Role of π-Electron Systems in Stabilization of the Oxidized Tetraheme Architecture in Cytochrome c3

Cytochrome c(3) possesses four hemes in a compact configuration and exhibits extremely low reduction potentials. Besides the well-characterized factors contributing to the reduction potentials, the aromatic rings have been suggested to be involved. To elucidate its mechanism, the effect of mutations at conserved and noncoordinated aromatic residues on the reduction/oxidation properties of heme irons was investigated on the basis of crystal structures, NMR spectra of coordinated His, and reduction potentials. Phe20 parallel to heme 1 is the most conserved residue. On its mutation, the change in each coordination structure was subtle in the crystal structures despite of significant changes in the NMR spectra and reduction potentials. Significant increases in the reduction potentials of heme I led to the conclusion that the aromatic ring of Phe20 stabilizes the polarization of the pi-electron density induced by the oxidized iron in the same heme. Furthermore, the reduction potential of heme 2 was also affected by the mutation at Phe20, revealing that the electrostatic interaction between the pi-electron system of the porphyrin and a distant iron contributes to the iron reduction potential. This kind of interaction provides new insight into the role of the heme architecture in regulation of the reduction potentials.