Purification and Characterization of an Arene cis -Dihydrodiol Dehydrogenase Endowed with Broad Substrate Specificity toward Polycyclic Aromatic Hydrocarbon Dihydrodiols

Abstract

ABSTRACT Initial reactions involved in the bacterial degradation of polycyclic aromatic hydrocarbons (PAHs) include a ring-dihydroxylation catalyzed by a dioxygenase and a subsequent oxidation of the dihydrodiol products by a dehydrogenase. In this study, the dihydrodiol dehydrogenase from the PAH-degrading Sphingomonas strain CHY-1 has been characterized. The bphB gene encoding PAH dihydrodiol dehydrogenase (PDDH) was cloned and overexpressed as a His-tagged protein. The recombinant protein was purified as a homotetramer with an apparent M r of 110,000. PDDH oxidized the cis -dihydrodiols derived from biphenyl and eight polycyclic hydrocarbons, including chrysene, benz[ a ]anthracene, and benzo[ a ]pyene, to corresponding catechols. Remarkably, the enzyme oxidized pyrene 4,5-dihydrodiol, whereas pyrene is not metabolized by strain CHY-1. The PAH catechols produced by PDDH rapidly auto-oxidized in air but were regenerated upon reaction of the o -quinones formed with NADH. Kinetic analyses performed under anoxic conditions revealed that the enzyme efficiently utilized two- to four-ring dihydrodiols, with K m values in the range of 1.4 to 7.1 μM, and exhibited a much higher Michaelis constant for NAD + ( K m of 160 μM). At pH 7.0, the specificity constant ranged from (1.3 ± 0.1) × 10 6 M −1 s −1 with benz[ a ]anthracene 1,2-dihydrodiol to (20.0 ± 0.8) × 10 6 M −1 s −1 with naphthalene 1,2-dihydrodiol. The catalytic activity of the enzyme was 13-fold higher at pH 9.5. PDDH was subjected to inhibition by NADH and by 3,4-dihydroxyphenanthrene, and the inhibition patterns suggested that the mechanism of the reaction was ordered Bi Bi. The regulation of PDDH activity appears as a means to prevent the accumulation of PAH catechols in bacterial cells.