Photochemical reaction of bacteriorhodopsin

Halobacterium halobium utilizes light both as a stimulus for phototaxis [l] and as an energy source for creating a proton gradient across the cell membrane which in turn produces ATP [2] . The photoreceptive pigment of this bacterium was first investigated by Oesterhelt and Stoeckenius [3] who showed that it is the only protein lying in so-called ‘purple membrane’ of the cell. Since this pigment, like visual pigments, has retinal as a prosthetic group [3] it is called ‘bacteriorhodopsin’. There are two forms of this pigment, dark-adapted bacteriorhodopsin (bRD, : 558 nm) and light-adapted bacteriorhodopsin $ll, x max: 568 nm). While Lozier et al. [4] and Kung et al. [S] have reported on the photochemical reactions of the light-adapted form, the photochemical reactions of the dark-adapted pigment have not been studied. We investigated the photochemical reaction of these forms of bacteriorhodopsin at liquid nitrogen temperatures.