Issue 5, 1994

Rate constants for intramolecular electron-transfer reactions of ruthenium-modified histidine mutants of cytochrome b2

Abstract

The cytochrome b2 core derived from flavocytochrome b2 has been expressed in Escherichia coli and four mutants Lys-56→His, Lys-51→His, Asn-42→His and Pro-41→His have been generated by site-directed mutagenesis. Ruthenium modification of the first three mutants with [Ru(NH3)5(H2O)]2+ led to the formation of singly modified His-56[Ru(NH3)5], His-51 [Ru(NH3)5] and His-42[Ru(NH3)5] derivatives in 30–45% yield. However no modification was observed with Pro41His. Moreover no evidence was obtained for the formation of His-19[Ru(NH3)5], where His-19 is the unco-ordinated histidine present in wild-type and mutant cytochrome b2 forms. Characterisation of the singly modified products by inductively coupled plasma analyses indicates an Fe : Ru ratio of 1 : 1 for all three derivatives. The NMR spectra of the Ru-modified proteins reveal specific broadening by the paramagnetic RuIII of the characteristically sharp CεH and CδH resonances assigned to His-56, His-51 and His-42, while the CεH resonance due to His-19 is unaffected. Whereas the titration of unmodified His-56, His-51, His-42 and His-41 residues by 1H NMR spectroscopy gave pKa 6.4, 6.2, 6.5 and 6.4 respectively, His-19 did not similarly titrate in the pH 4.8–10.0 range, which is attributed to hydrogen bonding to a nearby residue. Using pulse radiolysis to generate the methyl viologen radical, MV˙+, the metastable iron(II)ruthenium(III) form of the protein was obtained. Rate constants for intramolecular electron transfer from FeII to RuIII were determined; 3.5 (His-56), 2.8 (His-51) and 78 s–1(His-42) at I= 0.100 M. Edge-to-edge distances, from the nearest point on one or other of the axial haem ligands to the nearest point on the imidazole ring, are 10.8 (His-56), 9.8 (His-51) and 8.5 Å(His-42), and the driving force is close to 145 mV. Using the Beratan–Onuchic approach the most favourable pathways for electron transfer have been identified. For His-42 a direct through-bond pathway via axial His-43 to the haem Fe is indicated. In the case of the His-56 and His-51 derivatives, pathways that include through-space interactions appear to be dominant.

Article information

Article type
Paper

J. Chem. Soc., Dalton Trans., 1994, 675-681

Rate constants for intramolecular electron-transfer reactions of ruthenium-modified histidine mutants of cytochrome b2

E. Lloyd, K. Chapman, S. K. Chapman, Z. Jia, M. Lim, N. P. Tomkinson, G. A. Salmon and A. G. Sykes, J. Chem. Soc., Dalton Trans., 1994, 675 DOI: 10.1039/DT9940000675

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