Pheophytin-quinone reaction center

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

The functional reaction center contains two quinone molecules. One of these, Qb (Figure 12.15), is loosely bound and can be lost during purification. The reason for the difference in the strength of binding between Qa and Qb is unknown, but as we will see later, it probably reflects a functional asymmetry in the molecule as a whole. Qa is positioned between the Fe atom and one of the pheophytin molecules (Figure 12.15). The polar-head group is outside the membrane, bound to a loop region, whereas the hydrophobic tail is... 

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. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

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... 

Bacteria have a single reaction center in purple bacteria, it is of the pheophytin-quinone type, and in green sulfur bacteria, the Fe-S type. 

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 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.

Although the structures of the plant reaction centers are not yet known in detail, photosystem II reaction centers resemble reaction centers of purple bacteria in several ways. The amino acid sequences of their two major polypeptides are homologous to those of the two polypeptides that hold the pigments in the bacterial reaction center. Also, the reaction centers of photosystem II contain a nonheme iron atom and two molecules of plastoquinone, a quinone that is closely related to ubiquinone (see fig. 15.10), and they contain one or more molecules of pheophytin a and several... 

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. 

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.

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

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 resemblance between PS II and the reaction centers of purple bacteria and filamentous green bacteria has been established by extensive biochemical and spectroscopic studies. In addition to the analogous electron carriers (bacterio)pheophytin and quinones present in purple bacteria and PS II, their reaction-center-core protein subunits, the L and M subunits in purple bacteria and the DI and D2 subunits in PS II, show significant amino-acid sequence homologies. 

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. 

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.]... 

Somewhat surprisingly, the structure of the PSII reaction center, which removes electrons from H2O to form O2, resembles that of the reaction center of photosynthetic purple bacteria, which does not form O2. Like the bacterial reaction center, the PSII reaction center contains two molecules of chlorophyll a (Peso) - as well as two other chlorophylls, two pheophytins, two quinones (Qa and Qb), and... 

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