Kinetic studies on 1:1 electron-transfer reactions involving blue copper proteins. Part 9. Effects of chromium(III) modification on reactions of plastocyanin with [Co(phen)3]3+(phen = 1,10-phenanthroline), [Fe(CN)6]3–, [Co(dipic)2]–(dipic = dipicolinate)(oxidants) and [Ru(NH3)5(py)]2+(py = pyridine)(reductant)

Abstract

A 1:1 chromium(III)-modified form of parsley plastocyanin was obtained by reduction of oxidised protein, here referred to as PCu(II), with Cr2+(aq). At pH 7.5 a 21% decrease in rate constant is observed for the [Co(phen)3]3+ oxidation of PCu(I)·CrIII as compared to native protein. Biphasic kinetics (rate constants k1 and k2) are obtained for the same reaction at pH 5.8 suggesting that at this pH the CrIII is attached in two ways. The rate constant k1 is the same as that observed for native plastocyanin, with k2 some 72% less. On attachment of a second mol of CrIII by the same procedure to give PCu(I)·2CrIII, uniphasic kinetics with a rate constant k2 are observed. Within experimental error no effect of single modification is observed on rate constants for the [Fe(CN)6]3– oxidation at pH 7.5 and 5.8. At pH 5.8 the [Ru(NH3)5(py)]2+(py = pyridine) reduction of PCu(II)·CrIII gives a net 22% decrease, which increases to 38% for PCu(II)·2CrIII. No effect is observed for the [Co(dipic)2]–(dipic = dipicolinate) oxidation of PCu(I)·CrIII also at pH 5.8. These observations support the belief that positively charged complexes [Co(phen)3]3+(phen = 1,10-phenanthroline) and [Ru(NH3)5(py)]2+ react at a broad region on plastocyanin close to Tyr 83 and the negatively charged residues 42–45, whereas [Fe(CN)6]3– and [Co(dipic)2]-react at a site close to His 87. The reduction potential for the PCu(II)–PCu(I) couple (370 mV) is decreased by ca. 20 mV on attachment of CrIII.