ON THE MULTIPLE CYCLES OF BACTERIORHODOPSIN AT HIGH pH

Bacteriorhodopsin (bR) shows at least two parallel photocycles at pH 10.5, suggesting that more than one form of bR exists in alkaline bR sample. Upon the absorption of visible light, the different forms of bR at high pH yield different parallel intermediates: M₄₁₂ with two rise and two decay components; and R, an extremely fast rising and extremely slow decaying intermediate with an absorption peak at 350 nm. The kinetics and spectra do not agree with the proposal of Kouyama el al. (1988, Biochemistry 27,5855–5863) that the 350 nm-absorbing species is the N intermediate which follows M₄₁₂ and that the slow decaying M₄₁₂ is an M-like photoproduct of N. Our results basically agree with the proposal of Dacshazy et al. (1988, Proc. Natl. Acad. Sci. USA 85,6358–6361) that the fast and slow decaying M₄₁₂ intermediates and R are in independent photocycles arising from different forms of bR. The different forms of bR are probably in dynamic equilibrium with their ratios controlled by pH and ionic strength.