PcpA is a hydroquinone ring-cleaving dioxygenase that is homologous to the well-characterized catechol extradiol dioxygenases. 1H-NMR and steady-state kinetics were used to determine the regiospecificity of ring cleavage and the substrate specificity of the enzyme. PcpA is highly specific for 2,6-disubstituted hydroquinones, with halogens greatly preferred at those positions. Monosubstituted hydroquinones show a limited amount of ring cleavage, but rapidly inactivate the enzyme in an O2-dependent fashion. Ortho-disubstituted phenols and certain 3-substituted catechols are competitive inhibitors, which again show the same preference for halogen substituents. EPR studies of the Fe-NO adducts of PcpA with mono- and  disubstituted hydroquinones provide insight into the possible difference in reactivity of these classes of molecules with this enzyme. Halogen bonding and metal-halogen secondary bonding are suggested as possible sources for this enzyme's unusual preference for halogen substituents. A mechanism for PcpA is proposed that is consistent with its unique substrate specificity.

Defining a preference for halogen substituents in the hydroquinone ring-cleaving dioxygenase PcpA

243rd National Meeting (2012)

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