To characterize the protein-protein interaction during electron transfer, we used Zn-substituted cytochrome c (ZnCytc) as a model of ferrous Cytc and determined the volume change, ΔVdZn, for the dissociation of its complex with ferric cytochrome b5 (Cytb5) by the pressure dependence of its photoinduced electron-transfer kinetics. Under ambient pressure, the dissociation constant, KdZn, of the ZnCytc/Cytb5 complex was dependent on the buffer concentration, 1.5 and 12 μM in 2 and 10 mM Tris-HCl, pH 7.4, respectively, which was consistent with formation of salt bridges in its complexation. The dissociation of one salt bridge is usually associated with large volume changes of -10 to -30 cm3 mol-1, while pressure dependence of KdZn resulted in smaller value of ΔVdZn, -8.5 cm3 mol-1. Therefore, the interaction between ZnCytc and Cytb5 cannot be explained only by salt bridge interaction, and the partial cancellation by the positive volume change due to the additional hydrophobic interaction is a plausible explanation for the observed ΔVdZn. In addition, ΔVdZn of -8.5 cm3 mol-1 was considerably smaller than the previously reported volume change, ΔVdFe, of -122 cm3 mol-1 in the ferric Cytc/Cytb5 complex dissociation [Rodgers and Sligar (1991) J. Mol. Biol. 221, 1453-1460]. ZnCytc used here has been assumed to be a reliable model of ferrous Cytc, and thus the discrepancy between our present ΔVdZn and the previous ΔVdFe is discussed on the basis of the protein docking dependent on the oxidation states of heme iron in Cytc.
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