Photosynthetic bacteria electron acceptors

M. Bixon, J. Jortner, M.E. Michel-Beyerle, A. Ogrodnik, and W. Lersch, The role of the accessory bacteriochlorophyll in reaction centers of photosynthetic bacteria Intermediate acceptor in the primary electron transfer Chem. Phys. Lett. 140 626 (1987). 

Fds with conventional [Fe2-S2] clusters can undergo a one-electron transfer to a deeply valence-trapped FemFen species. For proteins of known structure (and presumably others) one iron atom is closer to the surface (by about 0.5 nm) and it has been established that the added electron resides on that atom. No instances are known where an [Fe2-S2] centre acts as a physiological two-electron donor or acceptor. In addition to the conventional [Fe2-S2] ferredoxins, the electron-transfer chains of mitochondria and photosynthetic bacteria contain Rieske proteins which have a cluster with the composition [(Cys.S)2FeS2Fe(N.His)2], in which the two imidazole groups are bound to the same iron atom (Figure 2.9). This atom is the site... 

Sulfate reducing bacteria were not antecedents of photosynthetic bacteria, but rather evolved from ancestral types which were photosynthetic bacteria. Although initially surprising, this evolntionary relationship is consistent with the idea that the accumulation of sulfate, the obligatory terminal electron acceptor for the sulfate reducing bacteria, was the resnlt of bacterial photosynthesis. 

Figure 5.6 Electron transport chain in photosynthetic bacteria. P = pigment A = electron acceptor D = electron donor...

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

On the reducing site of photosystem I, the initial electron acceptor appears to be a molecule of chlorophyll a (see fig. 15.17). The second acceptor probably is a quinone, phylloquinone (vitamin K, fig. 15.10). In these respects, photosystem I resembles photosystem II and purple photosynthetic bacteria, which use pheophytin a or bac-teriopheophytin a followed by a quinone. From this point on, photosystem I is different its next electron carriers consist of iron-sulfur proteins instead of additional quinones. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

Also, although we could clearly identify the photooxidation of P700, we have found no evidence for the existence of a chlorophyll molecule acting as a primary acceptor in PS1. This is in line with the work on photosynthetic bacteria where it has been shown that the related molecule (a monomeric bacteriochlorophyll) either does not receive an electron, or that it loses the electron at a faster rate than that at which it is gained. 

In many organisms, a cyclic process takes place, in which the reduced electron acceptor transfers its electron through a series of carriers back to the oxidized donor. Energy conservation is achieved by coupling proton translocation across a membrane to the electron flow. This type of cyclic electron flow occurs in eukaryotes under some conditions and in many anoxygenic photosynthetic bacteria. No NADPFl is produced, only ATP. This process occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPFl. In other organisms, noncyclic electron flow takes... 

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. 

Like the purple bacterial species mentioned above, Prostheocochloris aestuarii and other members of the Chlorobiaceae subgroup of the green photosynthetic bacteria appear to use a BChl dimer as an initial electron donor, but they evidently use BChl c istead of BPh as an initial electron acceptor [82-85]. The Chlorobiaceae also differ in using iron-sulfur proteins as the next electron carriers, instead of quinones. Their electron acceptor system appears to resemble that found in PS 1 of plants and cyanobacteria more than it does that of other groups of photosynthetic bacteria. 

A second bound form of cytochrome c is an integral part of the oxidoreductase complexes. Cytochrome c, present in photosynthetic bacteria has been distinguished from cyt. C2 (the soluble electron carrier) both thermodynamically and kinetically [121,122]. It is present in the isolated oxidoreductase with a stoicheiometry of one per two cytochromes of b type, and it is associated with the 34000 Da subunit. According to kinetic evidence this cytochrome acts as immediate electron donor to cyt. C2 and electron acceptor from the high potential Fe-S protein [122]. The midpoint potential of cyt. c, is 0.285 V at pH 7 [121,122]. 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

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. 

The Stable Primary Electron Acceptor (Qa) of Photosynthetic Bacteria... 

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