Activation volumes for intramolecular electron transfer in bovine heart cytochrome c oxidase

AbstractThe present work examines the activation volumes associated with intramolecular electron transfer (ET) within the CO-mixed-valence form of bovine heart cytochrome c oxidase (CcO). Activation volumes for intramolecular ET between cytochrome a3 and cytochrome a (k=(6.7±0.9)×105 s−1 at ambient pressure) and between cytochrome a and CuA (k=(5.9±1.7)×104 s−1) are found to be +41±5 ml/mol and +28±4 ml/mol, respectively. Examination of the crystal structures of both the fully oxidized and fully reduced forms of bovine heart CcO suggest that the activation volume for the ET between cytochrome a3 and cytochrome a arises from structural changes localized at cytochrome a3 upon heme reduction. Similarly, the activation volume for the ET between cytochrome a and CuA is primarily due to structural changes localized at CuA upon reduction of this site. Reduction/oxidation of cytochrome a does not appear to make any significant contribution to the activation volume. Overall, these results suggest conformational regulation of ET by both CuA and cytochrome a3 but not cytochrome a