Chromophore equilibria in bacteriorhodopsin

Abstract

An investigation of the dark equilibria between different chromophores of bacteriorhodopsin (BR) and studies of the kinetics of their interconversion and photochemical activity have led to the following conclusions. (a) A component of the 605-nm chromophore of BR decays in the millisecond range and is likely to be identical to the intermediate O of the photochemical cycle of BR and is assumed to be formed from the purple complex (PC) by the binding of one proton to BR. (b) An acidic form the PC, PCaL-, arises from the 605-nm chromophore by selective binding of anions L- (F- greater than Cl- greater than Br- greater than I- greater than Cl04-) to BR. (c) The isomeric equilibrium between 13-cis and all-trans retinal is approximately 0.15/0.85 in PCaCl-, 0.3/0.7 in the 605-nm chromophore as compared to 0.5/0.5 in the PC. (d) The 500-nm chromophore is formed from the PC by release of nearly one proton from BR. (e) The pH range in which the PC exists is reduced in a high-temperature structure of the purple membrane as compared to its low temperature structure. A model for the chromophore structure is proposed as a hypothesis, which allows a comprehensive interpretation of the results. In this model the absorption spectrum of the retinylidene lysine Schiff base is modulated by its protonation state and the interaction with an anionic group.

Asp85 is the only internal aspartic acid that gets protonated in the M intermediate and the purple‐to‐blue transition of bacteriorhodopsin A solid‐state13C CP‐MAS NMR investigation

“…In addition to the function of Asp ~5 as proton accepter in M, it has also been suggested that a carboxyl group near the Schiff base is involved in the purple-toblue transition of bacteriorhodopsin [12][13][14]. The blue form, BRbt,,c, is obtained either by acid titration of removal of cations [15--18].…”

Effects of genetic replacements of charged and H-bonding residues in the retinal pocket on Ca2+ binding to deionized bacteriorhodopsin.

“…Removal of these cations from purple membrane patches (6,7) or acidification (2,8,9) causes a color transition from purple to blue. The absorption maximum of the retinal chromophore is believed to be primarily regulated by the electrostatic interactions between the charged or polar side chains of amino acids and the PSB, as well as certain carbon atoms along the retinal electronic system (10)(11)(12)(13)(14).…”

Structural involvement of carboxyl residues in the photocycle of bacteriorhodopsin

“…Time-resolved resonance Raman spectroscopy has been used to show that the Schiff base linkage is protonated in BRsTo, unprotonated in M412, and protonated once again in 0640 [4 6]. Studies of an illumination-independent acid-induced species, which apparently represents stabilization of phototransient 0640 at low pH, suggest that protonation ofa carboxylgroup of an amino acid side chain is involved in the formation of the 0640 intermediate [7,8].…”

“…(i) Interacting directly with the Schiff base nitrogen [8,9]; (ii) Interacting with the/3-ionone ring of retinal to form a 'charge stabilized' intermediate [ 10]; (iii) Participating in a sequence of proton-translocating groups [4,11]; or (iv) Forming ion-pairs within the membrane with the positively charged groups of arginine and/or lysine [11 13]. Since the primary sequence of BR has been reported [14], it is now possible by chemical modification techniques to gain information on the role of specific amino acid residues in proton translocation.…”