Comparative Specificities of Two Evolutionarily Divergent Hydrolases Involved in Microbial Degradation of Polychlorinated Biphenyls

Abstract

2-Hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPDA) hydrolase (BphD) is a key determinant in the aerobic transformation of polychlorinated biphenyls (PCBs) by Burkholderia sp. strain LB400 (S. Y. K. Seah, G. Labbé, S. Nerdinger, M. Johnson, V. Snieckus, and L. D. Eltis, J. Biol. Chem. 275:15701–15708, 2000). To determine whether this is also true in divergent biphenyl degraders, the homologous hydrolase of Rhodococcus globerulus P6, BphD _P6 , was hyperexpressed, purified to apparent homogeneity, and studied by steady-state kinetics. BphD _P6 hydrolyzed HOPDA with a k _cat / K _m of 1.62 (± 0.03) × 10 ⁷ M ⁻¹ s ⁻¹ (100 mM phosphate [pH 7.5], 25°C), which is within 70% of that of BphD _LB400 . BphD _P6 was also similar to BphD _LB400 in that it catalyzed the hydrolysis of HOPDAs bearing chloro substituents on the phenyl moiety at least 25 times more specifically than those bearing chloro substituents on the dienoate moiety. However, the rhodococcal enzyme was significantly more specific for 9-Cl and 10-Cl HOPDAs, catalyzing the hydrolysis of 9-Cl, 10-Cl, and 9,10-diCl HOPDAs two- to threefold respectively, more specifically than HOPDA. Moreover, 4-Cl HOPDA competitively inhibited BphD _P6 more effectively than 3-Cl HOPDA, which is the inverse of what was observed in BphD _LB400 . These results demonstrate that BphD is a key determinant in the aerobic transformation of PCBs by divergent biphenyl degraders, but that there exists significant diversity in the specificity of these biphenyl hydrolases.