Photosynthetic reaction center purple bacteria

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Fig. 1. Schematic representation of the reaction center of purple photosynthetic bacteria (A) and a stereogram of the BChl molecules Pa and Pb of the primary electron donor, or "special pair (B). Fig. (B) source Huber (1988) A structural basis of light energy and electron transfer in biology [Nobel lecture]. Bloscience Reports 9 643.

The primary photochemical charge-separation process, i.e., P870-t-A -> P870 +A in purple photosynthetic bacteria requiresthat there is a reaction partner to accept the electron released by the primary donor. Again, using D-[P-A] to represent the core composition of the bacterial reaction center, we can write the following sequence of events ... 

Fig. 2. Natural selection of carotenoid configuration in the reaction center (RC) and light-harvesting complex (LHC) of purple photosynthetic bacteria. See text for discussion. Figure source Koyama (1991) Structure and function of carotenoids in photosynthetic systems. J Photochem Photobiol, B Biol 9 208.

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

Wang S, Lin S, Lin X, Woodbury NW and Allen IP (1994) Comparative study of reaction centers from purple photosynthetic bacteria Isolation and optical spectroscopy. Photosynth Res 42 203-215... 

Carotenoids function in photosynthetic reaction centers (RC) as triplet quenchers of the primary donor chlorophyll or bacteriochlorophyll triplet states. The best studied RCs are those of purple photosynthetic bacteria where atomic models are available based on X-ray crystallography and optical as well as magnetic resonance spectroscopies have yielded a detailed picture of the flow of triplet energy transfer. Good reviews of these topics can be found in (Frank, 1992, 1993 Frank and Cogdell, 1996). 

When a chromophore is excited, the absorption spectrum of a nearby chromophore may also be influenced owing to an interaction with the excited molecule, which is observed, for instance, in the photosynthetic bacterial reaction center of purple bacteria. In the same complex the... 

John C. Kendrew determined the first atomic-scale (2A resolution) crystallographic structure of a protein, myoglobin (molecular mass 16,900 Da or 16.9 KDa) in 1959 and Max Perutz followed shortly afterward with atomic scale resolution of the tetrameric protein hemoglobin (64,500 Da). The first crystallographic structure for an enzyme, lysozyme (13,900 Da), was determined by David Phillips in 1965. The crystallographic analysis of the structure of the supermolecular photosynthetic reaction center of purple photosynthetic bacteria in 1985 led to a Nobel Prize in chemistry for Robert Huber, Johann Diesenhofer, and Hartmut Michel. The reaction center, a complete nanomachine embedded in the cell membrane of purple photosynthetic bacteria, consists of 4 protein subunits and 14 cofactors. 

The solution of the crystal structure of reaction centers from purple photosynthetic bacteria (1-4) has raised a challenge Can we account for the directionality, speed and efficiency of the primary photochemical electron transfer reactions on the basis of the crystal structure ... 

Carotenoids in aerobically grown cells of Erythrobacter sp. OCh 114 have been identified [2]. Spheroidenone is dominant, and small amounts of 2,2 -diketospirilloxanthin and OH-spheroidenone are also found. All of these carotenoids are bound to a photosynthetic reaction center or light-harvesting complexes, whose properties are similar to those of the purple photosynthetic bacteria [1]. 

Breton J, Martin J-L, Fleming G R and Lambry J-C 1988 Low-temperature femtosecond spectroscopy of the initial step of electron transfer in reaction centers from photosynthetic purple bacteria Biochemistry 27 8276... 

Michel, H., Deisenhofer, J. Relevance of the photosynthetic reaction center from purple bacteria to the structure of photosystem II. BicKhemistry 27 1-7, 1988. 

The photosynthetic reaction center (RC) of purple nonsulfur bacteria is the core molecular assembly, located in a membrane of the bacteria, that initiates a series of electron transfer reactions subsequent to energy transfer events. The bacterial photosynthetic RCs have been characterized in more detail, both structurally and functionally, than have other transmembrane protein complexes [1-52]. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

Deisenhofer, J. Michel, H. (1991) Structures of bacterial photosynthetic reaction centers. Annu. Rev. Cell Biol. 7, 1-23. Description of the structure of the reaction center of purple bacteria and implications for the function of bacterial and plant reaction centers. 

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

Many cytochromes c are soluble but others are bound to membranes or to other proteins. A well-studied tetraheme protein binds to the reaction centers of many purple and green bacteria and transfers electrons to those photosynthetic centers.118 120 Cytochrome c2 plays a similar role in Rhodobacter, forming a complex of known three-dimensional structure.121 Additional cytochromes participate in both cyclic and noncyclic electron transport in photosynthetic bacteria and algae (see Chapter 23).120,122 124 Some bacterial membranes as well as those of mitochondria contain a cytochrome bct complex whose structure is shown in Fig. 18-8.125,126... 

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