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

Abstract

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.