Plastoquinone reduction

Mus, F., Cournac, L., Cardettini, V., Caruana, A. and Peltier, G. 2005. Inhibitor studies on non-photochemical plastoquinone reduction and H2 photoproduction in Chlamydomo-nas reinhardtii. Biochim. Biophys. Acta 1708, 322-332. 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

When the plastohydroquinone becomes fully oxidized at the lumenal surface ofthe membrane, it loses two electrons and also releases two protons to the (inside) lumen phase. Thus accompanying the reduction of one plastoquinone molecule by PS 11 and its subsequent reoxidation, there is a net transfer of two protons from the (outside) stromal phase into the (inside) lumen phase. Thus with the splitting of two water molecules by PS 11 to form one oxygen molecule, four protons are translocated across the membrane. As mentioned above, oxidation of two water molecules also releases an additional four protons into the lumen space. Thus water splitting and plastoquinone reduction/re-oxidation result in the generation of eight protons and the creation of a proton gradient across the thylakoid membrane. 

Fig. 16. Effect on the light-induced absorbance change at 515 nm (AAsis) and photophosphoiylation (ATP) and on electron transport (measured by oxygen evolution [Oj], P700 photooxidation [P700] and plastoquinone reduction [PQ]) as a function of (A) ethanol and (B) desaspidin concentrations. Figure source Witt (1971) Coupling of quanta, electrons, fields, ions and phosphorylation in the functional membrane of photosynthesis. Results by pulse spectroscopic methods. Quart Rev Biophys 4 438.

Chlorophyll, plastoquinone, and cytochrome are complicated molecules, but each has an extended pattern of single bonds alternating with double bonds. Molecules that contain such networks are particularly good at absorbing light and at undergoing reversible oxidation-reduction reactions. These properties are at the heart of photosynthesis. 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

The role of ubiquinone (coenzyme Q, 4) in transferring reducing equivalents in the respiratory chain is discussed on p. 140. During reduction, the quinone is converted into the hydroquinone (ubiquinol). The isoprenoid side chain of ubiquinone can have various lengths. It holds the molecule in the membrane, where it is freely mobile. Similar coenzymes are also found in photosynthesis (plastoquinone see p. 132). Vitamins E and K (see p. 52) also belong to the quinone/hydroquinone systems. 

Exercise 26-19 The biologically important quinone called plastoquinone is similar to CoQ, except that the CH30— groups of CoQ are replaced by CH3— groups. What differences in properties would you expect between plastoquinone and CoQ and their respective reduction products Consider half-cell potentials (see Exercise 26-15), solubility in polar and nonpolar solvents, and relative acidity. 

Photosystems I and II operate in concert. Their interaction is described in the Z scheme (shown in outline in Figure 18). In photosystem II, the primary oxidant is able to remove electrons from water. These electrons are transported to photosystem I via plastoquinone and plastocyanin to replace PSI electrons that have been used in the reduction of iron-sulfur proteins and transferred via NADP to 0O2. Electron flow between PSII and PSI is accompanied by the synthesis of Atp 367 These oxidizing and reducing aspects of photosynthesis can be separated and other substrates incorporated. 

The electron from P680 is transferred to a series of plas-tiquinone (PQ) derivatives, leaving behind an oxidized P680 molecule as shown in Figure 3-2. The reduction of plastoquinone is similar to that of Coenzyme Q in mitochondrial oxidation/reduction, in that PQ can accept either one or two electrons at a time. Plastiquinone molecules accept a proton (H+) from the stroma for each electron they accept. This leaves the stroma more basic than it was before, creating part of the gradient that will be used for ATP synthesis. 

Reduction of plastoquinone Qb by QA- and protonation at the acceptor side of PSII. The Qa is tightly bound to the protein, acting as a one electron acceptor. It passes electrons to a second plastoquinone, Qb, which can accept two electrons and two protons and acts as a mobile electron carrier connecting PSII to the next complex of the photosynthetic apparatus (i.e. the cytochrome b(f complex). After two electron-reductions and two protonation events, QbH2 leaves the reaction center and is replaced by an oxidized quinone from the pool in the membrane. 

Herbicides that inhibit photosynthetic electron flow prevent reduction of plastoquinone by the photosystem II acceptor complex. The properties of the photosystem II herbicide receptor proteins have been investigated by binding and displacement studies with radiolabeled herbicides. The herbicide receptor proteins have been identified with herbicide-derived photoaffinity labels. Herbicides, similar in their mode of action to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) bind to a 34 kDa protein, whereas phenolic herbicides bind to the 43-51 kDa photosystem II reaction center proteins. At these receptor proteins, plastoquinone/herbicide interactions and plastoquinone binding sites have been studied, the latter by means of a plastoquinone-deriv-ed photoaffinity label. For the 34 kDa herbicide binding protein, whose amino acid sequence is known, herbicide and plastoquinone binding are discussed at the molecular level. 

Mitochondria contain ubiquinone (also known as coenzyme Q), which differs from plastoquinone A (Chapter 5, Section 5.5B) by two methoxy groups in place of the methyl groups on the ring, and 10 instead of 9 isoprene units in the side chain. A c-type cytochrome, referred to as Cyt Ci in animal mitochondria, intervenes just before Cyt c a h-type cytochrome occurring in plant mitochondria is involved with an electron transfer that bypasses cytochrome oxidase on the way to 02. The cytochrome oxidase complex contains two Cyt a plus two Cyt a3 molecules and copper on an equimolar basis with the hemes (see Fig. 5-16). Both the Fe of the heme of Cyt a3 and the Cu are involved with the reduction of O2 to H20. Cytochromes a, >, and c are in approximately equal amounts in mitochondria (the ratios vary somewhat with plant species) flavoproteins are about 4 times, ubiquinones 7 to 10 times, and pyridine nucleotides 10 to 30 times more abundant than are individual cytochromes. Likewise, in chloro-plasts the quinones and the pyridine nucleotides are much more abundant than are the cytochromes (see Table 5-3). 

Isoprenylated Quinones.—Chemistry. An efficient method has been described for the preparation of ubiquinone-1 (221) and plastoquinone-1 (223) from the parent quinone and allyltributyltin. The synthesis of ubiquinone-10 by isoprenoid chain-elongation of a ubiquinone-1 derivative has been reported.The sul-phone derivative of the protected ubiquinol-1 (224) on reaction with solanesyl bromide (225) and McjCOK gave the sulphone (226) in 90% yield. Benseker reduction to remove the PhCH2- and PhS02-groups, followed by oxidation in air, afforded ubiquinone-10 (222). 

---