Photosynthesis proton pumping

Proton gradients can be built up in various ways. A very unusual type is represented by bacteriorhodopsin (1), a light-driven proton pump that various bacteria use to produce energy. As with rhodopsin in the eye, the light-sensitive component used here is covalently bound retinal (see p. 358). In photosynthesis (see p. 130), reduced plastoquinone (QH2) transports protons, as well as electrons, through the membrane (Q cycle, 2). The formation of the proton gradient by the respiratory chain is also coupled to redox processes (see p. 140). In complex III, a Q,cycle is responsible for proton translocation (not shown). In cytochrome c oxidase (complex IV, 3), trans-... 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

Light is indispensable for life. Green plants and some bacteria use solar energy for the energy source in their photosynthesis [1-3]. Archeal bacteriorhodopsin is a membrane bound protein and works as a light-driven proton pump [4, 5]. Another role of light is information carrier that is recognized in vision and photo-sensors. 

Sacksteder, C.A., Kanazawa, A., Jacoby, M.E., and Kramer, D.M. 2000. The proton to electron stoichiometry of steady-state photosynthesis in living plants A proton-pumping Q cycle is continuously engaged. Proc. Natl. Acad. Sci. USA 97 14,283-14,288. 

Allen, J. F. 2002. Photosynthesis of ATP—electrons, proton pumps, rotors, and poise. Cell 110 273-276. 

Cytochrome b(Jf complex An iron-rich protein complex involved proton pumping and ATP synthesis in photosynthesis. 

Proton-ATPases can be divided into three classes a) The plasma-membrane type, which operates via a phosphoenzyme intermediate and therefore is part of the P-ATPase superfamily. These proton pumps evolved from a common ancestor of the Ca" and Na pumps and are structurally distinct from the other two families of proton pumps (1-3). b) The eubacterial-type F-ATPases that are present in eubacteria, mitochondria and chloroplasts (3,4). c) The vacuolar-type V-ATPases that are present in archaebacteria and the vacuolar system of eukaryotic cells (2-6). F and V-ATPases are structurally and functionally related and have evolved from a common ancestral enzyme (3,4). This relationship was established from a wealth of sequence information regarding F-ATPases and by more recent studies on V-ATPases. The divergent pathways by which F and V-ATPases have evolved were recently elucidated by pai lel studies in several laboratories (3). It is the piupose of this communication to discuss aspects pertinent to the evolution of CFq-CFi, which is the F-ATPase functioning in photosynthesis. 

Of particular interest to us are the membrane proteins, numbered I-V. Four of them are part of the electron transport chain in accordance with the chemiosmotic model. FT in the chains is driven by the energy of food or by photosynthesis. Protons are pumped across the membrane to a more acid location. This is done in Complex I, Complex III, and Complex IV. Complex II is used in reduction of ubiquinone to ubiquinol. Another molecule of this type is Complex V, an ATP synthase where ATP is synthesized from ADP and P, as just mentioned. This complex does not have any electron transport chain. 

Crofts AR, Crowther D, Cells H, Cells SA and Tierney G (1977) Proton pumps in bacterial photosynthesis, Biochem.Soc.Transactions 5, 491-495,... 

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