Bacteriochlorophylls Purple bacteria reaction centers

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-... 

Figure 23-27 Illustration of proposed exciton transfer of the energy of light absorbed by bacteriochlorophyll a of purple bacteria. Energy absorbed by the light harvesting complex LH2 is transferred in steps to another LH2, to LH1 and to the reaction center. The short lines within the circles represent the edges of the BChla chromophores. After Kiihlbrandt300 with permission.

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

Reaction centers of purple bacteria typically contain three polypeptides, four molecules of bacteriochlorophyll, two bacteriopheophytins, two quinones, and one nonheme iron atom. In some bacterial species, both quinones are ubiquinone. In others, one of the quinones is menaquinone (vitamin K2), a naphthoquinone that resembles ubiquinone in having a long side chain (fig. 15.10). Reaction centers of some species, such as Rhodopseudomonas viridis, also have a cytochrome subunit with four c-type hemes. 

The linking characteristic between these groups is the use of various types of bacteriochlorophyll in a single stage process, involving either photosynthetic reaction center II (e.g., purple bacteria) or photosynthetic reaction center I... 

In purple bacteria, the constituent bacteriochlorophyll of the primary donor is the same as the principal bacteriochlorophyll pigment, namely, BChl a. The same is true for the green bacteria and also for photosystem II of green plants. However, as discussed later in Chapter 28, the primary donor of photosystem I(PSI),P700, is now known to consist of a 13 epimer of Chi a, designated as Chi o ]. Since the reaction centers of PS I and heliobacteria are both of the FeS-type [refer to Chapter 1], it is reasonable to anticipate that the primary electron donor of heliobacteria might also be an epimer. 

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. 

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). 

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.

The kinetics of primary processes occuring in the photosynthetic reaction center (RC) has been studied so far almost exclusively by transient absorption spectroscopy in the femto- to nanosecond time range. Most of these studies were carried out furthermore on the reaction centers of purple bacteria like, e.g., R sphaeroides and Rs. rubrum (for a review see (1)). Despite a large number of transient absorption studies being carried out over the last decade, no agreement has been reached as to the nature of the primary processes and the identity of the intermediates. At present in particular two topics are highly controversial i) Is the accessory bacteriochlorophyll (BChl) a anion an intermediate in the electron transfer process (2), and ii) are... 

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