Laser-flash-photolysis studies of p-cresol methylhydroxylase. Electron-transfer properties of the flavin and haem components

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Abstract
P-Cresol methylhydroxylase, a heterodimer consisting of 1 flavoprotein subunit and 1 cytochrome c subunit, may be resolved into its subunits, and the holoenzyme may then be fully reconstituted from the pure subunits. The reduction kinetics of the intact Pseudomonas putida enzyme and its subunits were studied by using exogenous 5-deazariboflavin semiquinone radical generated in the presence of EDTA by the laser-flash-photolysis technique. Under anaerobic conditions the 5-deazariboflavin semiquinone radical reacts rapidly with the native enzyme with a rate constant approaching that of a diffusion-controlled reaction (k = 2.8 .times. 109 M-1 .cntdot. s-1). Time-resolved difference spectra at pH 7.6 indicate that both flavin and heme are reduced initially by the deazariboflavin semiquinone radical, followed by an additional slower intramolecular electron transfer (k = 220 s-1) from the endogenous neutral flavin semiquinone radical to the oxidized heme moiety of the native enzyme. During the steady-state photochemical titration of the native enzyme at pH 7.6 with deazariboflavin semiquinone radical generated by light-irradiation the heme appeared to be reduced before the protein-bound flavin and was followed by the formation of the protein-bound anionic flavin radical. This result suggests that the redox potential of the heme is higher than that of the flavin, and that deprotonation of the flavin neutral radical occurred during the photochemical titration. Reduction kinetics of the flavoprotein and cytochrome subunits were also investigated by laser-flash photolysis. The protein-bound flavin of the isolated flavin subunit was reduced rapidly by the deazariboflavin semiquinone radical (k = 2.2 .times. 109 M-1 .cntdot. s-1), as was the heme of the pure cytochrome c subunit (k = 3.7 .times. 109 M-1 .cntdot. s-1). Flash-induced difference spectra obtained for the flavoprotein and cytochrome subunits at pH 7.6 were consistent with the formation of neutral flavin semiquinone radical and reduced heme, respectively. Investigation of the kinetic properties of the neutral flavin semiquinone radical of the flavoprotein subunit at pH 7.6 and at longer times (up to 5s) were consistent with a slow 1st-order deprotonation reaction (k = 1 s-1) of the neutral radical to its anionic form.