Intramolecular Proton Transfer in Bacteriorhodopsin (bR)

Abstract

Bacteriorhodopsin (bR), which pumps upon light excitation protons across the membrane is the only protein of the purple membrane from Halobacterium halobium (for reviews see 1,2). The 248 amino acids of the protein span the membrane in seven helices (A–G) with almost 80% embedded in the lipid phase. There are only a few charged amino acids within the hydrophobic core of the protein (3). Prom these residues one glutamic acid (Glu) and one aspartic acid (Asp) are protonated, whereas at least three Asp are deprotonated (4). These charges have to be stabilized by a counterion, one of which could be the protonated Schiff-base of the all-trans-retinal chromophore (4), which is linked to Lys-2l6 of the protein. The colour of the pigment (λ_max = 568 nm) is determined by the amino acid residue environment of the retinal and a cation (5,6), which is bound to helices C or D, most probably on the cytoplasmic side of the membrane (7).