Photosystem electron acceptors

In 1987, the iron-sulfur clusters Fa and Fb acting as terminal electron acceptors in photosystem I have been shown to be located on a... 

Fig. 5.8. When photosystem II is activated by absorbing photons, electrons are passed along an electron-acceptor chain and are eventually donated to photosystem I and finally to NAPD+. Photosystem II is responsible for the photolytic dissociation of water and the production of atmospheric oxygen. This pathway is sometimes referred to as the Z scheme because of its zigzag route, as depicted here, but the two arms are in fact remote in space. (Note Plastocyanin (Cu) is an alternative late replacement for an Fe cytochrome complex).

Figure 10.3 Z-scheme of oxygenic photosynthesis in green algae and cyanobacteria, showing links to hydrogenase. Q (plastoquinone) and X (an iron-sulfur cluster) are electron acceptors from photosystems II and I, respectively.The two hydrogenases shown are the NADP-dependent bidirectional hydrogenase and a ferredoxin-dependent enzyme.

Ferredoxins of the 2Fe-2S type play a role in the photosynthetic electron transport as an essential electron acceptor of photosystem I. The solution... 

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

It is established that the primary electron acceptor in photosystem 1 is the molecule ferredoxin, while in photosystem 2 it is a quinone. The identity of the primary electron donor in photosystem 2 is still unknown the oxidation of water must take place by electron transfer to this primary donor, X. 

In the overall scheme of the photosynthesis of green plants the electron transport cycle starts with the excitation of chlorophyll a in photosystem 2. The excited electron then follows a downward electron acceptor chain which eventually reaches the chlorophyll a of photosystem 1 (P700) in which it can fill the positive hole left by electronic excitation. The energy released in the electron transport chain which links photosystems 2 and 1 is used for other biochemical processes which are thereby related to photosynthesis. One of these is the process of photophosphorylation which is the production of molecules with phosphate chains used as energy transfer agents in many biochemical reactions. 

The electron acceptors on the reducing side of photosystem II resemble those of purple bacterial reaction centers. The acceptor that removes an electron from P680 is a molecule of pheophytin a. The second and third acceptors are plastoquinones (see fig. 15.10). As in bacterial reaction centers, electrons move one at a time from the first quinone to the second. When the second quinone becomes doubly reduced, it picks up protons from the stromal side of the thylakoid membrane and dissociates from the reaction center. 

On the reducing site of photosystem I, the initial electron acceptor appears to be a molecule of chlorophyll a (see fig. 15.17). The second acceptor probably is a quinone, phylloquinone (vitamin K, fig. 15.10). In these respects, photosystem I resembles photosystem II and purple photosynthetic bacteria, which use pheophytin a or bac-teriopheophytin a followed by a quinone. From this point on, photosystem I is different its next electron carriers consist of iron-sulfur proteins instead of additional quinones. 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

The observation of a photosynthetic reaction center in green sulfur bacteria dates back to 1963.39 Green sulfur bacteria RCs are of the type I or the Fe-S-type (photosystem I). Here the electron acceptor is not the quinine instead, chlorophyll molecules (BChl 663, 81 -OII-Chi a, or Chi a) serve as primary electron acceptors, and three Fe4S4 centers (ferredoxins) serve as secondary acceptors. A quinone molecule may or may not serve as an intermediate carrier between the primary electron acceptor (Chi) and the secondary acceptor (Fe-S centers).40 The process sequence leading to the energy conversion in RCI is shown in Figure 21. 

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,... 

Fig. 19. Schematic functions of a Pd-Si02 colloid in controlling charge separation and Hrevolution from a photosystem including eosin, Eo2-, as photosensitizer, iV,JV -dibenzyl-4,4 -(3,3 -dimethyl)bipyridi-nium, BMV2+, as electron acceptor and triethanolamine, TEOA, as electron donor...

TEOA, as electron donor yields upon illumination in the presence of C02 and the Ru-colloid, methane as major photoproduct, cp = 4 x 10 4, and ethylene and ethane at lower yields, cp = 7.5x 10 5 and cp = 4x 10 5 respectively. In this photosystem, reductive ET quenching of excited Ru(bpz) + yields the reduced photoproduct Ru(bpz)j (E° = —0.86 V vs. SCE) that mediates the reduction of COz to methane and hydrocarbon oligomers (Fig. 30a). Interestingly, the reduced photoproduct Ru(bpz)J although thermodynamically capable, does not effect H2-evolution from the system. On the other hand, a series of photosystems composed of Ru(bpy) + as photosensitizer, TEOA as sacrificial electron donor and different bipyridinium electron acceptors (23)-(26) exhibit non-specificity, and... 

Fig. 30a, b. Photosynthetic assemblies for C02 fixation to methane a) A photosystem composed of Ru(bpz)2+ as photosensitizer and electron carrier, b) A photosystem composed of Ru(bpy)2 + as photosensitizer and different bipyridinium salts, 23-26, as electron acceptors... 

C02-fixation to formate is catalyzed by formate dehydrogenase, ForDH. Photogenerated MV+ mediates the reduction of C02 to formate [200]. Other bipyridinium radicals, such as JV,j V -dimethyl-2,2 -bipyridinium or JV,Ar -trime-thylene-2,2 -bipyridinium radical cation act also as charge carriers for ForDH. The photosystem that Was utilized for generation of MV+ and 002-fixation includes Ru(bpy)f+ as photosensitizer, cysteine as sacrificial electron donor and MV2+ as electron acceptor. The net photosynthetic process accomplished in this photosystem (Fig. 40) corresponds to the reduction of 0O2 to formate by cysteine, see Eq. (70). This is an endoergic transformation by ca. 12.5 kcal mol-1. 

Through a series of oxidation-reduction reactions driven by two light reactions operating in series and involving several hundred chlorophyll molecules, electrons flow from water to NADP. Participating in the overall reaction is a water-splitting complex that includes a mangano-protein and chloride ions. An unidentified chlorophyll a molecule serves as the reaction center of photosystem II, with Q as the primary electron acceptor. Involved sequentially on the electron transport chain are plasto-... 

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

P700 (a special chlorophyll a molecule) serves as the reaction center of photosystem I, and a bound form of ferredoxin (ferredoxin-reducing substance) may be the electron acceptor. Electrons flow subsequently to NADP through ferredoxin (a nonheme iron protein) and a flavoprotein. 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

Fig. 6. Sites of inhibitory action of DCMU in photosynthetic electron transport chain. The abbreviations are as follows - Cyt f cytochrome f, Fd ferredoxin, Mn water-splitting complex (manganese-containing), P680 pigment complex of photosystem II, P700 pigment complex of photosystem I, PC plastocyanin, PQ plastquinone, Q quencher, Rd NADP reductase and X direct electron acceptor complex...

Brettel, K., Setif, P. and Mathis, P. 1986. Flash-induced absorption changes in photosystem I at low temperature evidence that the electron acceptor Aj is vitamin Kj. FEBS Lett. 203. 220-224. 

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