Electron Transport in Photosystem

What is known about the structure of photosynthetic reaction centers  

It is well established that there is a pair of bacteriochlorophyll molecules (designated Pgyg from the fact that light of 870 nm is the maximum excitation wavelength) in the reaction center of Rhodopseudomonas viridis the critical pair of chlorophylls is embedded in a protein complex that is in turn an integral part of the photosynthetic membrane. (We shall refer to the bacteriochloro-phylls simply as chlorophylls in the interest of simplifying the discussion.) 

Photosynthesis consists of two processes. The light reactions are electron transfer processes, in which water is oxidized to produce oxygen and NADP+ is reduced to produce NADPH. The dark reactions are also electron transfer processes, but here carbon dioxide is reduced to carbohydrates. 

The path of electrons in the light reactions of photosynthesis can be considered to have three parts. The first is the transfer of electrons from water to the reaction-center chlorophyll of photosystem II. 

---

Some responses, such as mortality, are irreversible. However, many sublethal responses may be reversible, such as the impact of the photosynthesis-inhibiting herbicide linuron on macrophytes (Snel et al. 1998). Linuron inhibits photosynthesis by disturbing electron transport in photosystem II. Table 6.2 presents the kinetics of photosynthesis inhibition when shoots of macrophytes are placed in water with 50 pg/L linuron, and subsequent recovery when placed in uncontaminated water. The EC50 values are remarkably similar between macrophytes, and half-life estimates for inhibition and recovery are less than 2 hours (Table 6.2). Except for Potamogeton... 

Redding K, van der Est A. The directionality of electron transport in photosystem I. In Photosystem I The Light Driven Plasto-cyanin Ferredoxin Oxidoreductase. Golbeck J, ed. 2006. Springer, Dordrecht, The Nedierlands. 

V. Picosecond Kinetics of Photochemical Charge Separation and Electron Transport in Photosystem II..316... 

In addition to the electron transfer reactions just described, it is possible for cyclic electron transport in photosystem I to be coupled to the production of ATP (Figure 22.9). No NADPH is produced in this process. Photosystem II is not involved, and no Og is generated. Gyclic phosphorylation takes place when... 

This strategy has led to commercial development of herbicide resistance for glu-fosinate, glyphosate and bromoxynil. Glufosinate and glyphosate resistance will be discussed in detail in later sections of this chapter (see also Chapter 6.2). Bro-moxynil s herbicide activity is due to inhibition of electron transport in photosystem II. Crops engineered with bromoxynil nitrilase metabolize the herbicide to a non-phytotoxic compound [5]. 

Bromoxynil (Buctril ) inhibits electron transport in photosystem II by binding to the D1 protein. BXN cotton, achieved via detoxification and introduced in 1996, peaked at about 7% of the total cotton acres in 1998 but has steadily declined in use and was last sold in 2004 (Fig. 6.1.3). 

Herbicides acting as inhibitors of photosynthesis by blocking of electron transport in photosystem II belong to the eldest classes of plant protection agents. These compounds are still of market relevance, especially in developing countries, but they are out of the focus of modern herbicide research due to their high application rates in response to the high enzyme concentration for photosynthesis in plants and their cross-resistance behavior. 

EFFECT OF THE SALINITY ON THE ATPase ACTIVITY (CF1,F1) ELECTRON TRANSPORT IN PHOTOSYSTEMS I,II AND RESPIRATORY CHAIN IN Medicago sativa AND Amaranthus hypochondriacus... 

The present study examines and compares the effects of NaCl in the growth medium and reaction mixture on the ATPase activity in chloroplast (CFl) and mitochondria (FI) and the electron transport in photosystems (PSI,PSII) and respiratory chain from two species with different salt tolerance, Amaranthus hypochondriacus and Medicago sativa. 

Effect of the Salinity on the ATPase Activity (CFl, FI) Electron Transport in Photosystems I, II and Respiratory Chain in Medicago sativa and Amaranthus hypochondriacus 47... 

In this connection it is interesting to realize that tricolorin A was also shown to uncouple photophosphorylation in spinach chloroplasts in a potent manner (Ujj = 0.33 jM) and to inhibit - in high concentrations (20pM) - electron transport in photosystem II. Again the intact macrolactone moiety turned out to be cmcial (Achnine et al. 1999). Glycoresins might be useful as leads for new herbicides (Vyvyan 2002). 

XXIX Linuron H Inhibits electron transport in photosystem II 1500 000 (R)... 

Barr R, Troxel KS and Crane FL (1982) Calmodulin antagonists inhibit electron transport in photosystem II of spinach chloroplaSts,Biochem. Biophys. Res. Commun. 104, 1182-1188... 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Metribuzin is a member of the substituted as-triazinone group of herbicides. Activity is due to interference with photosystem II electron transport in plant chloroplasts (Dodge, 1983). The metabolism of metribuzin in plants has been the subject of many short-term and long-term studies dating back to the early 1970s. 

In green plants, vitamin K (phyUoquinone) functions as a secondary electron acceptor in photosystem I, and in bacteria a variety of menaquinones (which also have vitamin K activity) have a role in the plasma membrane in electron transport, where they serve the same role as ubiquinone (Section 14.6) in mitochondrial electron transport. There is no evidence that vitamin K has any role in electron transport in animals. 

R1. Y Inoue (1996) Photosynthetic luminescence as a simple probe of photosystem II electron transport. In ... 

The presence of the two new chlorophyll molecules ( A and A ) is significant in that it points to the similarity between the photosystem-1 and the purple-bacterial reaction centers with regard to the electron-transport pathway and the kinds of pigment molecules involved, as well as their locations. While the involvement of an intermediary chlorophyll in electron transport in photosynthetic bacteria has gradually become clear (see Chapter 7), a similar involvement of an intermediary chlorophyll in photosystem 1 can only be surmised at present. With regard to the various cofactors involved, it is not known yet which ofthe two branches, primed or unprimed in Fig. 3, constitutes the photoactive electron-transport pathway. In any event, a unidirectional electron flow along a P700->(A[Chl] )->Ao->-A ->-FeS-X- FeS-(A/B) pathway is clearly indicated. 

Fig. 7. (A) Reaction scheme involving a cyclic electron transport around photosystem I mediated by the TMPDVTMPD couple. (B) Semi-logarithmic plots of decay of absorbance changes at 700 and 575 nm in spinach D144 particles following a flash at different TMPD concentrations [TMPD at 67 pM o for 0.2 pM, for 0.6 pM, and for 1.2 pM TMPD, respectively] Figure source (B) Hiyama and Ke (1972) Difference spectra and extinction coefficient of P700. Biochim Biophys Acta 267 162.

The extent of coupling between photophosphorylation and electron transport in chloroplasts is usually expressed by the ratio of ATP formed per pair of electrons transferred, written as ATP/Cj or P/Cj. This parameter expresses the amount of ATP formed divided by the number of pairs of electrons transferred through the electron-transfer chain. The P/ej ratio for phosphorylation coupled to the transfer of electrons from water to photosystem I can be computed by taking the HVe ratio of 2 (4 protons per electron-pair transferred) and the HVATP ratio of 3 (three protons required to flow through CFo F to produce one ATP), to obtain the P/c2 value of 1.33. 

Figure 8a. Photosystem-l-dependent phosphorylation (CPP), photosystem-I-dependent electron transport (MV, methylviologen reduction in the presence of an electron donor system), and electron transport through photosystems II and I (NADP and ferricyaniae reduction) in thylakoids after freezing for 3 hours to —25°C in solutions containing different ratios of sucrose to NaCl...

Blue copper proteins contain a minimum of one Type 1 Cu centre, and those in this class include plastocyanins and azurins. Plastocyanins are present in higher plants and blue-green algae, where they transport electrons between Photosystems I and II (see above). The protein chain in a plastocyanin comprises between 97 and 104 amino acid residues (most typically 99) and has 10 500. Azurins occur in some bacteria and are involved in electron transport in the conversion of [N03] to N2. Typically, the protein chain contains 128 or 129 amino acid residues (M 14600). 

Fig. 4. The effect of irradiance on the quantum yield for electron transport by Photosystems I ( ) and II (O) measured simultaneously with the irradiance response of CO2 assimilation (Fig. 3a) in Juanulloa aurantiaca.

Fig. 5. The relationship between the quantum efficiencies for electron transport by Photosystems I and II (Ops, ps, )whose irradiance response is shown in Fig. 4.

---