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

The 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 a(3) and cytochrome a (k=(6. 7+/-0.9)x10(5) s(-1) at ambient pressure) and between cytochrome a and Cu(A) (k=(5.9+/-1.7)x10(4) 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 a(3) and cytochrome a arises from structural changes localized at cytochrome a(3) upon heme reduction. Similarly, the activation volume for the ET between cytochrome a and Cu(A) is primarily due to structural changes localized at Cu(A) 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 Cu(A) and cytochrome a(3) but not cytochrome a.