[Changes in the contractibility of the cytochrome c globule during redox transition]

Changes of the volume and compressibility of cytochrome c molecule in solution during red-ox transition were investigated using differential measurements of density and ultrasound velocity. Small changes were obtained: intrinsic compressibility of the globule increases by (2.5 +/- 1)% and intrinsic volume increases by not more than 0.2%. The results are in contradiction with the recently reported data of Eden et al. claiming that oxidation of the protein is accompanied by a large, of about 40%, increase of compressibility. The validity of our results is verified by three different methods; by comparison of independently measured absolute values of apparent volumes and compressibilities of the oxidized and reduced protein (i); by differential densimetric and ultrasound velocimetric titrations of oxidized cytochrome with ascorbate (ii) and of reduced cytochrome with ferricyanide (iii). The obtained data lead to the conclusion that oxidation-induced-changes of the root mean square amplitude for intramolecular motion of atoms of cytochrome c globule is really 50-fold less than that estimated from X-ray data.