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Light energy transduction in halobacteria
BioeZechoclreuzistr~
and Bioenergetics
Light Energy Transduction
3,
SiI-3iz
(1976)
in Eaiobacteria
*
by W. STOECKEXIU~ School of Medicine, Cardiovascular San Francisco, US.1
Research
J.nstitute,
Universit>-
of California,
Bacteriorhodopsin is a protein found in the cell membrane of haIt contains one mole of retinal per mole of protein. The lobacteria. retinal is bound as a protonated SCHIFF base to a lysine residue of the protein. The absorption masimum of the bound retinal is red shifted to 570 nm. This apparently is caused through complesation with aromatic amino acid residues of the protein. Bacteriorhodopsin forms two-dimensional cq-stals in the cell membrane. These can be isolated as membrane fragments, termed the purple membrane, which contain bacteriorhodopsin as the only protein. It constitutes --i;s o/Oof the mass of the purple membrane ; the remainder is lipid. The purple membrane is -3.0 nm thick. and the crystalline at-rays appear in the surface membrane of the intact cells as round or X-ray diffraction and elongated patches with a width of -0.5 pm. electron microscopy show that the protein spans the membrane and is It has a high x-helix content uniformly oriented across the membrane. and the cx-helices are oriented at right angle to the plane of the membrane. When bacteriorhodopsin contained in the purple membrane absorbs light, it undergoes a cyclic photoreaction which can be observed by flash At room temperature and pH 4 to S, a cycle is completed spectroscopy. within 5 to IO ms. At least four - probably five - spectroscopically distinct intermediates in the c_vcle have been identified. During the cycle a proton is first releasea and subsequently a proton is bound by the The release and binding sites are not identical ; they are membrane. located on opposite sides of the membrane and the proton appears,to be transferred from the binding site to the release site through the protein. When bacteriorhodopsin cycles in the light. it therefore translocates protons across the membrane ; in other words, it acts as a light-driven I proton pump. This function can be demonstrated unequi\-ocally in a model system which consists of lipid vesicles fomled from purple membrane and synThe orientation of the bacteriorhodopsin thetic or natural phospholipids. * Plenac- lecture read at the 3rd International Symposium on BioelectroOctober 1975. chemist?-, Jiilich, ~7-31 The entire test of the lecture was not sent. by the author.
3’7’
Stoeckenius
molecules in this model system is the inverse of that in the cell. In the the vesicles pump protons inward, the cells outward. The lightdriven proton translocation generates an electrochemical gradient across the cell membrane and the cell can use this gradient to synthesize ATP or drive other enerbT-requiring process. In the lipid vesicles incorporation of mitochondrial ATPase from beef -hearts in addition to the purple membrane results in a model system capable of light-driven ATP synthesis. Intact cells often show a comples reaction to light because the light-driven proton transport is coupled to other ion movements and the net proton movement appears to be subject to control functions of the cell. A tentative model is proposed which accounts for the esperimental obsem*ations. A chain of proton-eschanging groups are assumed to esist in/or on the protein, estending from one side of the membrane to the other. A large transient change in the pK of one group in this chain during the photoreaction cycle could drive the translocation. Simultaneous conformational changes which move this group back and forth between the two neighboring groups in the chain could prevent back reactions and impart the vectorial character. light
and Bioenergetics
Light Energy Transduction
3,
SiI-3iz
(1976)
in Eaiobacteria
*
by W. STOECKEXIU~ School of Medicine, Cardiovascular San Francisco, US.1
Research
J.nstitute,
Universit>-
of California,
Bacteriorhodopsin is a protein found in the cell membrane of haIt contains one mole of retinal per mole of protein. The lobacteria. retinal is bound as a protonated SCHIFF base to a lysine residue of the protein. The absorption masimum of the bound retinal is red shifted to 570 nm. This apparently is caused through complesation with aromatic amino acid residues of the protein. Bacteriorhodopsin forms two-dimensional cq-stals in the cell membrane. These can be isolated as membrane fragments, termed the purple membrane, which contain bacteriorhodopsin as the only protein. It constitutes --i;s o/Oof the mass of the purple membrane ; the remainder is lipid. The purple membrane is -3.0 nm thick. and the crystalline at-rays appear in the surface membrane of the intact cells as round or X-ray diffraction and elongated patches with a width of -0.5 pm. electron microscopy show that the protein spans the membrane and is It has a high x-helix content uniformly oriented across the membrane. and the cx-helices are oriented at right angle to the plane of the membrane. When bacteriorhodopsin contained in the purple membrane absorbs light, it undergoes a cyclic photoreaction which can be observed by flash At room temperature and pH 4 to S, a cycle is completed spectroscopy. within 5 to IO ms. At least four - probably five - spectroscopically distinct intermediates in the c_vcle have been identified. During the cycle a proton is first releasea and subsequently a proton is bound by the The release and binding sites are not identical ; they are membrane. located on opposite sides of the membrane and the proton appears,to be transferred from the binding site to the release site through the protein. When bacteriorhodopsin cycles in the light. it therefore translocates protons across the membrane ; in other words, it acts as a light-driven I proton pump. This function can be demonstrated unequi\-ocally in a model system which consists of lipid vesicles fomled from purple membrane and synThe orientation of the bacteriorhodopsin thetic or natural phospholipids. * Plenac- lecture read at the 3rd International Symposium on BioelectroOctober 1975. chemist?-, Jiilich, ~7-31 The entire test of the lecture was not sent. by the author.
3’7’
Stoeckenius
molecules in this model system is the inverse of that in the cell. In the the vesicles pump protons inward, the cells outward. The lightdriven proton translocation generates an electrochemical gradient across the cell membrane and the cell can use this gradient to synthesize ATP or drive other enerbT-requiring process. In the lipid vesicles incorporation of mitochondrial ATPase from beef -hearts in addition to the purple membrane results in a model system capable of light-driven ATP synthesis. Intact cells often show a comples reaction to light because the light-driven proton transport is coupled to other ion movements and the net proton movement appears to be subject to control functions of the cell. A tentative model is proposed which accounts for the esperimental obsem*ations. A chain of proton-eschanging groups are assumed to esist in/or on the protein, estending from one side of the membrane to the other. A large transient change in the pK of one group in this chain during the photoreaction cycle could drive the translocation. Simultaneous conformational changes which move this group back and forth between the two neighboring groups in the chain could prevent back reactions and impart the vectorial character. light