Interpretation of the spectra observed during oxidation of p-hydroxybenzoate hydroxylase reconstituted with modified flavins

Oxidation of reduced p-hydroxybenzoate hydroxylase by oxygen in the presence of 2,4-dihydroxybenzoate and azide proceeds via three well established intermediates. Reconstitution of the apoprotein with either 8-thiophenyl-FAD, 8-fluoro-FAD, 8-chloro-FAD, or 8-sulfonyl-FAD does not alter this sequence of events. However, the peak positions of the intermediate spectra are somewhat shifted relative to those of native enzyme. Comparison of the spectra for intermediates II and III leads to the conclusion that the spectrum for intermediate II is a composite. One component is the spectrum of an intermediate III-like species, and the other appears to be related to the substrate. The substrate component is pH-dependent, having an absorbance maximum of 386 nm (extinction, ~6,000 M^-1 cm^-1) at pH 6.6 which shifts to ~430 nm (extinction, ~11-13,000 M^-1 cm^-1) at pH 9.2, with a pK of 7.9. The pH dependence for the spectrum of the substrate component combined with the pH independence of the intermediate III-like spectrum satisfactorily accounts for the pH dependence observed for intermediate II, including the fact that the high pH spectrum of native intermediate II is qualitatively quite different from that of 8-sulfonyl-FAD intermediate II.