Electron Transfer Reactions of Cytochrome ƒ from Brassica komatsuna with Hexacyanoferrate

Abstract

The electron transfer reactions of membrane-bound monomeric cytochrome ƒ 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 ƒ by Fe(CN)₆^3- at pH 7.0, μ 0.1M, and 20°C is 1.7×10⁵, M^-1•S^-1, which is similar to the value of oligomeric cytochrome ƒ from parsley. The activation parameters obtained were ΔH_??_=-0.87 kcal/mol and ΔS_??_-=38 cal/mol•deg. Respective rate constant and activation parameters obtained for the reduction of cytochrome ƒ by Fe(CN)₆^4- were k=1.7×10⁴ 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 ƒ markedly decreased with increasing ionic strength. The results indicate that the oxidation and the reduction take place at a positively charged site on the cytochrome ƒ surface, and electrostatic interactions are important for these reactions. The participation of protons and specific amino acid residues in electron transfer reactions of cytochrome ƒ is implied from the pH results. Alkaline isomerization of ferricytochrome ƒ was not observed. The midpoint potential of cytochrome ƒ 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.