Intermediary electron acceptor

Similar to bacterial RC there is spectral and ESR evidence that a pheophytin a molecule operates as an intermediary electron acceptor in PSII-RC. Optical absorbance changes, with a spectrum similar to that of a pheophytin a anion radical could be detected in PSII-enriched particles illuminated at low redox potentials (— 0.65 V) [57,77]. The appearance of the Ph signal could be correlated to a decrease in the extent of the rise in fluorescence of PSII of chlorophyll a observed upon illumination [78]. This apparent discrepancy (reduction of an electron acceptor is expected to cause an increase of fluorescence) is now explained by the fact that the fluorescence increase is in reality a delayed fluorescence emitted by the return to the ground state of P -682 regenerated by electron transfer from the pheophytin anion [79]. The lifetime, of this transient fluorescence rise is 2-4 ns, and that of electron transfer from Ph to P -6%2 = 4 ns, when PSII particles are poised at —0.45 V [73]. This transient fluorescence increase is, however, almost totally suppressed when A,j,(Ph) is prereduced chemically before illumination. Using this experimental criterium the midpoint potential of the Ph /Ph couple has been estimated to be -0.61 V [73,80]. 

Results from additional picosecond kinetic measurements of the photochemical and electron-transfer reactions in photosynthetic bacteria also gave support to the notion of the existence of an intermediary electron acceptor. This can best be illustrated with the kinetic studies of Kaufmann, Dutton, Netzel, Leigh and Rentzepis on the involvement of BO as a transient intermediary electron acceptor in photosynthetic bacteria. When Q is functional, i.e., Q is present in the oxidized state [see Fig. 1], flash illumination would be expected to produce first the [P BO"]-Q-state followed by the [P BO] Q -state. Examination of this reaction by picosecond spectroscopy revealed both the time it takes for electron donation from P to BO and the lifetime of BO , i.e., the time it takes for BO to transfer an electron to Q. 

The above discussions may be summarized as follows when the acceptor side of the reaction center is oxidized, i.e., it is in the [Pd] QA-state, light activation produces the P -state, i.e., the [P" -r]-Q -state, where I is the transient intermediary electron acceptor, namely a BO molecule. When the reaction center is pre-reduced to the [Pdj QA -state, light activation produces the P -state, i.e., the [ P-l]-Q -state with a lifetime of 10 ns. Most of the radical pairs recombine to reform the original [P-I] state, but some form the triplet state of P. In the initial excited singlet state [P 4 ], the spins on and r are antiparallel. During the 10-n lifetime of the excited singlet state, the spins of the unpaired electrons on the radical pair interact with nuclear spins on the two molecules, or with the electron spins on or the nonheme iron atom, but in any case there is a rephasing of these two unpaired spins. Recombination of these radical pairs, now with a predominantly triplet character, leads to the formation of the triplet state of P, i.e., theP -state [P -r] QA" [P -H-Qa" -> Pdl-QA. ... 

D. Picosecond Spectroscopic Measurement of the Intermediary Electron Acceptor (I) of... 

By analogy to the intermediary electron acceptor in purple bacteria, the absorbance change seen at 670 run in Fig. 5 (D) was tentatively assigned to a BO c molecule. 

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. 

W Klimov, E Dolan, ER Shaw and B Ke (1980) Interaction between the intermediary electron acceptor (pheo-phytin) and a possible plastoquinone-iron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA 77 7227-7231... 

Fig. 1. (A) Model of the photosystem-ll reaction center showing the location of

Fig. 4. (A) EPR spectra of TSF lla particles poised at -450 mV and after 90-s illumination at 295 or 220 K and measured at two different microwave powers. (B) shows effect of microwave power (P) on the amplitude of the photoinduced narrow (singlet) and doublet EPR signals at 7 K, Figure source Klimov, Dolan and Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 98,99 and Klimov, Dolan, Shaw and Ke (1980) Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-lron complex in photosystem-ll reaction centers. Proc Nat Acad Sci, USA. 77 7228...

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. 

VV Klimov, E Dolan and B Ke (1980) EPR properties of an intermediary electron acceptor (pheophytin) in photosystem II reaction centers at cryogenic temperatures. FEBS Lett 112 97-100... 

Fig. 1. Location of the intermediary electron acceptor FeS-X (a [4Fe 4S] cluster) in the reaction center of photosystem I (A) and in the sequence of electron acceptors (with the years of their discovery shown) (B).

In spite ofthe inherent difficulties cited above in sorting out spectral differences among the three iron-sulfur centers in the electron-transfer chain of photosystem I, two research groups" ° " have independently measured minute differences in the optical spectra ofthe two types of iron-sulfur centers, and used these differences to identify FeS-X as the intermediary electron acceptor located between A and FeS-A/B, and to confirm the electron-transfer sequence from FeS-X to FeS-A/B. We will first review the attempt made by Liineberg, Fromme, Jekow and Schlodder" to identify FeS-X as the intermediary electron carrier located between A] andFeS-A/B. 

VA Shuvalov, B Ke and E Dolan (1979) Kinetic and spectral properties of the intermediary electron acceptor A, In photosystem I. Subnanosecond spectroscopy. FEBS Lett 100 5-8... 

The spectral data presented to this point are consistent with the presence of an intermediary electron acceptor, most probably vitamin Ki, i.e. phylloquinone, located prior to the three iron-sulfur centers, but the chemical nature ofthe acceptor was not verified directly by optical spectroscopy. Consequently, Brettel, S6tif and Mathis extended the measurements to the ultraviolet and visible region in an attempt to demonstrate that the absorbance change was indeed due to phylloquinone. 

Since A] is an intermediary electron acceptor, its removal would block any forward electron transfer beyond Aq, and the reduction of all carriers beyond A]. Also, in the absence of A, the extra electron on reduced Ao would be expected to reverse its course and rapidly return to P700. Inthefollowing, wewill discuss the various effects on the properties ofthe PS-I complex resulting from phylloquinone extraction and reconstitution. 

Although the NADP -reduction activity ofthe PS-I particles was relatively low, the reconstitution results are significant in that the requirement for OQ in NADP -photoreduction has been unambiguously demonstrated by these experiments. These authors also used transient optical spectroscopy to probe the electron-transport kinetics in the absence and presence of kinetic data and the NADP -photoreduction results are consistent with phylloquinone, i.e., vitamin K, being an intermediary electron acceptor of photosystem I. 

Midpoint Potential of the Intermediary Electron Acceptor O Picosecond Kinetics of Photochemical Charge Separation and Electron Transport in Photosystem II... 

The bacteriopheophytin on the A branch, is an intermediary electron acceptor while <1>b is inactive in ET. Since the bacteriopheophytins lack a central magnesium ion, they are not bound to the polypeptide chain via histidine ligands. The sequence positions that correspond to the histidines in B and Bg, are methionine and leucine, respectively. and Og are surrounded by the transmembrane helices B, C, D, and E of the L- and M-subunit, respectively, and the interactions with the polypeptide chain are exclusively non-covalent. (4>g) is sandwiched at van der Waals distances between B (Bg) and (Qg). The... 

Indirect Measurements of Effects on PS II Primary Electron Acceptor. When photosynthetic organelles (microalgae or chloro-plasts) are illuminated, electrons extracted from water are transported to a final electron acceptor NADP" which becomes reduced. Intermediary electron acceptors of the photosynthetic electron transport chain undergo various redox transients. This chain may be represented simply as follows ... 

FTIR SPECTROSCOPY OF THE PS II INTERMEDIARY ELECTRON ACCEPTOR PHOTOREDUCTION IN D1D2 REACTION CENTER... 

Thus, the reaction center is composed of a dimer of two hydrophobic proteins which are denoted D1 and D2. These contain the redox components needed to transfer an electron from the primary donor, P6 o via the intermediary electron acceptor, a pheophytin molecule, to the first and second quinone electron acceptors, and Qg... 

FTIR Spectroscopy of the PS II Intermediary Electron Acceptor Photoreduction in D1D2 Reaction Center 463... 

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