Two forms of N intermediate (Nopen and Nclosed) in the bacteriorhodopsin photocycle

Abstract

Glutaraldehyde, aluminum ions and glycerol (that inhibit the M intermediate decay in the wild-type bacteriorhodopsin and azide-induced M decay in the D96N mutant by stabilization of the M_closed) accelerate the N decay in the D96N mutant. The aluminum ions, the most potent activator of the N decay, induce a blue shift of the N difference spectrum by ∽10 nm. Protonated azide as well as acetate and formate inhibit the N decay in both the D96N mutant and the wild-type protein. It is concluded that the N intermediate represents, in fact, an equilibrium mixture of the two (‘open’ and ‘closed’) forms. These two forms, like M_closed and M_open, come to an equilibrium in the microseconds range. The absorption spectrum of the N_open is slightly shifted to red in comparison to that of the N_closed. Again, this resembles the M forms. 13-cis-all-trans re-isomerization is assumed to occur in the N_closed form only. Binding of 1–2 molecules of protonated azide stabilizes the N_open form. Existence of the ‘open’ and ‘closed’ forms of the M and N intermediates provides the appropriate explanation of the cooperative phenomenon as well as some other effects on the bacteriorhodopsin photocycle. Summarizing the available data, we suggest that M_open is identical to the M_N form, whereas M₁ and M₂ are different substates of M_closed.