Quinones, primary electron acceptor

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

It is established that the primary electron acceptor in photosystem 1 is the molecule ferredoxin, while in photosystem 2 it is a quinone. The identity of the primary electron donor in photosystem 2 is still unknown the oxidation of water must take place by electron transfer to this primary donor, X. 

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. 

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. 

Fig. 4.4. (D and E) Primary electron acceptor (An 2) plastoquinone (or quencher Q). (D) Optical spectrum of the light induced plastosemiquinone anion (O), compared to the spectrum of PQ semi-quinone in non-aqueous solvent ( ). The additional spectral shifts at 545 and 685 nm are attributed to electrochromic effects on pheophytin (from Ref. 85). (E) ESR spectrum of A7, j reduced by light (a) or by dithionite (b) in Ch/amydomonas PSIl particles (from Ref. 87). (F) Secondary electron acceptor (An ) plastoquinone. (F) Flash-induced optical changes due to the reduction of the secondary electron acceptor plastoquinone the spectra oscillate in a dampened sequence following subsequent flashes indicating the production of semiquinone (1st and 3rd flash) and quinol species (2nd and 4th) (from Ref. 103).

In photosystem I, the primary electron acceptor is a monomeric chlorophyll called A. The secondary electron acceptor is a bound phylloquinone (abbreviated as OQ in this book), whose in situ quinone/ semiquinone redox potential has been estimated to be < -0.81 V. Phylloquinone has the same phytol side... 

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. 

The existence of Qa was initially inferred from chlorophyll-a fluorescence measurements. In 1963 when Duysens and Sweers found that the fluorescence yield of dark-adapted chloroplasts increases with time of illumination. These workers explained the phenomenon by suggesting that when the PS-II electron acceptor is present in the oxidized state, it can quench fluorescence, whereas it does not quench fluorescence once it is reduced, i.e., in its reduced state it inhibits the normal utilization of absorbed light energy to promote electron transport. Therefore, the electron acceptor was called Q, taken from the expression quencher of fluorescence. When it was subsequently established that the stable primary electron acceptor is a quinone molecule, the symbol Q became even more appropriate. 

M subunit (white) each form five transmembrane a helices and have a very similar structure overall the H subunit (light blue) is anchored to the membrane by a single transmembrane a helix. A fourth subunit (not shown) is a peripheral protein that binds to the exoplasmic segments of the other subunits. (Bottom) Within each reaction center is a special pair of bacteriochlorophyll a molecules (green), capable of initiating photoelectron transport two voyeur chlorophylls (purple) two pheophytins (dark blue), and two quinones, Qa and Qb (orange). Qb is the primary electron acceptor during photosynthesis. [After M. H. Stowell etal., 1997, Science 276 812.]... 

After the primary electron acceptor, Qb, in the bacterial reaction center accepts one electron, forming Qb , it accepts a second electron from the same reaction-center chlorophyll following its absorption of a second photon. The quinone then binds two protons from the cytosol, forming the reduced quinone (QHb), which is released from the reaction center (see Figure 8-36). QHb diffuses within the bacterial membrane to the Qo site on the exoplasmic face of a cytochrome bci complex, where it releases its two protons into the periplasmic space (the space between the plasma membrane and the bacterial cell wall). This process moves protons from the cytosol to the outside of the cell, generating a proton-motive force across the plasma membrane. Simultaneously, QHb releases its two electrons, which move through the cytochrome bci complex exactly as depicted for the mito-... 

The relation of the structure and organization of the Photosystem II reaction centers to those from Photosystem I or from the green or purple bacteria presents an interesting example of comparative biochemistry. Similarities between PS II and purple bacterial reaction centers include aspects of the reaction center proteins, the stoichiometry of chlorophyll and pheophytin in the reaction center and the complex of iron with quinones as the primary electron acceptor. In each of these respects the reaction centers of PS I or green bacteria, however, have no obvious similarity. 

FIGURE 1. Light-induced absorbance difference spectra (A) of the reduced minus oxidized forms of the primary quinone acceptor of PSII (QJ minus Qa.)> (B) of the reduced minus oxidized form of the primary electron acceptor pheophytin of PSII (Pheo minus Pheo), and (c) of the oxidized minus reduced forms of the reaction center P700 of PSI (P700" minus P700). 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

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