Photosystem secondary electron acceptor

In green plants, vitamin K (phyUoquinone) functions as a secondary electron acceptor in photosystem I, and in bacteria a variety of menaquinones (which also have vitamin K activity) have a role in the plasma membrane in electron transport, where they serve the same role as ubiquinone (Section 14.6) in mitochondrial electron transport. There is no evidence that vitamin K has any role in electron transport in animals. 

Of all commercial herbicides, a great many are inhibitors of photosynthesis (15). Of these, most prevent light-induced reduction of Qg, the secondary electron acceptor in Photosystem II (PS II)... 

In photosystem I, the primary electron acceptor is a monomeric chlorophyll called A. The secondary electron acceptor is a bound phylloquinone (abbreviated as OQ in this book), whose in situ quinone/ semiquinone redox potential has been estimated to be < -0.81 V. Phylloquinone has the same phytol side... 

In this chapter, we will look at how charge separation takes place in PS-II reaction centers after photoexcitation and at the properties of the PS-II primary electron donor P680. In the following chapter we will discuss the so-called stable primary electron acceptor and the secondary electron acceptor Qb. This will be followed by a discussion of the intermediary electron acceptor, the species that actually accepts the electrons from the photoexcited primary donor P680. We adopt this sequence of presentation because the reduction ofQ was experimentally more readily observed than that of and was quite naturally the first experimentally observed acceptor in the course of photosystem-II research. 

A donor-limited photoinhibition pathway in the Dl/D2/C5d b559 complex can be created by the addition of the artificial electron acceptor sihcomolybdate (SiMo), as shown in Fig. 3 (D). SiMo is an efficient, exogenous secondary electron acceptor of photosystem II. After light-induced charge separa-... 

The transient intermediary electron acceptor, reaction center is expected to be reduced very rapidly following a flash, perhaps on the order of picoseconds. Not surprisingly, its belated discovery in 1979 did not come about through rapid kinetic measurements, but rather by way of the rather slow process of photo-accumulation under conditions in which the secondary electron acceptor Qa is kept in its reduced state by electrochemical manipulation. After much of the chemical and physical properties ofO had become known, the question of its photoreduction rate naturally became of interest. 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

E Schlodder K Falkenberg M Gergeleit and K Brettel (1998) Temperature dependence of forward and reverse electron transfer from Ap, the reduced secondary electron acceptor in photosystem I. Biochemistry 37 9466-9476... 

Fig. 7. Picosecond kinetics of flash-induced absorbance changes at 432 and 380-390 nm in PS-1 core complex [PTOO-AqA,] isolated from Synechocystis sp. PCC 6803. Figure source Brettel and Vos (1998) Spectroscopic resolution of the picosecond reduction kineticsofthe secondary electron acceptor in photosystem I. FEBS Lett 447 316.

K Brettel and MH Vos (1999) Spectroscopic resolution of the picosecond reduction kinetics of the secondary electron acceptor in photosystem I. FEES Lett 447 315-317... 

S Itoh and M Iwaki (1991) Full replacement of the function of the secondary electron acceptor phylloquinone (=vitamin KO by non-quinone carbonyl compounds in green plant photosystem I photosynthetic reaction centers. Biochemistry 30 5340-5346... 

P430 - a secondary electron acceptor of photosystem I P680 - primary electron donor (chlorophyll special pair) of photoystem II see Chi a ... 

The Membrane-Bound Iron-Sulfur Proteins (FeS-A and FeS-B) Secondary Electron Acceptors of Photosystem I... 

Many commercially available herbicides have been demonstrated to interfere with one or more steps of photosynthesis, by reacting near the photosystem II (PS II) center [for a recent review see ( 1), among others]. DCMU ( ) and other chemical families of photosynthetic inhibitors ( 3, ) were shown to shift the potential of the PS II secondary electron acceptor B, a specialized plastoquinone molecule, bound to a protein ( 5). ... 

Photosystem II reaction centers (RC) consist of D1 and D2 polypeptides and a bound cytochrome b 559. Isolated RC, named D1/D2 particles, contain 4-6 Chi a. 1 j3-carotene and 1 or 2 cytochrome b 559 per two pheophytin a molecules (1). Polypeptides D1 and D2 exhibit marked homologies with the L and M subunits of the RC of purple bacteria, respectively (2). PS II shares a number of functional similarities with bacterial RCs. In these particles, absorption of a photon results in a rapid charge separation between the primary electron donor. Peso molecule because of the lack of a secondary electron acceptor, relaxation of the D1/D2 reaction centers occurs through the formation of a 900 /is-lived triplet state involving Chi a molecule(s), named P (3). 

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. 

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. 

Fig. 12.8. Schematic representation of events occurring during biogenesis of photosystem I reaction center. The subunits are designated as I to VII, the abbreviations are Ferr, ferredoxin P.C., plas-tocyanin A, Aj, Aj and A4, primary, secondary, tertiary and quaternary electron acceptors PMS, phenazine methosulfate DAD, diaminodurine.

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Transient difference spectra ofthevitamin Kj-depleted and vitamin K3-reconstituted particle recorded at the appropriate time after the flash can provide another approach for obtaining the difference spectmm ofthe primary electron acceptor Aq of photosystem 1, as illustrated by Kim et al in Fig. 9 (B). Difference spectra ofthe vitamin Ki-depleted (solid line) and the vitamin K3-reconstituted particles (dotted line) were recorded 2 ns after the flash. The former spectmm should represent the formation of the radical pair, i.e., AA [P700 - P700] + [Ao -Aq] in the absence of a secondary acceptor. The latter... 

Mohanty N, Vass I, Demeter S. Copper toxicity affects photosystem II electron transport at the secondary quinone acceptor, Qg. Plant Physiol 1989 90 175-179. 

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