Plastoquinone oxidation

Table I shows the maximum rates of oxygen evolution in Synechococ-cus oxygen-evolving PS II preparations determined in the presence of various quinone acceptors. The maximum rates were estimated by means of the reciprocal plot (rate" vs concentration" ). DCBQ and phe-BQ served as excellent electron acceptors, followed by DMBQs and DQ, and DBMIB, an inhibitor of plastoquinone oxidation, also supported a low rate of oxygen evolution. Large variations in the rate of oxygen evolution with quinones indicate that the entire PS II electron transport is limited by reduction of the acceptors.

Fig. 53. A scheme of the photosystem 2 complex arrangement in the thylakoid membrane. P680 is the reaction center electron acceptors Phe is the pheophytin Qa and Qb are bound plastoquinone PQH2 and PQ are free plastoquinol (reduced form) and plastoquinone (oxidized form) molecules, respectively 6559 is cytochrome 559 and Z is the electron donor for the oxidized reaction center P680 (after [206, 207]).

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 or Q (coenjyme Q) (Figure 12-5) finks the flavoproteins to cytochrome h, the member of the cytochrome chain of lowest redox potential. Q exists in the oxidized quinone or reduced quinol form under aerobic or anaerobic conditions, respectively. The structure of Q is very similar to that of vitamin K and vitamin E (Chapter 45) and of plastoquinone, found in chloroplasts. Q acts as a mobile component of the respiratory chain that collects reducing equivalents from the more fixed flavoprotein complexes and passes them on to the cytochromes. 

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. 

Under the conditions of low illumination (normally shady light, which has a high proportion of long-wavelength red light), PS I takes electrons faster than PS II can supply, leaving plastoquinone in its oxidized state. As a result, LHCs are dephosphorylated and migrate to the stacked portion of the thylakoid membrane where they drive to PS II. 

The water-plastoquinone photo-oxidoreductase, also known as photosystem II (PSII), embedded in the thylakoid membrane of plants, algae and cyanobacteria, uses solar energy to power the oxidation of water to dioxygen by a special centre containing four Mn ions. The overall reaction catalysed by PSII is outlined below ... 

PS II reduces plastoquinone (PQ) at the acceptor side, the required electrons are withdrawn from water leading to release of molecular oxygen and protons on the inside of the membrane. Conceptually, the complex can be divided into 2 parts, the photochemical one with the light-driven electron-transport chain, and the catalytic one, which is responsible for water oxidation. 

Eventually, the electrons in PQBH2 pass through the cytochrome b6f complex (Fig. 19-49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6f complex and preventing photosynthetic ATP production. 

Cyanobacteria can synthesize ATP by oxidative phosphorylation or by photophosphorylation, although they have neither mitochondria nor chloroplasts. The enzymatic machinery for both processes is in a highly convoluted plasma membrane (see Fig. 1-6). Two protein components function in both processes (Fig. 19-55). The proton-pumping cytochrome b6f complex carries electrons from plastoquinone to cytochrome c6 in photosynthesis, and also carries electrons from ubiquinone to cytochrome c6 in oxidative phosphorylation—the role played by cytochrome bct in mitochondria. Cytochrome c6, homologous to mitochondrial cytochrome c, carries electrons from Complex III to Complex IV in cyanobacteria it can also carry electrons from the cytochrome b f complex to PSI—a role performed in plants by plastocyanin. We therefore see the functional homology between the cyanobacterial cytochrome b f complex and the mitochondrial cytochrome bc1 complex, and between cyanobacterial cytochrome c6 and plant plastocyanin. 

FIGURE 19-55 Dual roles of cytochrome b6f and cytochrome c6 in cyanobacteria. Cyanobacteria use cytochrome bsf, cytochrome c6, and plastoquinone for both oxidative phosphorylation and photophosphorylation. (a) In photophosphorylation, electrons flow (top to bottom) from water to NADP+. (b) In oxidative phosphorylation, electrons flow from NADH to 02. Both processes are accompanied by proton movement across the membrane, accomplished by a Q cycle. 

The extent to which an electron carrier is oxidized or reduced during photosynthetic electron transfer can sometimes be observed directly with a spectrophotometer. When chloroplasts are illuminated with 700 nm light, cytochrome/, plastocyanin, and plastoquinone are oxidized. When chloroplasts are illuminated with 680 nm light, however, these electron carriers are reduced. Explain. 

The simpler cytochrome bc] complexes of bacteria such as E. coli,102 Paracoccus dentrificans,116 and the photosynthetic Rhodobacter capsulatus117 all appear to function in a manner similar to that of the large mitochondrial complex. The bc] complex of Bacillus subtilis oxidizes reduced menaquinone (Fig. 15-24) rather than ubiquinol.118 In chloroplasts of green plants photochemically reduced plastoquinone is oxidized by a similar complex of cytochrome b, c-type cytochrome /, and a Rieske Fe-S protein.119 120a This cytochrome b6f complex delivers electrons to the copper protein plastocyanin (Fig. 23-18). 

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 photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

Figure 4. Scheme for proton transfer by plastoquinone as a mobile carrier in membrane lipid. Electrons are transferred one by one to a bound plastoquinone A (PQA) which in turn reduces external plastoquinone. When reduced, the anionic plastoquinone takes up protons to become a hydroquinone which is oxidized by the cytochrome bb f complex on the inside of the membrane to release protons. A second quinone, vitamin K, (KQ) is also involved in chloroplast electron transport, but its role in proton movement is not known. 

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. 

PSI and PSII. PSII contains the site of water cleavage, and utilizes the electrons extracted from water to reduce plastoquinone to plastoquinol. The latter diffuses through the membrane until it is reoxidized by another membrane protein, the cytochrome bf complex, which transfers the electrons to a water-soluble electron carrier (plastocyanin or cytochrome c6). This carrier in turn is oxidized by PSI, which delivers the electrons via ferredoxin to the enzymes that produce NADPH (Figure 11.6) [12],... 

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