Structural changes of horse heart ferricytochrome C induced by changes of ionic strength and anion binding

To test the validity of the notion that changes in ionic strength and ion binding do not cause any major functionally relevant structural changes in cytochrome c, we measured the absorption and electronic circular dichroism (ECD) of horse heart ferricytochrome c for the Soret and 695 nm charge-transfer band as a function of dihydrogen phosphate and sodium acetate concentrations. This band is known to probe the integrity of the functionally pivotal Fe3+-M80 linkage. Spectral changes indicate that an ionic strength increase (via an increasing acetate ion concentration) affects only a subset of conformational substates of the Fe-M80 interface, probed by the 695 nm charge-transfer band, without a substantial modification of the heme environment. This result suggests that the substates probed by the 695 nm band differ with respect to their capability to transduce changes of solvent-protein interactions to the active site. The binding of H2PO4- ions causes more significant structural changes, which give rise to a large increase of the oscillator strength of the 695 nm band. This reflects a strengthening of the Fe-M80 bond in all substates, which probably destabilizes the oxidized state but stabilizes the folded state of the protein. Additional structural variations are likely to involve aromatic side chains, such as F82 and W59, and the hydrogen-bonding network in the heme pocket. In contrast to the current belief that anion binding to the binding domain of the protein for cytochrome c oxidase does not cause any functionally relevant structural changes, our results show that the structural variations that occur in the heme pocket are most likely of functional significance.