of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPDA) hydrolase (BphD) of Burkholderia cepacia LB400 to hydrolyze polychlorinated biphenyl (PCB) metabolites was assessed by determining its specificity for monochlorinated HOPDAs. The relative specificities of BphD for HOPDAs bearing chlorine substituents on the phenyl moiety were 0.28, 0.38, and 1.1 for 8-Cl, 9-Cl, and 10-Cl HOPDA, respectively, versus HOPDA (100 mm phosphate, pH 7.5, 25 degrees C). In contrast, HOPDAs bearing chlorine substituents on the dienoate moiety were poor substrates for BphD, which hydrolyzed 3-Cl, 4-Cl, and 5-Cl HOPDA at relative maximal rates of 2.1 x 10(-3), 1.4 x 10(-4), and 0.36, respectively, versus HOPDA. The enzymatic transformation of 3-, 5-, 8-, 9-, and 10-Cl HOPDAs yielded stoichiometric quantities of the corresponding benzoate, indicating that BphD catalyzes the hydrolysis of these HOPDAs in the same manner as unchlorinated HOPDA. HOPDAs also underwent a nonenzymatic transformation to products that included acetophenone. In the case of 4-Cl HOPDA, this transformation proceeded via the formation of 4-OH HOPDA (t(12) = 2.8 h; 100 mm phosphate, pH 7.5, 25 degrees C). 3-Cl HOPDA (t(12) = 504 h) was almost 3 times more stable than 4-OH HOPDA. Finally, 3-Cl, 4-Cl and 4-OH HOPDAs competitively inhibited the BphD-catalyzed hydrolysis of HOPDA (K(ic) values of 0.57 +/- 0. 04, 3.6 +/- 0.2, and 0.95 +/- 0.04 microm, respectively). These results explain the accumulation of HOPDAs and chloroacetophenones in the microbial degradation of certain PCB congeners. More significantly, they indicate that in the degradation of PCB mixtures, BphD would be inhibited, thereby slowing the mineralization of all congeners. BphD is thus a key determinant in the aerobic microbial degradation of PCBs.
Seah
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Department of Biochemistry, Pavillon Marchand, Université Laval, Quebec City, Quebec G1K 7P4, Canada.
Labbé
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Nerdinger
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Johnson
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MR
Snieckus
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Eltis
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LD
D001426:Bacterial Proteins / Q000378:metabolism*; D001673:Biodegradation, Environmental; D016956:Burkholderia cepacia / Q000201:enzymology*; D017606:Chlorine Compounds / Q000378:metabolism; D005231:Fatty Acids, Unsaturated / Q000378:metabolism; D006867:Hydrolases / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D011078:Polychlorinated Biphenyls / Q000378:metabolism*; D013053:Spectrophotometry; D013379:Substrate Specificity
D001426:Bacterial Proteins; D017606:Chlorine Compounds; D005231:Fatty Acids, Unsaturated; C018971:2-hydroxy-6-oxo-6-phenyl-2,4-hexadienoic acid; D011078:Polychlorinated Biphenyls; D006867:Hydrolases; C062907:2,6-dioxo-6-phenylhexa-3-enoate hydrolase