Equilibria between multiple spectral forms of bacteriorhodopsin effect of delipidation, anesthetics and solvents on their pH dependence

Purple membrane lipid control of bacteriorhodopsin conformational flexibility and photocycle activity

“…Figure 4 displays a plot of λ max vs. Δν 1/2 for Triton‐treated purple membrane systems reconstituted with native lipids at various NaCl concentrations. The shift in λ max on lipid perturbation by Triton originates from twists in the retinal structure and changes in its environment induced by a bacteriorhodopsin conformational change, which produces altered protein–retinal interactions [25], while the decrease in Δν 1/2 arises from decreased mobility of the bacteriorhodopsin α‐helix structures. The linear correlation between the two parameters demonstrates complete recovery to ≈ 569 nm as Δν 1/2 recovers to ≈ 30.5 cm −1 , approximately the half‐width observed in native purple membrane.…”

Reversible inhibition of proton release activity and the anesthetic-induced acid-base equilibrium between the 480 and 570 nm forms of bacteriorhodopsin

“…were previously found to have identical effects on the 570 and 480 nm chromophore equilibrium (Messaoudi et al, 1992).…”

“…At this pH, little 480 nm bacteriorhodopsin is formed, as indicated by the small absorbance increase at this wavelength, and some redshifted pigment (blue bacteriorhodopsin) appears, as indicated by the small absorbance increase in the 650 nm region. In anesthetic-treated purple membrane, the 480/570 nm chromophore equilibrium has an apparent pKa of 7.3 (see below) and that of the 570/605 nm chromophore equilibrium is about 4 (Messaoudi et al, 1992), the same value as that observed for purified bacteriorhodopsin (Baribeau and Boucher, 1985). It is thus not surprising to find a mixture of these three species in the mildly acidic pH range.…”

“…In addition, the well-known hydrogen bond-breaking properties of anesthetics (Sandorfy, 1978) would make them excellent candidates to impair the functionality of the proton pumping channel, but this implies that anesthetics interact close to the chromophore, which does not appear to be the case, as interaction between the retinal and, at least, aromatic neighboring residues are left intact during bR570 transformation into bR480 (Lee et al, 1991). On the other hand, one must not forget that the acid-base-type chromophore equilibration considered here is also i) a property of delipidated (Baribeau and Boucher, 1987) or reconstituted (Lozier et al, 1976;Pomerleau et al, 1995) bacteriorhodopsin; ii) that, when induced with anesthetic or solvents, the phenomenon obeys the lipid solubility rule (Messaoudi et al, 1992) and occurs irrespective of membrane sidedness (Uruga et al, 1991); and iii) that only minor tertiary structural differences have been observed between both spectral forms in the purified state and in anesthetic-treated membranes (Pande et al, 1989;Messaoudi et al 1993). In addition, general anesthetics preferentially locate near the interface and in the hydrocarbon domain of membranes in general (Barber et al, 1995) and of purple membrane in particular (Nakagawa et al, 1994).…”

“…Under these conditions, samples recovered their characteristic purple color. In some control experiments, anesthetics have been replaced by trace amounts of hexane or large amounts of acetone, as these solvents have been shown to produce similar chromophore equilibration (Messaoudi et al, 1992); in those cases, results were neither qualitatively nor quantitatively different.…”