Photosystem I electron acceptor

Note that pH 11 was used for the purely chemical titration in Fig. 8 (A) in order to reach a sufficiently negative potential while in the electrochemical titration in fig. 8 (B) pH 9.5 was sufficient to extend the potential to -700 mV. Previously, Lozier and Butler had performed a redox titration ofthe PS-I reaction and obtained results very similar to those shown in Fig. 8 (A). These workers, using the Clostridial H/HV hydrogenase system as the reductant and l,l -trimethylene-2-2 -dipyridylium dibromide as the mediator, achieved specific potentials by gradually varying the pH between 8 and 10 to effect reduction ofthe photosystem-I electron acceptors. 

Since these compounds were functioning as photosystem i electron acceptors, and the need for a proper reduction potential is well documented, reduction potentials were measured for these targets. Reduction potentials were determined in 50% ethanol/water with a dropping mercury electrode and are reported relative to the standard calomel electrode. (We are indebted to Manny Alvarez of FMC Corporation for these determinations.) Without exception, the reduction potentials were in the range expected for compounds serving as photosystem I electron acceptors (-300 to -714 mV)... 

Klughammer, C., Klughammer, B., and Pace, R., Deuteration effects on the in vivo EPR spectrum of the reduced secondary photosystem I electron acceptor A in cyanobacteria. Biochemistry, 38, 3726, 1999. 

Cuendet P and Gratzel M (1982a) Artificial photosynthetic systems in Experientia 38, 223-228 (1982b) New photosystem I electron acceptors Improvement of hydrogen production by chloroplasts in Photochem. Photobiol. 36, 203-210. 

The Hill reaction uses photosystem I. Electrons from P680 are excited and are replenished by electrons from water (leading to evolution of O2). The excited electrons in P680 pass to pheophytin and then to Q and hnally to the artihcial acceptor such as ferricyanide. 

The redox state of components of the photosystem 2 electron acceptor complex can be investigated by measurements of the EPR signals arising from the components (1 ), or by observing the potential dependence of P680 rereduction following flash excitation in samples in which water oxidation is inhibited (2 i. In spinach, both these approaches have indicated the presence of low potential acceptors in addition to tlie two iron quinones and Qt>) and pheophytin (I). 

PHOTOELECTROCHEMICAL MONITORING OF PHOTOSYSTEM I ELECTRON TRANSPORT WITH OXYGEN AS ACCEPTOR... 

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).

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. 

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... 

Tetranuclear iron-sulfur clusters are key relay stations in the electron flow in photosynthesis. Photosystem I comprises three subunits, PsaA, PsaB and PsaC. The latter contains two [Fe4S4] centres FA and FB. The core subunits PsaA and B, respectively, house a [Fe4S4] centre denoted FX in addition to other, organic cofactors. The role of this latter cluster was probed in preparations partially devoid of PsaC. It was concluded from the results that FX has a major role in controlling the electron transport through PS I.236 Since the final acceptor of the electrons in PS I is a ferredoxin with a [Fe2S2] cluster it was of interest to study a... 

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,... 

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. 

Bipyridyliums with redox potentials in the range of -300 to -500 mV, such as diquat and paraquat, can accept electrons in competition with the acceptor of photosystem I (Figure 2, site 4) and have herbicidal activity. Interception of electron flow from photosystem I essentially shunts the electron transport chain. 

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. 

Fig. 1 Schematic drawing of hydrogen peroxide photoproduction by the biological photosynthetic apparatus with electrons either from water or from an exogenous electron donor. A redox catalyst (RC) transfers electrons from the terminal acceptor of photosystem I to molecular oxygen.

Answer No NADPH is produced. Artificial electron acceptors can remove electrons from the photosynthetic system and stimulate 02 production. Ferricyanide competes with the cytochrome b6f complex for electrons and removes them from the system. Consequently, P700 (of photosystem I) does not receive any electrons that can be activated for NADP+ reduction. However, 02 is evolved because all components of photosystem II are oxidized (see Fig. 19-56). 

---