Early electron acceptor

Figure 23-17 The zigzag scheme (Z scheme) for a two-quantum per electron photoreduction system of chloroplasts. Abbreviations are P680 and P700, reaction center chlorophylls Ph, pheophytin acceptor of electrons from PSII QA, Qg, quinones bound to reaction center proteins PQ, plastoquinone (mobile pool) Cyt, cytochromes PC, plastocyanin A0 and Aj, early electron acceptors for PSI, possibly chlorophyll and quinone, respectively Fx, Fe2S2 center bound to reaction center proteins FA, FB, Fe4S4 centers Fd, soluble ferredoxin and DCMU, dichlorophenyldimethylurea. Note that the positions of P682, P700, Ph, Qa/ Qb/ Ay and A, on the E° scale are uncertain. The E° values for P682 and P700 should be for the (chlorophyll / chlorophyll cation radical) pair in the reaction center environment. These may be lower than are shown.

In contrast to Photosystem II, which operates in a highly oxidizing regime. Photosystem I is much more reducing. The redox potentials of the early electron acceptors in Photosystem I are approximately -1V, with the excited state of the photoactive chlorophyll P700, estimated to be -1.26 V (Table 3). 

The Early Electron Acceptors of Photosynthetic Bacteria Bacteriochlorophyll and Bacteriopheophytin... 

Chapter 7 The Early Electron Acceptors of Photosynthetic Bacteria II. Photochemical Accumulation of Reduced BO (Ba> or r)... 

At this point, a brief mention will be made regarding the redox potential of the [Brb/BO"] couple. Being an early electron acceptor in bacterial reaction centers, its redox potential is an important property for the understanding of the energetics ofthe photochemical reactions. Klimov, Shuvalov, Krakhmaleva, Klevanik and Krasnovsky determined the redox potential of the [BO/BO"] couple by redox potentiometry. The approach taken was to gradually decrease in a straightforward manner the ambient... 

W Nitschke, U Feller, W Lockau and G Flauska (1987) The photosystem of the green sulfur bacterium Chbrobium limicola contains two early electron acceptors similar to photosystem I. FEBS Lett 218 283-286... 

Fig. 4. (A) Top light-minus-dark EPR spectrum of TSF-I particles poised at -625 mV and 9 K in the g=1.78 region ([FeS-X -FeS-X] spectrum) middle and bottom kinetics of flash-induced EPR-signal at g=1.78 on two different time scales. (B) Kinetics of the dark decay of the of the EPR signal of FeS-X" at g=1.79 (top) and of P700 at g=2.0026 (bottom) in an LDS-fractionated PS-1 core complex from spinach. Figure sources (A) Shuvalov, Dolan and Ke (1979) Spectral and kinetic evidence for two early electron acceptors in photosystem I. Proc Nat Acad Sci, USA 76 772 (B) Warden and Golbeck (1986) Photosystem I charge separation in the absence of centers A and B. II. ESR spectral characterization of center X and correlation with optical signal A. Biochim Bioohvs Acta 849 28.

VA Shuvalov, E Dolan and B Ke (1979) Spectral and kinetic evidence for two early electron acceptors in photosystem I. Proc Nat Acad Scl, USA 76 770-773. 

Fig. 3. (A) Difference spectrum constructed from absorbance change transients in PS-1 particles poised at —625 mV [in the presence of dithionite] in a pH 10 buffer. Each transient was induced by a 3-s iliumination (B) AA obtained directiy with a P700-enriched, PS-i particie in a conventionai, commerciai spectrophotometer in the light-minus-dark mode (C) AA between 350 and 750 nm measured at 200 K with a Triton-fractionated, PS-1 particle from pea chloroplasts the spectrum shown was obtained by first illuminating at 200 K for 45 m and then at 215 K for three 30-m incremental periods. Figure source (A) Swarthoff, Gast, Amesz and Buisman (1982) Photoaccumulation of reduced primary electron acceptors of photosystem I of photosynthesis. FEBS Lett 146 131 (B) Ikegami and Ke (1984) A 160-kilodalton photosystem-l reaction-center complex. Low temperature absorption and EPR spectroscopy of the early electron acceptors. Biochim Biophys Acta 764 75 (C) Mansfield and Evans (1985) Optical difference spectrum of the electron acceptor Ao in photosystem I. FEBS Lett 190 239.

I Ikegami and B Ke (1984) A 160-kllodalton photosystem-l reaction-center complex. Low temperature absorption and EPR spectroscopy of the early electron acceptors. Biochim Biophys Acta 764 70-79... 

FIGURE 1 Schematic models of chlorosomes from the green gliding bacteria, e.g. Chloroflexus aurantiacus (A) and the green sulfur bacteria, e.g. Chlorobium vibrioforme (B). The numbers refer to wavelength maxima of antenna bacteriochlorophyll complexes. The reaction centers are of the "quinone type in A, with a QA Qb acceptor complex and the Fe-S type in B, with Fe-S proteins as early electron acceptors. The rod elements probably contain oligomers of bacteriochlorophyll c, d or e, as discussed in Ae text. 

Photosystem I (PS I) reaction center complex consists of two large subunits which carry antenna chlorophyll P-700 and early electron acceptors and several small subunits, one of which contains the iron-sulfur centers, F and Fg. Because the PS I complexes are usually isolated with detergents, careful evaluation of detergent-effects, such as solubilization of chlorophyll a, or inactivation of P-700, is essential for determination of stoichiometry of subunit polypeptides and functional constituents in the complex. Recently, we have shown that the differential extinction coefficient of P-700 is markedly affected by sodium dodecyl sulfate (SDS) which induces a band-shift of chlorophyll molecules at 690 nm [1]. Herein, we report effects of other detergents on the extinction coefficient of P-700 and provide a simple method to estimate the extinction coefficient in PS I preparations treated with various detergents. Using the extinction values thus determined,... 

This suggests that BPheo acts as an early electron acceptor, as was also suggested by flash spectroscopy (C. Kirmayer et al., personal communication). 

The reaction center of . aurantiacus contains the following components the primary donor, P865, is a BChl a dimer while two additional BChl a molecules are present, which are responsible for the absorption bands around 800 nm. Three molecules of BPheo are present in contrast to two in the reaction center of purple bacteria. The one with absorption band at 540 nm at 4 K probably acts as an early electron acceptor. Menaquinone (vitamin K2) is a secondary electron acceptor. The reaction center pigment arrangement and the organization of the electron acceptor chain show clear similarities between the reaction center of C. aurantiacus and those of purple bacteria. 

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