Electron flow from reduced photosystem

Figure 1. Possible pathways of electron flow from reduced photosystem I.

Figure 19.20. Electron Flow Through Photosystem I to Ferredoxin. Light absorption induces electron transfer from P700 down an electron-transfer pathway that includes a chlorophyll molecule, a quinone molecule, and three 4Fe-4S clusters to reach ferredoxin. The positive charge left on P700 is neutralized by electron transfer from reduced plastocyanin.

The classical inhibitor for the Q, site is another antibiotic, antimycin, that is produced by various species of Streptomyces. Antimycin blocks electron transfer from reduced Cyt 6(HP) to the oxidized quinone at the Qr site [see Pig. 12 (B, b)]. Binding of antimycin shifts the a-band of reduced Cyt ( (HP) from 562 to 564 nm in the Cyt bc complex. It has been demonstrated that antimycin also prevents the binding of the semiquinone radical in the Cyt 6(LP) domain. When both of these inhibitors are present, all electron transfers to and from either ofthe 6-cytochromes are blocked, a situation that has often been called a double kill of cytochrome 6 [see Fig. 12 (B, c)]. It should be noted, however, that even though myxothiazol and antimycin are very effective inhibitors for the be complexes, these compounds have virtually no inhibitory action on the A /complex, suggesting that some important structural differences must exist among these complexes. However, antimycin at a high concentration does inhibit cyclic electron flow around photosystem II, presumably by acting on a different protein. 

There seems to be no question about the functional Importance of double bonds content of the acyl lipids. On the other hand, the contention that the fluidity per se Is a key factor governing photosynthetic capacity does not have any experimental support. The rate of the electron flow between the photosystems was Inhibited by saturation up to the level of cca. 20%, but the fluidity of the lipid matrix was even higher than at the beginning. Further Increase In lipid saturation led to an effective enhancement of mlcrovlscoslty In the deep hydrophobic core, but this has no apparent Influence on the rate of steady-state electron flow from PS II to PS I. Instead a gradual decrease observed In the rate of electron transport confined to PS II. According to our most recent studies (16), catalytic hydrogenation affects the dissociation of reduced plastoqulnone from the B protein and decreases electron transfer between the primary electron acceptor Q and the secondary acceptor, B. 

Cyclic photophosphorylation is also a highly energetic reaction. The bipyridyliums, paraquat and diquat (Figure 2.2), divert the electron flow of cyclic photophosphorylation (photosystem I). The capture of an electron from the chlorophyll reduces the herbicide and the reduced herbicide reacts with oxygen to form superoxide. Superoxide produces hydrogen peroxide within the chloroplast and these two compounds interact to form hydroxyl radicals in the presence of an iron catalyst. Hydroxyl radicals are very damaging and lead to the destruction of the cellular components leading to rapid plant death. 

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. 

Electrons from ferredoxin may also reduce O2, which yields H2O2 and eventually H20 (o2 + 2e + 2H+ -> H202 - H20 +102). (The light-de-pendent consumption of O2, as occurs when electrons from ferredoxin or from one of the iron-plus-sulfur-containing intermediates of Photosystem I move to O2, is termed the Mehler reaction.) Because equal amounts of O2 are evolved at Photosystem II and then consumed using reduced ferredoxin in a separate reaction, such electron flow is termed pseudocyclic (see Figs. 5-18 and 6-4). No net O2 change accompanies pseudocyclic electron flow,... 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

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). 

In vivo, most electrons from reduced ferredoxin are passed onto nicotinamide adenine dinucleotide phosphate cation (NADP ), via ferredoxin-NADP reductase, to generate the NADPH needed to drive carbon dioxide fixation by the Calvin cycle. Thus electrons from photosystem I can pass through at least three routes (Figure 1), of which route C is preferred (II). However, if the supply of NADP were limited, for example, because of a poor supply of carbon dioxide causing a slow turnover of the Calvin cycle, the electron flow rate along pathway C would be expected to be decreased and more 02" should be made by route B and, to a lesser extent, by route A (15-17), Some oxygen reduction takes place even when carbon dioxide is present in ample amounts (18). 

Herbicides inhibit photosynthesis by interrupting electron flow on the reducing side of the reaction center of photosystem 11. As originally proposed by Wraight and by Velthuy , based on several lines of evidence, that inhibition by a herbicide occurs in the Qe-binding site in D1 protein and that the action arises from the ability of the herbicide molecule to compete with Qb forthebinding site, thus resulting in the disruption of electron transfer from to Qb-... 

Electrons also can flow through the single photosystem of purple bacteria via a linear (noncyclic) pathway. In this case, electrons removed from reaction-center chlorophylls ultimately are transferred to NAD" (rather than NADP" as in plants), forming NADH. To reduce the oxidized reaction-center chlorophyll a back to its ground state, an electron is transferred from a reduced cytochrome c the oxidized cytochrome c that is formed is reduced by electrons removed from hydrogen sulfide (H2S), forming elemental sulfur (S), or from hydrogen gas (H2). Since H2O is not the electron donor, no O2 is formed. 

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