Activation volumes for intramolecular electron transfer in Escherichia coli cytochrome bo3

AbstractIn 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 CuB 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