Oxygen-linked equilibrium Cu B-CO species in cytochrome ba 3 oxidase from Thermus thermophilus: implications for an oxygen channel at the Cu B site

Abstract

We report the first study of O 2 migration in the putative O 2 channel of cytochrome ba 3 and its effect to the properties of the binuclear heme a 3-Cu B center of cytochrome ba 3 from Thermus thermophilus. The Fourier transform infrared spectra of the ba 3-CO complex demonstrate that in the presence of 60-80 μM O 2, the ν(C-O) of Cu B 1+-C-O at 2053 cm -1 (complex A) shifts to 2045 cm -1 and remains unchanged in H 2O/D 2O exchanges and in the pH 6.5-9.0 range. The frequencies but not the intensities of the C-O stretching modes of heme a 3-CO (complex B), however, remain unchanged. The change in the ν(C-O) of complex A results in an increase of k -2, and thus in a higher affinity of Cu B for exogenous ligands. The time-resolved step-scan Fourier transform infrared difference spectra indicate that the rate of decay of the transient Cu B 1+-CO complex at pH 6.5 is 30.4 s -1 and 28.3 s -1 in the presence of O 2. Similarly, the rebinding to heme a 3 is slightly affected and occurs with k 2 = 26.3 s -1 and 24.6 s -1 in the presence of O 2. These results provide solid evidence that in cytochrome ba 3, the ligand delivery channel is located at the Cu B site, which is the ligand entry to the heme a 3 pocket. We suggest that the properties of the O 2 channel are not limited to facilitating ligand diffusion to the active site but are extended in controlling the dynamics and reactivity of the reactions of ba 3 with O 2 and NO