Pheophytin-quinone electron

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

Structural studies of the reaction center of a purple bacterium have provided information about light-driven electron flow from an excited special pair of chlorophyll molecules, through pheophytin, to quinones. Electrons then pass from quinones through the cytochrome bci complex, and back to the photoreaction center. 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

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. 

Figure 1. Pheophytin-quinone and iron-sulfiir Reaction Centers. The dotted line represents the absorption of light by theprimary electron donor (Chl2 or BChl2).Thelineshows the energy transfers in the Reaction Center, from the PSII tyrosine residue (Yz), through the monomer bacttriochlorophyll (BChl), A) the monomer bacterio-pheophytin (BPhe), or B) pheophytin (Phe) and quinone transfer components, QA and QB, in the pheophytin-quinone type of Reaction Center, and Q through the monomer chlorophyll (Chi), quinone (Q) and F components in the iron-sulfur Reaction Centers.

From a proteic point of view, the location of the light-induced reactions in PS1 appears to be very different from that in PS2 or in bacterial RCs. Indeed, in the bacterial RCs the membrane-embedded part of the photosystems that carry the first electron donors and acceptors consists of two proteic subunits of 250-350 amino acids, whereas psa A and psa B are each constituted of 750 amino acids. If one compares the function of the different photosystems, it clearly appears that many of the electron tranfer steps are similar between PS2 and bacterial RCs. In these RC, after excitation of the primary electron donor, the electron rapidly jumps from a chlorophyllic structure (a dimer of BChl in bacterial RCs) to a (bacterio)pheophytin. From the (bacterio)pheophytin the electron is transferred to a quinone then to a second quinone. In PS1, after the excitation of the primary electron donor, the electron jumps rapidly from the primary electron donor P700 (most likely a dimer of Chls) to a chlorophyll (Aq) from Aq it is transferred to the A acceptor, and thence to a series of iron-sulfur clusters (Fx. Fg and Fb) (4). If some structural analogy may be found between all the photosystems. It obviously will concern the proteic features related to the first electronic steps, e.g. those which are located in the local environment of the primary donor and/or primary acceptors of electrons. 

Redox titrations of the complex confirmed the absence of the quinone electron acceptor but suggested that electron transfer beyond the pheophytin could occur under oxidising conditions, resulting in the formation of a chlorophyll radical at g=2 (5).It has been shown that following the addition of artificial quinones the photoreduction of cytochrome b559 may occur, in the presence of added electron donors (6,7). 

Thus, the reaction center is composed of a dimer of two hydrophobic proteins which are denoted D1 and D2. These contain the redox components needed to transfer an electron from the primary donor, P6 o via the intermediary electron acceptor, a pheophytin molecule, to the first and second quinone electron acceptors, and Qg... 

The D1/D2/cytochrome b-559 complex contains 4 chlorophylLa molecules, 2 pheophytin-a molecules, 1 cytochrome b-559 and some p-carotene it contains no plastoquinone. In the absence of the secondary acceptors (quinone), electron transfer within this complex Is limited to the formation of the primary radical pair P680+Pheophytin". Absorption spectroscopy of this preparation has indicated the presence of a component decaying with a lifetime of 32-36 ns, corresponding to the lifetime of the primary radical pair (Danielius et al., 1987 Takahashi et al., 1988). More recently, time-resolved fluorescence studies (MImuro et al., 1988 Seibert et al., 1988) have shown that this complex exhibits a lifetime of 25-35 nanoseconds this has also been attributed to charge recombination of the primary radical pair, however, the fluorescence from this component was observed to be less than 2% of the total light emitted. 

Radicals are formed in electron transfer reactions involving organic molecules such as chlorins [P, (bacterio)chlorophylls, (bacterio)pheophytins], quinones (Q, Q, A, of Type I RCs), tyrosine (TyrZ and TyrD in Photosystem II), carotenoids, etc. Classical EPR spectroscopy, now complemented by more elaborate techniques such as pulse EPR, ENDOR, or high-field EPR, is the most efficient way to study them (see examples of applications in References 3,4,14,42—45). 

The flow of electrons occurs in a similar manner from the excited pigment to cytochromes, quinones, pheophytins, ferridoxins, etc. The ATP synthase in the mitochondria of a bacterial system resembles that of the chloroplast—chloroplast proton translocating ATP synthase [37]. 

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

The pheophytin radical now passes its electron to a tightly bound molecule of quinone (Qa), converting it to a semiquinone radical, which immediately donates its... 

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

A simple system for modelling the intermediate step of the charge separation process during photosynthesis [the stage of electron transfer from the reduced pheophytin (i.e. chlorophyll deprived of the Mg atom) to quinone] has been advanced and studied [67]. In this work the charge photoseparation process was studied in solutions of P-L-Q compounds in electron-donor, Et3N, solvent at 77 K. The structure of one of the P-L-Q compounds studied is given in Fig. 13. We will consider briefly the main results of ref. 67. 

The results of ref. 67 show that it is indeed possible to use solutions of porphyrin-quinone compounds in electron donor solvents for modelling the stage of electron transfer from pheophytin to quinone during photosynthesis (cf. Chap. 8, Sect. 1.2). Further research on these relatively simple model systems may provide still deeper insight into the mechanisms of this stage of photosynthesis. 

The electron acceptors on the reducing side of photosystem II resemble those of purple bacterial reaction centers. The acceptor that removes an electron from P680 is a molecule of pheophytin a. The second and third acceptors are plastoquinones (see fig. 15.10). As in bacterial reaction centers, electrons move one at a time from the first quinone to the second. When the second quinone becomes doubly reduced, it picks up protons from the stromal side of the thylakoid membrane and dissociates from the reaction center. 

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. 

A simple system for modelling the intermediate step of the charge separation process during photosynthesis (the stage of electron transfer from the reduced pheophytin, i.e. chlorophyll deprived of the Mg atom, to quinone) has been... 

Natural photosynthesis provides the most dramatic demonstration of the potential hidden in this basic photoreaction. In (bacterial) photosynthesis a chlorophyll-dimer (BC)2—the special pair —receives the radiation energy and thereby gains the energy required to enable it to transfer an electron to a pheophytin moiety (BP), an act occurring within 2-3 picoseconds (Martin et al. 1986) even at very low temperatures. Subsequently the electron is transferred to a quinone acceptor (MQ), which once again occurs (Holten et al. 1978) on a very short time scale of about 230 ps. 

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