Before discussing the structural and functional properties of the reaction centres of different types of photosynthetic organisms, it is necessary to appreciate their specific electron-transfer pathways in terms of redox potentials. Figure 3.2 compares the Type I and Type I1 reaction centres of anoxygenic and oxygenic organisms. In purple photosynthetic bacteria (specifically R. sphaeroides), the primary donor is called P870, because the long wavelength absorption peak of its special pair of bacteriochlorophylls is at 870 nm. Similar notation is used for other primary donors e.g. P840 (green sulphur bacteria), P870 (green non-sulphur bacteria), P700 (PSI) and P680 (PSII). However, as hinted in Section 1.1.4, P680 differs from the other primary electron donors in that it seems not to be a special pair (Barber and Archer, 2001).
As we noted in Section 1.3.1, when excitation arrives at the RC from the LH system, primary charge separation occurs and this is followed by secondary electron flow to a terminal electron acceptor, ferredoxin (Fd) in the case of green sulphur bacteria and PSI (Type I RCs), or quinone (QB), in the case of purple bacteria and PSII (Type I1 RCs). In the case of anoxygenic bacteria, some of the reducing potential is used to convert NAD' to the NADH needed for C 0 2 fixation and some is utilised in cyclic electron flow, whereby the reductant indirectly reduces the oxidised primary donor. This cyclic electron flow involves the cytochrome bc complex, which is also embedded in the chromatophore membrane and which couples the electron flow to the vectorial movement of protons across the membrane, as shown in Fig. 3.6. The resulting pH and electrical gradients are then used to drive the conversion of ADP to ATP in accordance with the chemiosmotic mechanism of Peter Mitchell (1966), a contribution for which he received the Nobel Prize for Chemistry in 1978.
As already mentioned and shown diagrammatically in Figs. 1.4, 1.7 and 3.2, Photosystem I and Photosystem I1 work together in oxygenic photosynthetic organisms to oxidise water and reduce ferredoxin. PSII functions as the waterplastoquinone oxidoreductase while PSI is a plastocyanin-ferredoxin oxidoreductase. The redox coupling between the two reaction centres is accomplished by a cytochrome bc complex rather like that found in anaerobic photosynthetic bacteria but called, for historical reasons, the cytochrome b6f complex. An important feature of this scheme is that two photons are used to drive one electron from water to ferredoxin. The cytochrome b6f complex acts as a plastoquinol-plastocyanin oxidoreductase and, like its counter part in photosynthetic bacteria, facilitates the maintenance of the electrochemical potential gradient of protons across the thylakoid membrane needed to convert ADP to ATP. In oxygenic photosynthesis, the reduced ferredoxin is used to convert NADP' to NADPH, which together with ATP is required to convert C02 to carbohydrate.
Green sulphur bacteria also use reduced ferredoxin in the same way as PSI except that they, like purple bacteria, use non-phosphorylated nicotinamide adenine dinucleotide (NAD') rather than NADP'. The similarity in the redox properties and electron transport pathways of the Type I and Type I1 RCs is evident in Fig. 3.2 except for the important fact that P680' is a much stronger oxidant (with a midpoint potential of -1 V) than P700', P840'and P870'(-0.4 V). This is because P680'must be sufficiently oxidising to remove electrons from water, which is a very stable molecule and difficult to oxidise compared with the substrates oxidised by other reaction centres. This oxidation reaction involves a cluster of 4 Mn atoms and the transfer of electrons and protons from the substrate water molecules is facilitated by a redox-active tyrosine, named Yz, positioned between the (Mn)4-cluster and P680 (see Fig. 1.7). As the production of dioxygen from water is a four-electron process
and a dioxygen molecule is produced at a single PSII reaction centre, the Mn cluster must accumulate four oxidising equivalents. This is why the evolution of 0 2 oscillates with a period of four when oxygenic organisms are subjected to single turnover flashes of light, as discovered by Pierre Joliot and colleagues in 1969. This discovery caused Kok et al. (1970) to propose the S-state cycle, whereby the absorption of four successive photons drives the series of reactions
When S4 is formed, dioxygen is released and the cycle resets itself to the So-state. Although the precise chemical mechanism of the S-state cycle is unknown, it is generally believed that the two water substrate molecules bind at the So-state and that H' and electrons are extracted before arriving at the S4-state. The late Jerry Babcock and colleagues (Tommos and Babcock, 2000) have suggested an attractive 'hydrogenatom abstraction' hypothesis for the water oxidation mechanism.
Not surprisingly, the high redox potential of P680' and the possibility of forming reactive oxygen species during the water-splitting reaction give rise to oxidative damage of the PSII RC. This manifests itself as rapid degradation and regular replacement of protein, as Godde and Bornman describe in Chapter 5. Plants and other oxygenic organisms have evolved a range of protective strategies that reduce the frequency of photoinduced PSI1 damage and allow the repair process to cope under normal conditions. The effect of this intrinsic and detrimental property of PSII is, however, observed when organisms are exposed to environmental stress, when the rate of repair does not match the rate of damage and photoinhibition occurs. When this happens, the efficiency of photosynthesis and biomasskrop productivity decline.
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