Photosynthetic bacteria Primary electron donor

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

Photosynthetic eubacteria are classified as filamentous, green sulfur, gram-positive linked, purple, and cyanobacteria. All contain membrane-bound RCs in which (B)Chl serves as the primary electron donor. The RCs may be divided into two main types RC-1, in which the initial electron acceptor is a (B)Chl molecule and the secondary acceptor is an Fe-S center, and RC-2, in which the initial acceptor is a (B)Ph molecule and the secondary acceptor is a quinone. RC-1 centers are found in green sulfur and gram-positive linked bacteria, while RC-2 centers are found in filamentous bacteria and purple bacteria. Cyanobacteria contain both RC-1 and RC-2 centers in which the chlorophyll is Chi a. BChl a is found in filamentous, green sulfur and purple bacteria, while BChl g is characteristic of the grampositive line. BChl b is found in certain purple bacteria instead of BChl a. 

Fig. 4.2. Optical and ESR spectra of electron carriers in the RC of purple photosynthetic bacteria. (A and B) Primary electron donor (D,) bacteriochlorophyll dimer. (A) Light-dark optical spectrum (recorded at 30 °C) and (B) ESR spectrum of D, in Rps. sphaeroides. The ESR spectrum is y/2-times narrower than the corresponding spectrum of the Bchl cation, indicating a dimeric structure (from Ref. 3).

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.

Chapter 4 The Primary Electron Donor (P) of Photosynthetic Bacteria... 

Although the question ofthe role ofBA in electron transfer has been controversial for sometime, there have been some new developments, which will be discussed in Chapter 7. The question ofthe nature of the currently recognized reaction partner of photooxidized P870, i.e., the primary electron acceptor BOa, and of how P870 is re-reduced by the secondary electron donor will be dealt with in Chapters 7 and 10, respectively. In the remainder of this chapter we will discuss the physical and chemical properties ofthe primary electron donor of photosynthetic bacteria. 

Fig. 2. Two kinds of photosynthetic bacterial reaction centers based on the nature of binding of the cytochromes to the membrane. P is the primary electron donor T is the intermediate electron acceptor No." refers to Cyt per RC. See text for discussion. Figure adapted from PL Dutton and RC Prince (1978) Reaction center-driven cytochrome interactions in electron and proton translocation and energy coupling. In RK Clayton and WR Sistrom (eds) Photosynthetic Bacteria, p 525. Plenum Press.

The subject matter of this chapter is confined to the role of cytochrome as a secondary electron donor, D, i.e., the interaction with the photooxidized primary electron donor formed during the photochemical charge-separation process in photosynthetic bacteria. Another cytochrome, present essentially as a ubiquinone-cytochrome c oxidoreductase in the cytochrome-6ci complex, is particularly important in energy conservation and the creation of a proton gradient for ATP synthesis in of photosynthetic bacteria. This cytochrome fee, complex, is discussed in Chapter 35 dealing with proton transport. 

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. 

The primary electron donor D in reaction centers (RCs) of photosynthetic bacteria is a dimer ( special pair ) of Bacteriochlorophyll (BChl) molecules (Fig. 1). The structure and spectral (EPR, ENDOR) characteristics of the dimer have been investigated extensively in RCs of Rhodopseu-domonas viridis [1-4] and Rhodobacter sphaeraides [5-9]. 

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 ... 

Energy Conversion by Plants and Bacteria, pp 195-272. Acad Press R3. RC Prince and PL Dutton (1978) Protonation and the reducing potential of the primary electron acceptor. In RK Clayton and WR Sistrom (eds) Photosynthetic Bacteria, pp 439-453. Plenum press R4. G Feher (1992) Identification and characterization of the primary donor in bacterial photosynthesis a chrono-iogicai account of an EPR/ENDOR investigation (The Bruker Lecture) J Chem Soc Perkin Trans 2 1861-1874... 

The second kind of reaction center, as represented by that of Chromatium vinosum or Rhodopseudo-monas viridis, has a tightly bound c-type cytochrome [see Fig. 2, right]. This so-called reaction center-associated cytochrome is a tetraheme of molecular mass of 40 kDa and structurally quite different from the other known, c-type cytochromes. One of the hemes in this RC-associated, c-type cytochrome also serves as the immediate electron donor to the photooxidized primary donor of the photosynthetic bacteria (either P870 in C. vinosum or P960 in Rp. viridis). The oxidized cytochrome in the tetraheme is in turn reduced by the soluble cytochrome C2. The RC-associated cytochromes are not easily dissociated from the RC, even at high ionic strength. 

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