Estimation of microscopic redox potentials of a tetraheme protein, cytochrome c3 of Desulfovibrio vulgaris, Miyazaki F, and partial assignments of heme groups.

The microscopic formal redox potentials of a tetraheme protein, cytochrome c3 from Desulfovibrio vulgaris, Miyazaki F, were estimated from the chemical shifts of the heme methyl signals in its 1H NMR spectrum. All chemical shifts in the five macroscopic oxidation states were determined for eight of the heme methyl protons by the saturation-transfer method. The electron-distribution probability at each heme in each oxidation state was estimated directly from the chemical shifts. To minimize errors due to interheme pseudocontact contributions, the average electron-distribution probability was used for calculation of the microscopic formal redox potentials. By introducing interacting potentials, 32 parameters were reduced to 10. The 10 parameters were determined analytically from the 9 independent electron-distribution probabilities and 2 macroscopic formal redox potentials. The results showed the presence of a strong positive interaction between a pair of particular hemes. The microscopic formal redox potential changes dramatically with the extent of reduction because of the intramolecular interheme interactions. NMR signals of two hemes were assigned to particular hemes in the crystal structures by nuclear Overhauser effect experiments. The results showed that the hemes with the highest and lowest redox potentials in the one-electron reduction process correspond to hemes I and IV in the crystal structure.