Plasto quinone

FIGURE 22.15 The structures of plasto-quiuoue and its reduced form, plastohydro-quiuoue (or plastoquiuol). The oxidation of the hydroquiuoue releases 2 as well as 2 c. The form shown (plastoquinone A) has nine isoprene units and is the most abundant plastoquinone in plants and algae. Other plasto-quinones have different numbers of isoprene units and may vary in the substitutions on the quinone ring. 

Ubiquinone (also called coenzyme Q) and plasto-quinone (Fig. 10-22d, e) are isoprenoids that function as lipophilic electron carriers in the oxidation-reduction reactions that drive ATP synthesis in mitochondria and chloroplasts, respectively. Both ubiquinone and plasto-quinone can accept either one or two electrons and either one or two protons (see Fig. 19-54). 

Ubiquinones function as electron transport agents within the inner mitochondrial membranes496 and also within the reaction centers of the photosynthetic membranes of bacteria (Eq. 23-32).484/488/494 The plasto-quinones also function in electron transport within... 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

Labeling experiments have shown that the plasto-quinones of chloroplasts as well as the tocopherols each bear one methyl group (marked with an asterisk in Fig. 25-4) that originates from chorismate. The dihydroxy compound homogentisate is probably an intermediate.80 83 It is a normal catabolite of tyrosine in the animal body (Fig. 25-5, Eq. 18-49). Both pren-ylation and methylation by AdoMet are required to complete the synthesis of the plastoquinones and tocopherols. Possible biosynthetic intermediates with one or more double bonds in the polyprenyl side chain have been found in plants and also in fish oils.83a... 

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

This complex catalyzes the reaction by proceeding through the Q cycle (p. 513). In the first half of the Q cycle, plastoquinoi is oxidized to plasto-quinone, one electron at a time. The electrons from plastoquinoi flow through the Fe-S protein to convert oxidized plastocyanin into its reduced form. 

Fig. 27. Electron flow in green plant photosynthesis. Vertical wavy arrows represent excitation of chlorophyll molecules by absorbed light. Reaction intermediates are as follows Z , unknown intermediate donating electrons to photocenter II Q , unknown intermediate accepting electrons from excited chlorophyll PQ, plasto-quinone, structurally similar to coenzyme Q or ubiquinone of Fig. 4 bss>, haa,, cytochrome components PC, plastocyanin, a copper-containing nonheme protein FR8, unknown ferredoxin reducing substance FD, ferredoxin FP, flavoprotein mediating reduction of NADP. ...

Copper amine oxidases Ubiquinones, plasto-quinones, vitamin K Vitamin K-dependent carboxylation Vitamin E... 

Gong H, Ohad I. The PQ/PQHBzb ratio and occupancy of photosystem II QBbb site by plasto-quinone control the d adadon of the D1 protein during photoinhibition in vivo. J Biol Chem 1991 266 21293-21252. 

Figure 1. Chemical structure for plasto-quinone. The value of n in the polyiso-prene tail is 9 for the most abundant plastoquinone in green plants, plasto-quinone A.

Secondary photosynthetic electron transfer was detected after addition of artificial electron acceptors and donors, including a quinone dependent activit y on adding decylplastoquinone. An equivalent activity was obtained on addition of the much more hydrophobic plastoquinone-9 molecule but only when a reconstitution procedure was adopted in which a diacyl glycerolipid extract of thylakoids was used. Thermoluminescence measurements showed that reconstitution with plasto-quinone-9 and lipid involved the binding of the quinone to some of the reaction centres in a preparation. Thus a limited reconstitution of quinone-reaction centre interactions could be achieved without proteins other than those already present in the isolated reaction centre. 

In order to verify that the PSII-beta is specifically responsible for the 100 ps PSII lifetime component observed at Fo conditions, we performed experiments to close PSII-beta but leave PSII-alpha open. It has been shown that PSII-beta is only weakly coupled to the plasto-quinone pool so that in the absence of DCMU it is possible to create a state, Fpl, in which the PSII-alpha centers are predominantly open but most PSII-beta centers are closed (7). If the fast, 100 ps, signal at Fo is really a composite of PSI and open PSII-beta centers then the amplitude of this signal should diminish during the Fo - Fpl transition but be constant afterwards. Fig 3a,b,c shows that such behavior is observed during the Fo - Fpl - Fm transition. 

LHC II) plays a major role in this regulation (1). The mechanism of regulation of the excitation energy distribution is based on a balance between phosphorylated and dephosphorylated LHC II, with the redox-state of the plasto-quinone as a modulator of kinase/phosphatase activity. 

Simple quinones or benzoquinones are found commonly in nature. Some quinonoid compounds such as the electron carrier ubiquinone (13), sometimes known as coenzyme Q, play important primary roles in plants. Structurally related plasto-quinones are important as electron carriers in photosynthesis. 

In contrast to photosynthetic bacteria the photosynthetic apparatus of algae and higher plants comprises a uniform prenylquinone composition (Table 3) with plastoquinone-9 + its hydroquinone (PQ-9 + PQ-9.H2) as the major component and smaller amounts of phylloquinone (vitamin K ) and o<-tocoquinone (o -TQ) (1,2). All three prenylquinones are potential photosynthetic electron carriers. The role of -tocopherol, the cyclic form of the reduced ol-tocoquinone, seems to be primarily that of a lipid antioxidant. In older, in senescent or chromoplast-bearing tissue other plastoquinone-forms (plasto-quinone B and C) may show up, or in etiolated tissue there are biosynthetic precursors of and o(-TQ (for literature see 2,17,18), but these minor quinone components do not play any role in photosynthesis. 

Key LGP, light-gathering pigments (accessory pigments and chlorophylls other than P700 and P690) PQ, plasto-quinone CYT, cytochrome PC, plastocyanin FD, ferredoxin. 

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