Two tyrosyl radicals stabilize high oxidation states in cytochrome C oxidase for efficient energy conservation and proton translocation.

The reaction of oxidized bovine cytochrome c oxidase (bCcO) with hydrogen peroxide (H2O2) was studied by electron paramagnetic resonance (EPR) to determine the properties of radical intermediates. Two distinct radicals with widths of 12 and 46 G are directly observed by X-band EPR in the reaction of bCcO with H2O2 at pH 6 and pH 8. High-frequency EPR (D-band) provides assignments to tyrosine for both radicals based on well-resolved g-tensors. The wide radical (46 G) exhibits g-values similar to a radical generated on l-Tyr by UV-irradiation and to tyrosyl radicals identified in many other enzyme systems. In contrast, the g-values of the narrow radical (12 G) deviate from l-Tyr in a trend akin to the radicals on tyrosines with substitutions at the ortho position. X-band EPR demonstrates that the two tyrosyl radicals differ in the orientation of their β-methylene protons. The 12 G wide radical has minimal hyperfine structure and can be fit using parameters unique to the post-translationally modified Y244 in...