A photosynthetic reaction centre (or photosynthetic reaction center) is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and phaeophytin, as well as quinones. The energy of the photon is used to promote an electron to a higher molecular energy level of a pigment. The free energy created is then used to reduce a chain of nearby electron acceptors, which have subsequently higher redox-potentials. These electron transfer steps are the initial phase of a series of energy conversion reactions, ultimately resulting in the production of chemical energy during photosynthesis.
The electron on the reduced Pheo is transferred very quickly (after about 0.4 ns) onto plastoquinone QA (P680+ Pheo– QA → P680+ Pheo QA– ). Plastoquinone QA , located on protein D2, is the first stable acceptor of electrons in PSII . At the same time, the oxidized molecule of chlorophyll P680+ is neutralized by an electron taken from tyrosine (Tyrz) in the water-splitting complex on the electron donor side of PSII (Tyrz P680+ → Tyrz+ P680). The localization of separated charges on the stable acceptor and the stable donor prevents their recombination, which ensures effective use of energy during the following stages of photosynthesis
Structures of Two Bacterial Reaction Centers
In 1984, Hartmut Michel, Johann Deisenhofer, Robert Huber, and coworkers in Munich solved the three-dimensional structure of the reaction center from the purple photosynthetic bacterium Rhodopseudomonas viridis (Deisenhofer and Michel 1989). This landmark achievement, for which a Nobel prize was awarded in 1988, was the first high-resolution X-ray structure determination for an integral membrane protein and the first structure determination for a reaction center complex.
Structure of the reaction center of the purple bacterium Rhodopseudo-monas viridis, resolved by X-ray crystallography. The 11 tubes within the membrane represent transmembrane protein helices. The chlorophyll-type pigments are shown in light green; the heme groups of the tightly bound iron-containing cytochrome are shown in dark gray at the top of the diagram. The C (cytochrome subunit) points into the periplasm or outside the cell; the H subunit is in contact with the cytoplasm of the cell. The quinones and the cytochrome participate in electron transfer reactions with the bacteriochlorophylls. (Courtesy of J. Richardson.)
The protein part of the complex consists of four separate polypeptides. Two of them, called L and M (for light and medium mass) bind all of the bacteriochlorophyll, quinone, and carotenoid cofactors of the complex. The structure has a twofold symmetry about an axis perpendicular to the plane of the membrane, hinting at a dimeric nature of the reaction center. The ten transmembrane portions of the L and M peptides (five from each) are arranged in α helices, and there are almost no charged amino acid residues in the interior of the membrane. The H (heavy) protein has a single transmembrane helix and is localized mostly on the cytoplasmic side of the membrane. The C (cytochrome) subunit is located in the periplasmic region (the region between the bacterial plasma membrane and the outer membrane). The geometric arrangement of the pigments and the quinones (electron acceptors) is shown in Web Figure 7.5.B, with the protein removed. A similar arrangement is found in the reaction center of another purple photosynthetic bacterium, Rhodobacter sphaeroides, except that the C subunit is not present.
Web Figure 7.5.B Geometric arrangement of pigments and other prosthetic groups in the bacterial reaction center: bacteriochlorophyll (BChl); bacteriopheophytin (BPh), a chlorophyll molecule in which the magnesium atom has been replaced by two hydrogen atoms; and QA and QB, the first and second quinone acceptors. The cytochrome heme groups have been deleted from this diagram; if present, they would be at the top. The two bacteriochlorophylls at the top (P870) are the “special pair” that react with light within the reaction center. The electron transfer sequence begins at the special pair and proceeds down the right side of the diagram to bacteriopheophytin. Next, the electrons are transferred to QA and then to QB. The distances between the depicted molecules and their angles within the planes of symmetry of the reaction center have been determined with precision, making it possible to analyze the path of photons and electrons in the reaction center in remarkable detail.
Two of the bacteriochlorophyll molecules are in intimate contact with each other and are known as the special pair. This dimer, whose existence was predicted from magnetic-resonance studies, is the photoactive portion of the complex. An electron is transferred from this dimer along the sequence of electron carriers on the right side of the complex. Detailed analysis of these structures, along with analysis of numerous mutants, has revealed many of the principles involved in the energy storage processes that are carried out by all reaction centers.
The bacterial reaction center structure is thought to be similar in many ways to that found in photosystem II from oxygen-evolving organisms, especially in the electron acceptor portion of the chain (see textbook Figure 7.25). The proteins that make up the core of the bacterial reaction center are relatively similar in sequence to their photosystem II counterparts, implying an evolutionary relatedness.
The chemical mechanism of photosynthetic water oxidation is not yet known, although there is a great deal of indirect evidence about the process. If a sample of dark-adapted photosynthetic membrane is exposed to a sequence of very brief, intense flashes, a characteristic pattern of oxygen production is observed. Little or no oxygen is produced on the first two flashes, and maximal oxygen is released on the third flash and every fourth flash thereafter, until eventually the yield per flash damps to a constant value. This remarkable result was first observed by Pierre Joliot in the 1960s.
A schematic model explaining these observations, proposed by Kok and coworkers, has been widely accepted (Kok et al. 1970). This model for the photooxidation of water, called the S state mechanism, consists of a series of five states, known as S0 to S4, which represent successively more oxidized forms of the water-oxidizing enzyme system, or oxygen-evolving complex (Web Figure 7.7.A). The light flashes advance the system from one S state to the next, until state S4 is reached. State S4 produces O2 without further light input and returns the system to S0. Occasionally, a center does not advance to the next S state upon flash excitation, and less frequently, a center is activated twice by a single flash. These misses and double hits cause the synchrony achieved by dark adaptation to be lost and the oxygen yield eventually to damp to a constant value. After this steady state has been reached, a complex has the same probability of being in any of the states S0 to S3 (S4 is unstable and occurs only transiently), and the yield of O2 becomes constant. States S2 and S3 decay in the dark, but only as far back as S1, which is stable in the dark. Therefore, after adaptation to the dark, approximately three-fourths of the oxygen-evolving complexes appear to be in state S1 and one-fourth in state S0. This distribution of states explains why the maximum yield of O2 is observed after the third of a series of flashes given to dark-adapted chloroplasts.
Web Figure 7.7.A
Rensselaer researchers to map step-by-step mechanism of photosynthesis
"Photosystem II powers the planet with solar energy. If there is a design that is perfect for harnessing the energy of the sun, this is it.
This S state mechanism explains the observed pattern of O2 release, but not the chemical nature of the S states or whether any partly oxidized intermediates, such as H2O2, are formed. Additional information has been obtained by measurement of the pattern of proton release as the S states are advanced with flashes. In addition to oxygen, hydrogen ions are a product of water oxidation. The release of protons is not strictly coupled to O2 release.[/b][/b]
The PS-I reaction center
The PS-I reaction center is composed of a multiprotein complex. The reaction center chlorophyll P700 and about 100 core antenna chlorophylls are bound to two proteins, PsaA and PsaB, with molecular masses in the range of 66 to 70 kDa (Golbeck 1992; Krauss et al. 1993, 1996; Chitnis 1996; Jordan et al. 2001). PS-I reaction center complexes have been isolated from several organisms and found to contain the 66 to 70 kDa proteins, along with a variable number of smaller proteins in the range of 4 to 25 kDa (see textbook Figure 7.29). Some of these proteins serve as binding sites for the soluble electron carriers plastocyanin and ferredoxin. The functions of some of the other proteins are not well understood. An 8-kDa protein contains some of the bound iron-sulfur centers that serve as early electron acceptors in photosystem I. The structure of the PS-I complex from pea has been determined to a resolution of 4.4 Å, and the positions of many of the chlorophylls and electron transfer components have been located (Krauss et al. 1996; Schubert et al. 1997; Jordan et al. 2001; Nelson and Ben-Shem 2004; see textbook Figure 7.29).
In their reduced form, the electron carriers that function in the acceptor region of photosystem I are all extremely strong reducing agents. These reduced species are very unstable and thus difficult to identify. Evidence indicates that one of these early acceptors is a chlorophyll molecule, and another is a quinone species, phylloquinone, also known as vitamin K1 (Nugent 1996).
Additional electron acceptors include a series of three membrane-associated iron–sulfur proteins, or bound ferredoxins, also known as Fe–S centers Fe–SX, Fe–SA, and Fe–SB. Fe–S center Fe–SX is part of the P700-binding protein; centers Fe–SA and Fe–SB reside on an 8 kDa protein that is part of the PS-I reaction center complex. Electrons are transferred through centers Fe–SA and Fe–SB to ferredoxin, a small, water-soluble iron–sulfur protein. The membrane-associated flavoprotein ferredoxin–NADP reductase (FNR) reduces NADP+ to NADPH, thus completing the sequence of noncyclic electron transport that begins with the oxidation of water (Karplus et al. 1991).
Electron transport between PSII and PSI is mediated by plastohydroquinone, the cytochrome b6f complex, and plastocyanin (see textbook Figure 7.28). The cytochrome b6f complex contains two b-type hemes and one c-type heme (Web Figure 7.8.A)
Web Figure 7.8.A Structure of prosthetic groups of b- and c-type cytochromes. The protoheme group (also called protoporphyrin IX) is found in b-type cytochromes, the heme c group in c-type cytochromes. The heme c group is covalently attached to the protein by thioether linkages with two cysteine residues in the protein; the protoheme group is not covalently attached to the protein. The Fe ion is in the 2+ oxidation state in reduced cytochromes and in the 3+ oxidation state in oxidized cytochromes.
A model for the organization of electron carriers in PS-I is shown in Web Figure 7.8.B. The P700 dimer is located at the bottom of the structure and two symmetrical arms radiate from P700 (Malkin and Niyogi 2000). Each arm includes an accessory chlorophyll a and another chlorophyll molecule tentatively identified as A0. Other structures include the Fe-S center FX, and two other Fe-S centers, F1 and F2. The distance between the carriers is shown at the right.
Web Figure 7.8.B A model for the organization of electron carriers in PS-I
The PS-I reaction center appears to have some functional similarity to the reaction center found in the anaerobic green sulfur bacteria and the heliobacteria. These bacteria contain low-potential Fe–S centers as early electron acceptors and are probably capable of ferredoxin-mediated NAD+ reduction similar to the NADP+ reduction function of photosystem I. There is almost certainly an evolutionary relationship between these complexes and photosystem I of oxygen-evolving organisms.
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