Photosynthetic bacteria Secondary electron acceptor

Chapter 6 The Secondary Electron Acceptor(Qb) of Photosynthetic Bacteria... 

Subsequently Halsey and Parson pursued this issue further by removing ubiquinone from Chromatium chromatophores and examining the consequences. The authors found that while the extracted chromato-phores were photochemically active on the first flash, they lost the ability entirely to perform photochemistry on the second flash, even when delayed by several seconds after the first. Reconstitution of the extracted chromatophores with the Mpid extracts restored the photochemical activity. Taking these results together with the chemical identity ofthe extracts allowed these workers to conclude that ubiquinone serves as the secondary electron acceptor, Qb, in such photosynthetic bacteria as Chromatium and Rb. sphaeroides. 

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 absorption of light in the reaction center (RC) of photosynthetic bacteria induces electron transfer from the special bacteriochlorophyll pair (P) through a series of one-electron acceptors (bacteriopheophytin, and a primary quinone, Q ) to a two-electron acceptor quinone, Qg [1], In RCs from sphaeroides, both and Qg are ubiquinone-10. It is generally believed that the doubly reduced secondary quinone (hydroquinone dianion) will form quinol (hydroquinone) by taking up two protons before being released from the RC and replaced by another quinone from the quinone-pool. The rate of quinol formation can be limi ted by either of these processes the second electron transfer from Qb to Q/vQb the... 

The only known function of PhQ in cyanobacteria and plants is to function as an electron transfer cofactor in PS I. In spite of its importance in cyanobacteria, the biosynthetic route of PhQ was not previously elucidated. Many prokaryotes contain the metabolic pathway for the biosynthesis of menaquinone (MQ), a PhQ-Hke molecule (Figure 119.1). In certain bacteria, MQ is used during fumarate reduction in anaerobic respiration. - In green sulfur bacteria and in heliobacteria, MQ may function as a loosely bound secondary electron acceptor in the photosynthetic reaction center. The genes encoding enzymes involved in the conversion of chorismate to MQ were cloned in a variety of organisms. MQ differs from PhQ only in the tail portion of the molecule an unsaturated C-40 side chain is present, rather than a mostly saturated C-20 phytyl side chain. Therefore, the synthesis of the naphthalene rings in PhQ and MQ involves similar steps in both pathways. 

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. 

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. 

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. 

Reaction centers in photosynthetic bacteria typically contain three membrane-bound subunits (L, M, and H), and the following cofactors four bacteriochlo-rophyll (Bchl or B), two bacteriopheophytin (Bphe or 4>), two quinones (Q), and one Fe atom 28, 178). The sequence of electron transfer steps along the various cofactors has been established largely by spectroscopic methods. The primary donor, D, which initially absorbs light (creating the excited state D ) is a dimer of Bchl molecules [also designated (Bchl)2 or P]. Electron transfer proceeds from D to an intermediate acceptor (a Bphe molecule), to a primary acceptor, Qa, and finally to the secondary acceptor Qb. After these initial events, the RC... 

Those photosynthetic eubacteria with RC-2 centers (filamentous and purple bacteria) reduce NAD" for CO2 fixation by reverse electron flow from the quinone pool, whereas the green sulfur bacteria (RC-1 center) reduce ferredoxin and NAD directly from the secondary acceptor (Fe-S center) of the RC. In both cases an external reductant such as H2S is required. The mechanism of NAD reduction in the gram-positive line has not yet been investigated, but H. chlorum is a het-erotroph rather than an autotroph, and may not need to fix CO2. 

One-electron redox reactions are most important in photosynthetic RC. BChls a, b, and g, and Chi a, and in certain bacteria, also [Zn] -BChl a or Chi d, act as electron donors. The same or similar (B)Chls act as primary and secondary acceptors in Type I RC, while in Type II RC, the secondary acceptors are the respective metal-free (B)Phe. ° (B)Chl-sensitized reductions of redox partners, in particular, across membranes are of considerable interest as model reactions for photosynthesis. These reactions were also studied in solution in covalently Knked systems Kke caroteno-chlorophyUo-quinones and more elaborate systems. ... 

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