Kinetic studies on 1 : 1 electron-transfer reactions involving blue copper proteins. Part 14. Reactions of poplar plastocyanin with inorganic complexes

Abstract

The reactions of poplar plastocyanin, PCu(I), with [Fe(CN)₆]^3– and [Co(phen)₃]³⁺(phen = 1,10-phenanthroline) as oxidants, and of PCu(II) with [Co(phen)₃]²⁺ as reductant, have been studied at 25 °C, I= 0.10 M (NaCl). At pH 7.5, 69% of the reaction of [Co(phen)₃]³⁺ is at the Tyr 83 site, which incorporates the acidic patch, in this case at 42–44. With [Fe(CN)₆]^3– as oxidant the behaviour observed is consistent with reaction at the His 87 (north) site. From the effects of pH (4.5–7.5) on rate constants, active site (pK_a 4.7) and (acidic patch) binding site (pK_a′ 5.25) deprotonation/protonation equilibria are identified. For PCu(II) no active site pK_a is observed, and the binding site pK_a′ is 5.1. Comparisons are made with plastocyanin from parsley, spinach, and French bean. With [Co(phen)₃]³⁺ as oxidant no limiting kinetic behaviour is observed, and association of PCu(I) with [Co(phen)₃]³⁺ is at a lower level (K < 50 M^–1). From competitive inhibition studies association with redox inactive [(NH₃)₅Co(NH₂)Co(NH₃)₅]⁵⁺ is also at a lower level (K= 5 650 M^–1). The origin of differences in pK_a′ for PCu(I) and PCu (II) states is discussed.