Valinomycin modifies phosphorescence quenching in cytochrome c oxidase.
The spectrum of resting cytochrome c oxidase is modulated by valinomycin addition, which induces a red shift of the Soret band. The enzyme is known both to fluoresce and phosphorescence, effects which can be modulated by certain protein reagents and quenchers. Valinomycin had little effect upon fluorescence at 335 nm, whether excited at 295 nm or 280 nm (tryptophans or both tryptophans and tyrosines, respectively). Phosphorescence at 445 nm was slightly enhanced upon the binding of valinomycin to the enzyme, suggesting a small conformational change accompanying the spectral shift of the heme groups. The quenching by nitrite of the phosphorescence excited at 260 nm, but not that excited at 295 nm, was diminished by valinomycin. This suggests that the valinomycin-induced conformational change may involve (i) a change in the accessibility of tyrosines to the quencher and/or (ii) a change in distribution of distances and/or orientations between populations of tyrosines and tryptophans.