Interheme Electron Transfer in cytochrome C_3 and The Role of Aromatic Amino Acids.

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Cytochrome c_3 is a tetraheme protein that participates in the electron-transfer system involved in the metabolism of sulfur compounds and hydrogen in the sulfate reducing bacteria. This protein is of great interest not only from a biological point of view but also because of its physicochemical properties. In this project, the mechanism of interheme electron transfer of cytochrome c_3 from D. vulgaris Miyazaki F was investigated with a special enphasis on the roles of aromatic amino acid residues. Since the crystal structure of cytochrome c_3 is established, nuclear magnetic resonance was used for this investigation with close refference to the crystal structure. In the first place, the proton signals due to the aromatic residues were identified by amino acid-selective deuterataion of the protein in vivo. To carry out the deuteration, a chemical medium for the growth of the sulfate reducing bacteria was developed. Most of the aromatic signals were successfully identified by this method. Furthermore, many of them were site-specifically assigned by the nuclear Overhauser effect experiments and two-dimensional NMR. On the basis of this assignment, the role of some aromatic residues in the electron transfer was partially elucidated. One of the fundamental parameters in the electron transfer is the redox potential of a redox site. A method to estimate the 32 microscopic redox potentials of cytochrome c_3 on the basis of NMR and electrochemical experiments were developed, leading to the successful estimation of the 32 redox potentials. Furthermore, the microscopic potentials were ascribed to specifichemes in the crystal structure on the basis of the assignment of the heme methyl signals. This would provide the firm basis for the further development in the elucidation of the mechanism of electron transfer in terms of physicochemical and structural factors.