Iron-sulfur centers photosystem

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

The reaction center of photosystem I is larger and more complex. It contains two large polypeptides and at least seven other smaller subunits. The reactive chlorophyll a dimer P700 resides on the two main polypeptides, along with about 60 additional molecules of chlorophyll a, two quinones, and an iron-sulfur center. 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Photooxidation of P700 in photosystem I reduces a chlorophyll, which transfers electrons to a series of membrane-bound iron-sulfur centers, probably by way of a quinone. From the iron-sulfur centers, electrons move to the soluble iron-sulfur protein, ferre-... 

TheP7007P700 couple has a redox potential of+0.45 V [c/. redox-potential scale in Fig. 2]. The of the Aq/Aq" couple is probably less than -1 V if it is consistent with the in vitro redox-potential value of Chl/Chl of < -1.0 V. The initial charge separation into P700 and Ao would store approximately 1.5 eV out of 1.8 eV of energy of the absorbed 700-nm photon. The redox potential ofthe A,/A," couple is probably -0.8 V. The redox potentials ofthe iron-sulfur centers FeS-X, FeS-B and FeS-A have been experimentally determined to be -0.73, -0.58 and -0.53 V , respectively. The final electron acceptor in photosystem 1 is the [2Fe-2S]-type ferredoxin (Fd) present in the stroma region of the chloroplasts and having a redox potential of -0.4 V. Under iron-deficient growth conditions, a flavoprotein called flavodoxin is synthesized as a replacement acceptor for ferredoxin. 

The membrane-bound iron-sulfiir centers were discovered by Dick Malkin and Alan Bearden in 1971 in spinach chloroplasts using EPR spectroscopy. Since the EPR spectrum was found to resemble that of the iron-sulfur protein ferredoxin and since the soluble ferredoxin had already been removed from the chloroplast sample used in the measurement, the substance represented by the newly found EPR spectrum was initially called membrane-bound ferredoxin. And since the iron-sulfur center was also found to be photo-reducible at cryogenic temperature, it was therefore suggested that it was the primary electron acceptor of photosystem I. 

Fig. 12. (A) Stereograms of the electron-density map showing part of the PsaC-subunit in the region of the three iron-sulfur centers and (B) the corresponding model, both based on the two-[4Fe 4S] structure in P. aarogenes (P.a.-fd). See text for details. Figure source Schubert, Klukas, KrauU, Saenger, Fromme and Witt (1997) Photosystem I of Synechococcus elongatus at 4 A resolution Comparative structure analysis. J Mol Biol 272 752. Also see Color Plate 10.

A Kamlowski A Van der Est PFromme and D Stehlik (1997) Low-temperature EPR on photosystem I single crystals orientation of the iron-sulfur centers Fp andFg. Biochem Biophys Acta 1319 185-198... 

R Hootkins R Malkin and AJ Bearden (1981) EPR properties of photosystem I Iron-sulfur centers in the halophilic alga Dunaliella prava. FEBS Lett 123 229-234... 

JH Golbeck and JT Warden (1982) Electron spin resonance studies of the bound Iron-sulfur centers In photosystem I. Photoreduction of center A occurs In the absence of center B. Biochim Biophys Acta 681 77-84... 

R Malkin (1984) Diazonium modification of photosystem I. A specific effect on Iron-sulfur center B. Biochim Biophys Acta 764 63-69... 

Y Kojima YNiinomi STsuboi THIyama and H Sakurai (1987) Destruction of photosystem-i iron-sulfur centers of spinach and Anacystis nidulans by mercurials. Bot Mag Tokyo 100 243-253... 

H Sakurai K Inoue TFujii and P Mathis (1991) Effects of selective destruction of iron-sulfur center B on electron transfer and charge recombination in photosystem I. Photosynthesis Res 27 65-71... 

IR Vassiliev Y-S Jung Pi ang and JH Golbeck (1998) PsaC subunit of photosystem I is oriented with iron-sulfur dusterpB as the immediate electron donor to ferredoxin and flavodoxin. Biophys J 74 2029-2035 Y-S Jung L Yu and JH Golbeck (1995) Reconstitution of iron-sulfur center Fb results in complete restoration of NADP photoreduction in Hg-treated photosystem I complexes from Synechococcus sp PCC 6301. Photosynthesis Res 46 249-255... 

Fig. 10. Top Charge recombination reactions in photosystem I (left) and in the reaction center of green-sulfur bacteria (right). Bottom Flash-induced absorbance changes and kinetics of charge recombination in the dark. [The left panel is reproduced from Fig. 3 (A). Data in lower, right panel are taken from Kusumoto, Inoue and Sakurai (1995) Spectroscopic studies of bound cytochrome c and an iron-sulfur center in a purified reaction center from the green sulfur bacterium Chlorobium tepidum. Photosynthesis Res 43 109.

K Sigfridsson, Hansson and P Brzezinski (1995) Electrogenic light reactions in photosystem I Resolution of electron-transferrates between the iron-sulfur centers. Proc Nat Acad Sci, USA 92 3458-3462 JE Franke, L Ciesia and JT Warden (1995) Kinetics ofPsaC reduction in photosystem I. in P Mathis (ed) Photosynthesis from Light to Biosphere. Vol 2 75-78. Kluwer 27. T Hiyama and B Ke (1972) Difference spectra and extinction coefficients of P700. Biochim Biophys Acta 267 160-171... 

J Rawlings, 0 Siiman and HB Gray (1974) Low temperature electronic absorption spectra of oxidized and reduced spinach ferredoxins. Evidence for nonequivalent iron (III) sites. Proc Nat Acad Sci, USA 71 125-127 K Brettel (1988) Electron transfer from Af to an iron-sulfur center with t% = 200 ns at room temperature in photosystem I. FEBS Lett 239 93-98. 

The Iron-Sulfur Center FeS-X of Photosystem I, the Photosystem-I Core Complex, and Interaction of the FeS-X Domain with FeS-A/FeS-B... 

This chapter deals with another electron carrier on the reducing side of the electron-transfer chain of photosystem I, namely, the iron-sulfur center FeS-X (also abbreviated as Fx in the literature). As described below, studies indicate that it is a [4Fe 4S] cluster that is uniquely in being coordinated to both the two major polypeptide subunits PsaA and PsaB that make up the protein heterodimer of the PS-I reaction center. For reference, we show in Fig. 1 a model of the location of this electron carrier in the photosystem-I reaction center, in terms of both its physical locale (A) and its position in the electron-transport chain (B). 

Fig. 10. Mdssbauer spectra of reduced (left panels) and oxidized (right panels) PS-I core complex at 80 K (top panels) and 4.2 K (bottom panels), The lower spectrum in each of the left panels was corrected for the unreduced component. The table at top right lists the experimentally determined isomer-shift (IS) and quadrupole-splitting (QS) parameters for oxidized and reduced core complexes at 77 K lower table lists isomer-shift values reported in the literature for other [4Fe 4S] and [2Fe 2S] clusters. Data source Petrouleas, Brand, Parrett and Golbeck (1989) A Mossbauer analysis of the low-potential iron-sulfur center in photosystem I Spectroscopic evidence that Fx is a [4Fe-4S] cluster. Biochemistry 28 8982.

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. 

S Hoshina, R Sakural, N Kunishima, K Wada and S Itoh (1990) Selective destruction of iron-sulfur centers by heat/ethylene glycol treatment and isolation of photosystem 1 core protein. Biochim Biophys Acta 1015 61-68... 

B Lagoutte, P Setif and J Duranton (1984) Tentative identification of the apoprotein of iron-sulfur centers of photosystem 1. FEBS Lett 174 24-29... 

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