pcpB

PcpB is the FAD-dependent pentachlorophenol 4-monooxygenase (PCP hydroxylase) of Sphingobium chlorophenolicum, a dimeric member of the PheA/TfdB family of flavin-dependent phenol hydroxylases. Using NADPH and O2 it converts pentachlorophenol to 2,3,5,6-tetrachloro-1,4-benzoquinone with release of chloride, the first step of PCP degradation. It also acts on other para-substituted phenols, removing hydrogen, nitro, amino and cyano groups. The enzyme turns over slowly, with a kcat for PCP of about 0.024 per second, it uncouples extensively so that the C4a-hydroxyflavin intermediate decomposes to H2O2 in a futile cycle consuming NADPH, and it acts promiscuously on the downstream metabolite tetrachlorohydroquinone, which reverses pathway flux. PcpB is therefore the rate-limiting step of a pathway that appears to have been assembled recently by patching horizontally acquired enzymes into existing metabolism. The slow turnover is not simply a lack of optimization: because the product tetrachlorobenzoquinone is a potent alkylating agent, slow release may itself be selected, allowing the TCBQ reductase PcpD to sequester and reduce the intermediate before it escapes the active site. Its substrate range extends to other para-substituted phenols, including p-nitrophenol, p-nitrocatechol and 2,4-dinitrophenol for a near-identical ortholog, with specificity for the para position relative to the phenolic hydroxyl.