PURPLE MEMBRANE: SURFACE CHARGE DENSITY and THE MULTIPLE EFFECT OF pH and CATIONS

The purple membrane of Halobacterium halobium is one of the most intensively studied biological membranes. It is a well-defined and relatively uncomplicated system which uses light to pump protons from its cytoplasmic to its extracellular surface. The proton gradient is then used for adenosine triphosphate (ATP)* synthesis (reviewed in Stoeckenius et al., 1979; Ebrey, 1982; Oesterhelt and Tittor, 1989). The purple membrane is unusually stable and readily available. Much is known about its structure and its chromophore is well characterized. These properties make the purple membrane an excellent model for studying ion pumps as well as membrane structure and the effects of cations and surface potential on its properties and function. The isolated membrane, which does not vesiculate but rather forms planar sheets (ca 0.5 pm in diameter), contains only one type of protein, the 26 kDa bacteriorhodopsin (bR). Most of the lipids are quite polar, and there are only 10 per bR. The color of the membrane is due to a retinal covalently attached as a protonated Schiff base to Lys-216 inside the proton channel. A number of factors contribute to the modulation of the retinal chromophore’s light absorbing properties, including the protein’s counter-ion environment near the positively charged Schiff base. After absorbing a photon, various intermediate states can be observed which result in proton release and reuptake, leading to net proton transport, before bR returns to its original state (simplified model; maximal absorbances in parentheses) (see reviews; Drachev et al., 1987):

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