Bacteriorhodopsin: the Light-Driven Proton Pump

Halobacterium halobium is an archaebacterium which requires high concentrations of NaCl for growth."High" meaning an optimum of 4.3 M NaCl. In comparison, seawater contains about 0.6 M NaCl. Halobacteria ("halo"-salt) are found in natural salt lakes, and in places such as enclosed lagoons where evaporation causes seawater salinity to increase. Walter Stoeckenius has seperated the bacterial cell membrane into yellow, red and purple fractions. The purple fraction consists of a 26-kd protein with associated lipids. The purple-membrane protein was named bacteriorhodopsin (bR) as it contains retinal as its light-absorbing group.

The structure of bR was first determined in 1975 by Richard Henderson and Nigel Unwin using electron micrographs of the purple membrane. This technique was effectively used since bR occurs naturally as 2D crystals in the purple membrane. Crystalline sheets with diameters as large as 1um may be isolated. The resulting 7-Angstrom resolution map showed that bR contains seven closely packed alpha-helices extending perpendicular to the plane of the membrane. The initial model that was presented is shown.

Henderson & Unwin's 7-A model.
The interpretative diagram showing arrangement of the alpha helices. A suggestion for the connection between the helices was formed later, and will be discussed.
The latest structural map I've encountered in the literature was published by Henderson, Baldwin & Ceska in 1990. Data from 72 images, tilted and untilted specimens, were analyzed to produce a map with a horizontal resolution of 3.5-A, and a vertical resolution of 10-A.

In the prescence of O2, halobacteria generate ATP by oxidative phosphorylation. Oxygen-stress conditions leads them to switch to photosynthesis. The protons needed for the photosynthetic reaction are translocated across the membrane by bR. As stated in the title, bR is a light driven proton pump. Two protons per absorbed photon are translocated across the cell membrane from the cytosol to the outside. The proton-motive force thus generated drives the synthesis of ATP that powers other ion pumps.

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