Electron transfer rates and equilibrium within cytochrome c oxidase

Intramolecular electron transfer (ET) between the Cu-A center and heme a in bovine cytochrome c oxidase was investigated by pulse radiolysis. Cu-A, th e initial electron acceptor, was reduced by 1-methyl nicotinamide radicals in a diffusion-controlled reaction, as monitored by absorption changes at 8 30 nm. After the initial reduction phase, the 830 nm absorption was partial ly restored, corresponding to reoxidation of the Cu-A center. Concomitantly , the absorption at 445 nm and 605 nm increased, indicating reduction of he me a. The rate constants for heme a reduction and Cu-A reoxidation were ide ntical within experimental error and independent of the enzyme concentratio n. This demonstrates that a fast intramolecular electron equilibration is t aking place between Cu-A and heme a. The rate constants for Cu-A --> heme a ET and the reverse (heme a --> Cu-A) process were found to be 13 000 s(-1) and 3700 s(-1), respectively, at 25 degrees C and pH 7.4. This corresponds to an equilibrium constant of 3.4 under these conditions. Thermodynamic an d activation parameters of the ET reactions were determined. The significan ce of these results, particularly the observed low activation barriers, are discussed within the framework of the known three-dimensional structure, E T pathways and reorganization energies.