Photosystem signals

The question of the molecular basis for the S states has existed since the original proposal by Kok and coworkers. As first formulated, the S state designation referred to the oxidation state of the O2-evolving center which could, in principle, include all of photosystem II and its associated components. Indeed, there are a number of redox-active components on the electron-donor side of photosystem II in addition to the Mn complex, such as the tyrosine radical that gives rise to EPR signal, and cytochrome b jg. 

If we return to the possible arrangements of Mn in photosystem II that are consistent with the EXAFS data, one can rule out the possibility that the Mn ions are arranged as two isolated inequivalent Mn dimers because this arrangement cannot account for the Sj -state g - 4.1 EPR signal. This leaves two possible arrangements of the Mn ions a Mn trimer plus a mononuclear Mn center, or a Mn tetramer. Consequently, one should look for an assignment for the 2-state multiline EPR signal from either a mixed-valence Mn trimer or tetramer rather than from a mixed-valence Mn dimer. 

When the primary electron donation pathway in photosystem II is inhibited, chlorophyll and p-carotene are alternate electron donors and EPR signals for Chl+ and Car+ radicals are observed.102 At 130 GHz the signals from the two species are sufficiently resolved to permit relaxation time measurements to be performed individually. Samples were Mn-depleted to remove the relaxation effects of the Mn cluster. Echo-detected saturation-recovery experiments were performed with pump pulses up to 10 ms long to suppress contributions from cross relaxation and spin or spectral diffusion. The difference between relaxation curves in the absence of cyanide, where the Fe(II) is S = 0, and in the presence of cyanide, where the Fe(II) is S = 2, demonstrated that the relaxation enhancement is due to the Fe(II). The known distance of 37 A between Fe(ll) and Tyrz and the decrease of the relaxation enhancement in the order Tyrz > Car+ > Chl+ led to the proposal of 38 A and > 40A for the Fe(II)-Car+ and Fe(II)-Chl+ distances, respectively. Based on these distances, locations of the Car+ and Chl+ were proposed. 

Styring S, Rutherford AW. In the oxygen-evolving complex of photosystem II the So state is oxidized to the Si state by D+ (signal IIsiow). Biochemistry. 1987 26(9) 2401-5. 

Chlorophyll a fluorescence was measured with a dual-modulated fluorometer (Photosystem Instruments, Trtilek et al. 1997). Minimum (Fa) and maximum fluorescence (Fm) was recorded after dark adaptation (5 min at 4°C, sufficient to attain stabilization of the fluorescence signal for all light regimes). The maximum quantum yield of photosynthesis FvIFm was calculated as (Fm-F0)/Fm (Krause and Weis 1991). 

Trebitsh, T. and Danon, A. 2001. Translation of chloroplastpsbA mRNA is regulated by signals initiated by both photosystems II and I. Proc. Natl. Acad. Scie. 98, 12289-12294. 

FTR plays the central role in light regulation of the activity of enzymes involved in oxygenic photosynthesis. The light signal is converted into reducing equivalents in the form of reduced 2Fe Fd by photosystem I and FTR catalyzes the reduction of disulfides in thioredoxin (Trx) / and m using the... 

Fig. 2. (A) Light-induced EPR transient of P680 in PS-II particles containing 2 mM NH2OH and (B) EPR spectrum constructed from the type of transient signal in (A) (C) Flash-induced EPR transients at 3318 G [trace (b)] and 3330 G [trace (a)] in PS-II particles treated with both acetate and FCCP, and (D) EPR spectra constructed from the transients in (C) (a) and (b) in (D) refer to magnetic fields for traces in (C). See text for discussion. (A, B) from Ghanotakis and Babcock (1983) Hydroxylamine as an inhibitor between Z and P680 in phofosysfem II. FEBS Lett 153 233 (C, D) from Bock, Gerken, Stehlick and Witt (1988) Time resolved EPR on photosystem II particles after irreversible and reversible inhibition of water cleavage with high concentrations of acetate. FEBS Lett 227 143, 144.

Fig. 7. (A) Photooxidation pathways of the alternate electron donors Cyt b559 and Chiz in PS II. (B) effect of illumination temperature on the photooxidation of competing electron donors in PS II as monitored by EPR signal intensities. See text for discussion. Plot (B) reproduced from Thompson and Brudvig (1988) Cytochrome b559 may function to protect photosystem II from photoinhibition. Biochemistry 27 6656.

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...

Fig. 6. (A) Plot of amplitude of light-induced pheophytin-reduction signal vs. ambient potential of the medium. (B) Effed of ambient redox potential on the extent of light-induced PS-II reaction-center triplet signal in pea chloroplast particles. (C) Plot of the extent of the light-induced triplet EPR signal in (B) vs. redox potential. Open and closed circles are for reductive and oxidative titrations, respedively. The solid curve is a computer fit of the Nernst equation with n=1 and E , was estimated to be -604 mV. Figure source (A) Klimov. Allakhverdiev. Demeter and Krasnovsky (1979) Photoreduction ofpheophytin in photosystem 2 ofchloroplasts with respect to the redox potential of the medium. DokI Akad NaukSSSR 249 229 (B and C) Rutherford, Mullet and Crofts (1981) Measurement of the midpoint potential of the pheophytin acceptor of photosystem II. FEBS Lett 123 236,237...

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