Modulation excitation spectrophotometry of purple membrane of Halobacterium halobium

Abstract

WHEN Halobacterium halobium is grown at low oxygen concentrations in the light, it synthesises patches of membrane containing a purple pigment. If exposed to low salt concentrations, the cell membrane dissociates into fragments which differ in their protein and pigment composition and can be separated¹. The most conspicuous of these fragments is the so-called ‘purple membrane’². The isolated purple membrane contains 25% lipid and 75% protein²; only a single species of protein has been found. This protein, bacteriorhodopsin, is apparently similar to the animal visual pigment; it contains 1 mol of retinal per mol protein bound as a Schiff base to a lysine residue³. The protein forms a planar lattice in the membrane⁴ and shows a broad absorption maximum at 570 nm^3,4. Absorption of light converts the 570 nm species to a second species which absorbs maximally at 412 nm, and in the dark reconverts to the 570 nm complex within a few milliseconds. This is apparently accompanied by a conformational change in the protein, which cycles rapidly between the two conformations, releasing protons on one side of the membrane in the first transition and taking them up on the other in the second⁵. Thus, in the intact cells the bacteriorhodopsin seems to act as a light-driven proton pump^6,7.