Kinetic studies on 1 : 1 electron-transfer reactions involving blue copper proteins. Part 7. Effects of pH and redox-inactive [Pt(NH3)6]4+ on reactions of parsley plastocyanin with different inorganic redox partners
Abstract
The effect of pH on the reduction of parsley plastocyanin, PCu(II), by a positively charged complex, [Ru(NH3)5(py)]2+(py = pyridine), has been studied for the first time. Protonation at the negative patch of the protein, which includes residues 42–45, is proposed as an explanation of the 60%(maximum) decrease in rate constant, pKa 5.0. Inactivation of PCu(I) by H+ is apparent with the complexes [Co(bipy)2(O2CMe)2]+(bipy = 2,2′-bipyridine) and [Co(dipic)2]–[dipic = dipicolinate (pyridine-2,6-dicarboxylate)] as oxidants, as noted previously for [Co(phen)3](phen = 1,10-phenanthroline), [Co(bipy)3]3+, and [Fe(CN)6]3–(pKa 5.7–6.1) and assigned as an H+-induced change effective at the copper active site. The strong competitive inhibition exhibited by redox-inactive [Pt(NH3)6]4+ on the PCu(I)+[Co(phen)3]3+ reaction is tested further at pH 5.8 using other oxidants, [Co(bipy)3]3+, [Ru(NH3)5(py)]3+, [Co(bipy)2(O2CMe)2]+, and [Co(dipic)2]–. Partial inhibition of all three 3+ complexes is observed (53% maximum effect at high inhibitor concentrations), and of the 1+ complex (25% decrease) consistent with these complexes interacting at the negative patch on the protein incorporating Tyr 83. The small ca. 8% acceleration observed with [Co(dipic)2]– does not lead to a clear-cut interpretation. The reduction of PCu(II) by [Ru(NH3)5(py)]2+ is inhibited by [Pt(NH3)6]4+(40% decrease) clearly designating the Tyr 83 locality as the dominant site for reaction, and not as previously supposed the His 87 site.
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