Photosystem interaction with

The light reactions consist of two parts, accomplished hy two distinct hut related photosystems. One part of the reaction is the reduction of NADP to NADPH, carried out by photosystem I (PSI). The second part of the reaction is the oxidation of water to produce oxygen, carried out by photosystem II (PSII). Both photosystems carry out redox (electron transfer) reactions. The two photosystems interact with each other indirectly through an electron transport chain that links the two photosystems. The production of ATP is linked to electron transport in a process similar to that seen in the production of ATP by mitochondrial electron transport. 

Photosystem I is a membrane pigment-protein complex in green plants, algae as well as cyanobacteria, and undergoes redox reactions by using the electrons transferred from photosystem II (PS II) [1], These membrane proteins are considered to be especially interesting in the study of monomolecular assemblies, because their structure contains hydrophilic area that can interact with the subphase as well as hydrophobic domains that can interact either with each other or with detergent and lipids [2], Moreover, studies with such proteins directly at the air-water interface are expected to be a valuable approach for their two-dimensional crystallization. 

Figure 4. Interaction of structural elements of herbicidal inhibitors of photosystem 11 with a postulated receptor...

The phenolic photoaffinity label azidodinoseb (Figure 4) binds less specifically than either azidoatrazine or azidotriazinone (14). In addition to other proteins, it labels predominantly the photosystem II reaction center proteins (spinach 43 and 47 kDa Chlamydomo-nas 47 and 51 kDa) (17). Because of the unspecific binding of azidodinoseb, this can best be seen in photosystem II preparations (17). Thus, the phenolic herbicides bind predominantly to the photosystem II reaction center, which might explain many of the differences observed between "DCMU-type" and phenolic herbicides (9). The photosystem II reaction center proteins and the 34 kDa herbicide binding protein must be located closely to and interact with each other in order to explain the mutual displacement of both types of herbicides (8,12,21). Furthermore, it should be noted that for phenolic herbicides, some effects at the donor side of photosystem II (22) and on carotenoid oxidation in the photosystem II reaction center have been found (23). 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

Fromme, P., Melkozemov, A., Jordan, P., Krauss, N. (2003). Structure and function of photosystem I interaction with its soluble electron carriers and external antenna systems. FEBS Lett. 555,40-44. 

The doubly reduced Qb can take up two protons from the surrounding medium, forming plastohydro-quinone, QbH2. Eventually QbH21s released from its binding site to the thylakoid matrix where it can interact with the cytochrome Z>6/complex and thus transfer its two electrons to photosystem I eventually. The site vacated by QbH2 is then replenished with an unreduced PQ molecule from the plastoquinone pool, this replacement plastoquinone now ready to function as Qb and accept electron from newly formed Qa . The plastoquinone pool contains 5-10 PQ molecules per reaction center, with each PQ able to store two electrons. This series of reactions is summarized in Pig. 6. 

SM Rodday, AN Webber, SE Bingham and J Biggins (1995) Evidence that the Fx domain in photosystem I interacts with the subunit PsaC. site-directed changes in PsaB destabilizes the subunit interaction in Chlamy-domonas reinhardtii. Biochemistry 34 6328-6334... 

N Krau, W Hinrichs, I Witt, P Fromme, W Pritzkow, Z Dauter, C Betzel, KS Wilson, HT Witt and W Saenger (1993) 3-dimensional structure of system I of photosynthesis at 6 ngstrom resolution. Nature 361 326-331 SM Rodday, LT Do, V Chynwat, HA Frank and J Biggins (1996) Site-directed mutagenesis of the subunit PsaC establishes a surface exposed domain interacting with the photosystem I core binding site. Biochemistry 35 11832-11838... 

Direct interaction of sHsps other than a-crystallin with membranes has been reported only for Synechocystis Hsp 16.6 (Torok et al., 2001). Purified recombinant protein interacted with lipid vesicles, as monitored by fluorescence anisotropy, to reduce lipid fluidity and also increased the surface pressure of lipid monolayers. There was evidence for both lipid specificity and membrane penetration of the sHsp. Thylakoids isolated from cells deleted for Hsp 16.6 were more fluid membranes than wild-type cells, even in the presence of very little sHsp. Ability to stabilize directly photosynthetic membranes by lipid interaction could explain data supporting a role for sHsps in photosystem II protection, but the specificity of this effect has not been very well documented (Harndahl and Sundby, 2001 Heckathorn et al., 1998 Lee et al., 2000 Nakamoto et al., 2000). 

Rashid A, Carpentier R. The 16 and 23 kDa extrinsic polypeptides and the associated Ca and Cl modify atrazine interaction with the Photosystem II core complex. Photosynth Res 1990 24 221-227. 

Due to the fact that the arylutea type herbicides, such as diuron and monuron, did not inhibit the wild type bacterial reaction centers, the predictions have been based mainly upon mutations of the Qp-binding domain, which was affected by interaction with diuron. For example, the characterization of the herbicide-resistant mutants from Bps. viridis has revealed that one of the mutants, T4 (Tyr L222 to Phe) was sensitive to the urea type inhibitors similar to the D1 protein of PSII reaction centre. The semiquinone-iron electron paramagnetic resonance (EPR) signal of Qp in viridisTA mutants was also similar to that reported for photosystem II. 

The toxic effect of nickel (Ni) on photosynthetic electron transport was investigated by monitoring the Hill activity, fluorescence, oxygen evolution and thermo-luminescence properties in the green algae Scenedesmus obliquus. The results obtained surest that Ni modify the Qp site or interact with the nonheme iron between the Qa and Qp inhibiting the photosystem II functions. ... 

Approximately half of all commercial herbicides act by inhibiting photosynthesis by interacting with specific sites along the photosynthetic electron transport chain. A number of diverse chemicals including the ureas, amides, triazines, triazinones, uracils, pyridazinones, quinazolines, thiadiazoles, and certain phenols are thought to act specifically at a common inhibitory site at the reducing side of photosystem II (PS II) (U ). 

The notion that DCMU/metribuzin and phenol/hydroxyquinoline react with the same binding area on the acceptor side of photosystem II, but do not use identical binding interactions with the amino acids in the binding niche, is supported by the behaviour of the hydroxyquinolines in a metribuzin tolerant mutant of Cblaaydoaoaas where Ser 264 is replaced by Ala (13,19) as shown in Table III. In this mutant there is no decrease of the inhibitory potency of hydroxyquinolines contrary to that of metribuzin and DCMU, but in line with other phenol-type inhibitors (13,19). This is further explored in ref. (y.) with other mutations as well. 

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