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Initial reactions involved in the bacterial degradation of polycyclicaromatic hydrocarbons (PAHs) include a ring-dihydroxylation catalyzed by adioxygenase and a subsequent oxidation of the dihydrodiol products by adehydrogenase. In this study, the dihydrodiol dehydrogenase from thePAH-degrading Sphingomonas strain CHY-1 has been characterized. The bphB geneencoding PAH dihydrodiol dehydrogenase (PDDH) was cloned and overexpressed as aHis-tagged protein. The recombinant protein was purified as a homotetramer withan apparent Mr of 110,000. PDDH oxidized the cis-dihydrodiols derived frombiphenyl 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 metabolizedby strain CHY-1. The PAH catechols produced by PDDH rapidly auto-oxidized inair but were regenerated upon reaction of the o-quinones formed with NADH.Kinetic analyses performed under anoxic conditions revealed that the enzymeefficiently utilized two- to four-ring dihydrodiols, with Km values in therange of 1.4 to 7.1 $\mu$M, and exhibited a much higher Michaelis constant forNAD+ (Km of 160 $\mu$M). At pH 7.0, the specificity constant ranged from (1.3$\pm$ 0.1) x 106 M?1 s?1 with benz[a]anthracene 1,2-dihydrodiol to (20.0 $\pm$0.8) x 106 M?1 s?1 with naphthalene 1,2-dihydrodiol. The catalytic activity ofthe enzyme was 13-fold higher at pH 9.5. PDDH was subjected to inhibition byNADH and by 3,4-dihydroxyphenanthrene, and the inhibition patterns suggestedthat the mechanism of the reaction was ordered Bi Bi. The regulation of PDDHactivity appears as a means to prevent the accumulation of PAH catechols inbacterial cells.

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