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Biophys J, September 1998, p. 1483-1490, Vol. 75, No. 3

Ionic Strength-Dependent Physicochemical Factors in Cytochrome c3 Regulating the Electron Transfer Rate

Tomoaki Ohmura,* Haruki Nakamura,# Katsumi Niki,§ Michael A. Cusanovich, and Hideo Akutsu§

 *Advanced Technology Research Center, Mitsubishi Heavy Industries, Ltd., Yokohama 236-8515, Japan;  #Biomolecular Engineering Research Institute, Suita 565-0874, Japan;  §Department of Chemistry and Biotechnology, Yokohama National University, Yokohama 240-8501, Japan; and  Department of Biochemistry, University of Arizona, Tucson, Arizona 85721 USA

The effect of ionic strength on the macroscopic and microscopic redox potentials and the heme environment of cytochrome c3 from Desulfovibrio vulgaris Miyazaki F have been investigated by NMR and electrochemical methods. The redox potentials of this tetraheme protein are found to be ionic strength-dependent. Especially, the microscopic redox potentials of hemes 2 and 3 at the fourth reduction step increase significantly with increasing ionic strength, which is in contradiction to the theoretical expectation. The coordinated imidazole proton signals are unaffected by ionic strength. However, the methyl and propionate proton signals of hemes 1 and 4 showed significant ionic strength dependencies that are distinct from those for hemes 2 and 3. This heme classification is the same as that found in the ionic strength dependencies of the microscopic redox potentials at the fourth reduction step. Furthermore, the effect of ionic strength on the electrostatic potentials at the heme irons has been examined on the theoretical basis. The electrostatic potential at heme 4 changes up to 1 M ionic strength, which was not expected from the observations reported on cytochromes so far. These results are discussed in connection with the reported anomalous ionic strength dependency of the reduction rate of cytochrome c3.

Biophys J, September 1998, p. 1483-1490, Vol. 75, No. 3
© 1998 by the Biophysical Society   0006-3495/98/09/1483/08  $2.00


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