Electron Transfer Reactions of Cytochrome f from Brassica komatsuna with Hexacyanoferrate1

Abstract

The electron transfer reactions of membrane-bound monomeric cytochrome f from Brassica komatsuna (Brassica rapa L. var. perviridis Bailey) with hexacyanoferrate (II}-(III) have been studied as a function of pH, ionic strength and temperature. The second-order rate constant for the oxidation of cytochrome f by Fe(CN)₆³⁻ at pH 7.0, μ 0.1 M, and 20°C is 1.7 × 10⁵ M⁻¹ S⁻¹ which is similar to the value of oligomeric cytochrome f from parsley. The activation parameters obtained were ΔH= –l0.87 kcal/mol and ΔS°= –38 cal/mol.deg. Respective rate constant and activation parameters obtained for the reduction of cytochrome f by Fe(CN)₅⁴ were k = 1.7 × 10 M⁻¹ S⁻¹ ΔH* = +6.7 kcal/mol, and ΔS = −16 cal/mol . deg. Both the rate constants for the oxidation and the reduction of cytochrome f markedly de creased with increasing ionic strength. The results indicate that the oxidation and the reduction take place at a positively charged site on the cytochrome f surface, and electrostatic interactions are important for these reactions. The participation of protons and specific amino acid residues in electron transfer reactions of cytochrome f is implied from the pH results. Alkaline isomerization of ferricytochrome f was not observed. The midpoint potential of cytochrome f has a constant value of 360 mV between pH 5.0–8.9, and decreases by about 55 mV per pH unit above 8.9. The results are compared with the data for horse heart cytochrome c and Euglena gracilis cytochrome c-552. These data are discussed in relation to the theories of electrostatic corrected outer-sphere electron transfer of Marcus and multiphonon nonadiabatic electron tunneling of Jortner and Hopfield.