Special pairs purple bacteria

Cyclic photophosphorylation in purple bacteria. QH2 is eventually dehydrogenated in the cytochrome bc1 complex, and the electrons can be returned to the reaction center by the small soluble cytochrome c2, where it reduces the bound tetraheme cytochrome or reacts directly with the special pair in Rhodobacter spheroides. The overall reaction provides for a cyclic photophosphorylation (Fig. 23-32) that pumps 3-4 H+ across the membrane into the periplasmic space utilizing the energy of the two photoexcited electrons. 

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 reaction center of purple bacteria contains three protein subunits (L, M, and H) located in the plasma membrane (Figure 8-35). Bound to these proteins are the prosthetic groups that absorb light and transport electrons during photosynthesis. The prosthetic groups include a special pair of bacteriochlorophyll a molecules equivalent to the reaction-center chlorophyll a molecules in plants, as well as several other pigments and two quinones, termed Qa and Qb, that are structurally similar to mitochondrial ubiquinone. 

As in the reaction center of green and purple bacteria, each chloroplast photosystem contains a pair of specialized reaction-center chlorophyll a molecules, which are capable of Initiating photoelectron transport. The reaction-center chlorophylls in PSI and PSII differ in their light-absorption maxima because of differences in their protein environment. For this reason, these chlorophylls are often denoted Peso... 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

Fig. 3. The Q-cycle operating with the reaction center of the pigment system of the purple sulfur-, purple non-sulfur and green non-sulfur bacteria, B870 = special pair of bacteriochlorophyll (BChl) a or b B870 = B870 in the S, excited state BChl = bacteriochlorophyll a or b molecules directly reduced by B870 BPheo = bacteriopheophytin a or b Q = bound quinone [ubiquinone (UQ) in Rhodobacter sphaeroides menaquinone (MQ) in Rhodobacter viridis and Chloroflexus spp.l Qb = mobile quinone [UQ in Rb. sphaeroides and Rb. virldis MQ in Chloroflexus spp.l which exchanges with QbHj at the sites marked with an X QbHj = fully reduced (quinol) form of Qg QbH- = semiquinone form of Qb Fe-S = Rieske iron-sufur center b(Fe ) or c(Fe +) = reduced forms of cytochromes b or o b(Fe ) or c(Fe ) = oxidized forms of cytochromes b or c X = Qb/QbH2 exchange site.

Compared to the special pair of the purple bacteria (Sp. viridis), Chi a molecules in the reaction center of PS II are weakly coupled to each other [51]. Therefore, the presence of a certain distance (1.20 nm), between the Mg centers in the dimer can be considered a reasonable argument for weak coupling. Using the exciton formula of McRae and Kasha [52] and assuming a distance of 1.20 nm between the Mg centers, this dimer would be expected to absorb at... 

FIGURE 15.7 Antenna system of purple bacteria, determined by x-ray crystallography by R. J. Cogdell et al. The light-harvesting smaller circles (LHl) donate the excitation to the inner ring (LH2). LH2 donates the excitation to the special pair of the reaction center. 

It performs water oxidation, resulting in Oj evolution and in proton release. The redox equilibrium 2H2O —> 4H+ -I- 4e + Oj has an Em of -1-0.82 V at pH 7. The reaction is pulled by the oxidized primary donor P (named P-680 in Photosystem II). P-680 is a special pair of chlorophyll a molecules, but these molecules are not as close as in purple bacteria, and they are surrounded by two other chlorophyll a, so that P-680 can also be described as a special tetramer. The P-680/P-680 redox couple must have an Em (perhaps around -1-1.2 V) much more positive than -1-0.82 V, in order to oxidize water irreversibly. P-680 is directly reduced by a tyrosine residue named TyrZ it is Tyr 161 of the polypeptide Dl. Oxidized TyrZ then oxidizes a cluster of four manganese atoms, which is the catalytic site for water oxidation. A histidine residue is probably involved in the process, which also requires Ca and Ck ions. The details of the structure and mechanism of this ensemble are still under intense investigation. "... 

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