Bacteriorhodopsin is a protein generally found in archaea. It is most notably studied in Halobacterium. This prokaryote is unusual because this can trap light energy photosynthetically without the presence of chlorophyll. When exposed to low oxygen concentration, some strains of Halobacterium synthesize a modified cell membrane called the purple membrane, which contains the protein bacteriorhdopsin. ATP is produced by unique type of photosynthesis without the participation of bacteriophyll or chlorophyll.
The bacteriorhodopsin closely resembles the sensory pigment rhodopsin from the rods and cones of vertebrate eyes. Bacteriorhodopsin�s chromophore is the carotenoid derivative retinal (the aldehyde of vitamin A) that is covalently attached to the pigment protein by a Schiff base with the amino group of lysine. The protein has seven membrane spanning helices connected by loops on either side; its retinal rests in the center of the membrane to form crystalline patches called the purple membrane.
Bacteriorhodopsin functions as a light driven proton pump. When retinal absorbs light, the double bond between carbons 13 and 14 changes from Trans to a Cis configuration and the Schiff base loses a proton. Protons are moved across the plasma membrane to the periplasmic space during these alterations, and the Schiff base changes are directly involved in this movement. The bacteriorhodopsin protein undergoes several conformational changes during the photo cycle. These conformational changes also are involved in proton transport. The light driven proton pumping generates a pH gradient that can be used to power the ATP synthesis by a chemiosmotic mechanism.
This photosynthetic capacity is particularly useful to Halobacteriumbecause oxygen is not very soluble in high concentrated salt solutions and may decrease to extremely low levels in its habitat. When the surrounding becomes temporarily anaerobic the bacterium uses light energy to synthesize sufficient ATP to remain alive until the oxygen level rises again. Halobacterium cannot grow anaerobically because it requires oxygen for continued retinal synthesis, but it can survive the stress of temporary oxygen limitation by means of photosynthesis.
Halobacterium actually has four rhodopsins, each with different functions. Bacteriorhodopsin drives outward proton transport for purposes of ATP synthesis. Halorhodopsin uses light energy to transport chloride ions into the cell and maintain a 4 to 5 M intracellular KCl concentrations. Finally there are two rhodopsins that act as photoreceptors, one for red light and one for blue. They control flagellar activity to position the organism optimally in the water column. Halobacterium moves to a location of high light intensity, but one in which ultraviolet light is not sufficiently intense to be lethal.
The Photocycle of Bacteriorhodopsin
In this mechanism the retinal component of bacteriorhodopsin is buried in the mebrane and retinal interacts with two amino acids (A1 and A2), aspartates 96 and 85, that can reversibly accept and donate protons. A2 is connected to the cell exterior, and A1 is closer to cell interior. The two amino acids may be special aspartic acid residues in bacteriorhodopsin. Light absorption by retinal tiggers an isomerization from 13-trans retinal to 13- cis retinal. The retinal then donates a proton to A2 while A1 is picking another proton from the interior and A2 is releasing a proton to the outside. Retinal obtains a proton from A1 and isomerizes back to the 13-trans form. The cycle is ready to begin again.
