Increase in apparent compressibility of cytochrome c upon oxidation.

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Abstract
The apparent molal adiabatic compressibilities of [horse heart] ferri- and ferrocytochrome c were determined from measurements of density and sound velocity. The values found were +2.99 .times. 10-8 and -2.40 .times. 10-8 cm5 mol-1 dyn-1 for the ferri and ferro forms, respectively. Experiments were performed on identical solutions containing either the oxidized or reduced form of the protein. Solutions of ferricytochrome c were found to have significantly greater adiabatic compressibility than equivalent solutions of ferrocytochrome c at 25.degree. C and pH 7.15. The remarkable similarity of the 3-dimensional structures of the ferri and ferro proteins strongly suggests that this difference in compressibility is due to an increase in volume fluctuations within ferricytochrome c relative to the ferro form rather than a change in equilibrium structure or hydration. Such a difference in the dynamic properties of the structures is consistent with both the crystallographic thermal B factors and the observed increase in amide hydrogen exchange kinetics when ferrocytochrome c is oxidized. The relative magnitude of the root mean square volume fluctuations is approximated from an ideal solution treatment of the compressibility data and yields a ratio of .delta.Vrms (ferri cyt c)/.delta.Vrms (ferro cyt c) = 1.3.