Structure/function correlations among coupled binuclear copper proteins through spectroscopic and reactivity studies of NspF

The terminal step of 4-hydroxy-3-nitrosobenzamide biosynthesis inStreptomyces murayamaensis is performed by NspF, a mono-oxygenase that convertso -aminophenols to the corresponding nitroso product (hydroxyanilinase activity). Previous biochemical characterization of the resting form of NspF suggested that this enzyme belonged to the coupled binuclear copper enzyme (CBC) family. Another member of this enzyme family, tyrosinase, is able to mono-oxygenate monophenols (monophenolase activity) but noto -aminophenols. To gain insight into the unique reactivity of NspF, we have generated and characterized the oxy form of its active site. The observation of spectral features identical to those of oxy-tyrosinase indicates that oxy-NspF contains a Cu2 O2 core where peroxide is coordinated in aμ -η 2 ∶ η 2 mode, confirming that NspF is a CBC enzyme. This oxy form is found to react with monophenols, indicating that, like tyrosinase, NspF also possesses monophenolase activity. A comparison of the two electrophilic mechanisms for the monophenolase and hydroxyanilinase activity indicates a large geometric change between their respective transition states. The potential for specific interactions between the protein pocket and the substrate in each transition state is discussed within the context of the differential reactivity of this family of enzymes with equivalentμ -η 2 ∶η 2 peroxy bridged coupled binuclear copper active sites.