Kinetic studies on 1 : 1 electron-transfer reactions involving blue copper proteins. Part 6. Competitive inhibition of the [Co(phen)3]3+(phen = 1,10-phenanthroline) oxidation of parsley plastocyanin PCu(I) by redox-inactive complexes

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Abstract
A search has been carried out for redox-inactive inorganic complexes which exhibit competitive inhibition (i.e. blocking) of the [Co(phen)3]3+(phen = 1,10-phenanthroline) oxidation of negatively charged parsley plastocyanin, PCu(I), at 25 °C and I= 0.10 M (NaCl). Three complexes have been identified which give greater association (KB/M–1) with PCu(I) than that previously determined for [Cr(phen)3]3+(176 M–1). Association constants (M–1) obtained at pH 7.5 are: [Co(NH3)3]3+, 470; [Pt(NH3)6]4+, 22 300; and [(NH3)5Co(NH2)Co(NH3)5]5+, 16 000. Acid dissociation of [Pt(NH3)6]4+, pKa 7.1, requires that a lower pH be used for maximum effectiveness of this complex. Partial blocking only is observed in all three cases, and it is concluded that the site for electron transfer involves a broad region of protein surface incorporating Tyr 83 and the negative patch of residues including 42–45. The size of the effect for the 4+ and 5+ complexes makes these complexes most appropriate for further studies in which use of this site is tested for. It has been demonstrated with [Pt(NH3)6]4+ that association with PCu(I) becomes less effective at pH < 6.6. The protein acid dissociation pKa of 5.8 which is obtained is coincident with pKa values reported previously for the inactivation of PCu(I). A possible explanation involving a co-operative effect between the active site and negative patch 42–45 is proposed. No similar competitive inhibition of the [Fe(CN)6]3– oxidation of PCu(I) by redox-inactive complexes [Zr(C2O4)4]4– and [Mo(CN)8]4– is observed at pH 7.5.

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