Electron transfer among the CuA-, heme b- and a3-centers of Thermus thermophilus cytochrome ba3

The 1-methyl-nicotinamide radical (MNA(*)), produced by pulse radiolysis has previously been shown to reduce the Cu-A-site of cytochromes aa(3), a process followed by intramolecular electron transfer (ET) to the heme a but not to the heme a(3) [Farver, O., Grell, E., Ludwig, B., Michel, H. and Pecht, I. (2006) Rates and equilibrium of CuA to heme a electron transfer in Paracoccus denitrificans cytochrome c oxidase. Biophys. J. 90, 2131-2137]. Investigating this process in the cytochrome ba(3) of Thermus thermophilus (D), we now show that MNA* also reduces Cu-A with a subsequent ET to the heme b and then to heme a(3), with first-order rate constants 11200s(-1), and 770 s(-1), respectively. The results provide clear evidence for ET among the three spectroscopically distinguishable centers and indicate that the binuclear a(3)-Cu-B center can be reduced in molecules containing a single reduction equivalent. (c) 2006 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies.