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 CuA-site of cytochromes aa3, a process followed by intramolecular electron transfer (ET) to the heme a but not to the heme a3 [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 ba3 of Thermus thermophilus (Tt), we now show that MNA∗ also reduces CuA with a subsequent ET to the heme b and then to heme a3, with first-order rate constants 11 200 s−1, and 770 s−1, respectively. The results provide clear evidence for ET among the three spectroscopically distinguishable centers and indicate that the binuclear a3-CuB center can be reduced in molecules containing a single reduction equivalent.