Activation volumes for intramolecular electron transfer in Escherichia coli cytochrome bo₃

In this report we describe the activation volumes associated with the heme-heme electron transfer (ET) and CO rebinding to the binuclear center subsequent to photolysis of the CO-mixed-valence derivative of Escherichia coli cytochrome bo₃ (Cbo). The activation volumes associated with the heme-heme ET (k=1.2×10⁵ s^-1), and CO rebinding (k=57 s^-1) are found to be +27.4 ml/mol and -2.6 ml/mol, respectively. The activation volume associated with the rebinding of CO is consistent with previous Cu X-ray absorption studies of Cbo where a structural change was observed at the Cu_B site (loss of a histidine ligand) due to a change in the redox state of the binuclear center. In addition, the volume of activation for the heme-heme ET was found to be quite distinct from the activation volumes obtained for heme-heme ET in bovine heart Cytochrome c oxidase. Differences in mechanisms/pathways for heme b/heme o₃ and heme a/heme a₃ ET are suggested based on the associated activation volumes and previously obtained Marcus parameters.