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 (pKa 4.7) and (acidic patch) binding site (pKa′ 5.25) deprotonation/protonation equilibria are identified. For PCu(II) no active site pKa is observed, and the binding site pKa′ 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 pKa′ for PCu(I) and PCu (II) states is discussed.
|Number of pages||5|
|Journal||Journal of the Chemical Society, Dalton Transactions|
|Publication status||Published - 1 Jan 1987|