Electron transfer among the Cu A ‐, heme b ‐ and a 3 ‐centers of Thermus thermophilus cytochrome ba 3

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 ( Tt ), 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 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 a 3 ‐Cu B center can be reduced in molecules containing a single reduction equivalent.