Exposure of highly resolved Photosystem II preparations to 2 M NaCl produces an 8OYo inhibition of oxygen-evolution activity concomitant with extensive loss of two water-soluble polypeptides (23 and 17 kDa). Addition of Ca'+ to salt-washed PS II membranes causes an acceleration in the decay of Zt, the primary donor to P-680+, and we show here that this acceleration is due to reconstitution of oxygenevolution activity by Ca'+. Other cations (Mg'+, Mn2+, Sr'+) are much less effective in restoring oxygen evolution. On the basis of these observations we propose that Ca2+, perhaps in concert with the 23 kDa polypeptide, is an essential cofactor for electron transfer from the 'S'-states to Z on the oxidizing side of PS II.Photosystem II Oxygen evolution Polypeptide Calcium

Exposure of detergent-isolated preparations of the Photosystem II complex to 2 M NaCl releases watersoluble 17 and 23 kDa polypeptides; the inhibited rate of oxygen evolution activity is stimulated by addition of Ca2+ ) FEBS Lett. 167, 127-1301. Reactivation of oxygen evolution activity by Ca2+ requires the presence of the ion in high (mM) non-physiological concentrations. Using a new dialysisreconstitution procedure we have shown that rebinding of the 17 and 23 kDa polypeptides restores oxygen evolution activity only when the system has not been pretreated with EGTA. Removal of loosely-bound Ca2+ from the salt-extracted PS II complex and from the polypeptide solution, by dialysis against EGTA, blocks reconstitution of oxygen evolution activity even though the two polypeptides do rebind; restoration of Ca*+ to EGTA-treated systems, after rebinding of the 17 and 23 kDa polypeptides, results in a strong reconstitution of oxygen evolution activity. The effect of rebound 17 and 23 kDa polypeptides is to promote high affinity binding of Ca2+ to the reconstituted membrane.Photosystem II Oxygen evolution Calcium Poiypeptide

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X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca2+ replacement in the reaction center complex of the photosynthetic oxygen-evolving complex (OEC). EPR and activity measurements demonstrate that, in the absence of the 17 and 23 kDa extrinsic polypeptides, it is not necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca2+ by Sr2+, Dy3+, or La3+. The extended X-ray absorption fine structure (EXAFS) spectra for the OEC show evidence for a Mn···M interaction at ca. 3.3 Å that could arise either from Mn···Mn scattering within the Mn cluster or Mn···Ca scattering between the Mn cluster and the inorganic Ca2+ cofactor. There is no significant change in the either the amplitude or the phase of this feature when Ca2+ is replaced by Sr2+ or Dy3+, thus demonstrating that there is no EXAFS-detectable Mn···Ca contribution at ca. 3.3 Å in these samples. The only significant consequence of Ca2+ replacement is a small change in the ca. 2.7 Å Mn···Mn distance. The average Mn···Mn distance decreases 0.014 Å when Ca2+ is replaced by Sr2+ and increases 0.012 Å when Ca2+ is replaced by Dy3+. A structural model which can account both for the variation in Mn···Mn distance and for the known properties of Ca2+-substituted samples is one in which there is a hydrogen bond between a Ca2+-bound water and a Mn2(μ-O)2 unit. This scheme suggests that an important role for the Ca2+ may be to modulate the protonation state, and thus the redox potential, of the Mn cluster.

Inhibition of photosystem II electron transport by UV-B radiation has been studied in isolated spinach photosystem II membrane particles using low-temperature EPR spectroscopy and chlorophyll fluorescence measurements. UV-B irradiation results in the rapid inhibition of oxygen evolution and the decline of variable chlorophyll fluorescence. These effects are accompanied by the loss of the multiline EPR signal arising from the S2 state of the water-oxidizing complex and the induction of Signal IIfast originating from stabilized Try-Z+. The EPR signals from the QA-Fe2+ acceptor complex, Tyr-D+, and the oxidized non-heme iron (Fe3+) are also decreased during the course of UV-B irradiation, but at a significantly slower rate than oxygen evolution and the multiline signal. The decrease of the Fe3+ signal at high g values (g = 8.06, g = 5.6) is accompanied by the induction of another EPR signal at g = 4.26 that arises most likely from the same Fe3+ ion in a modified ligand environment. UV-B irradiation also affects cytochrome b-559. The g = 2.94 EPR signal that arises from the dark- oxidized form is enhanced, whereas the light inducible g = 3.04 signal that arises from the photo-oxidizable population of cytochrome b-559 is diminished. UV-B irradiation also induces the degradation of the D1 reaction center protein. The rate of the D1 protein loss is slower than the inhibition of oxygen evolution and of the multiline signal but follows closely the loss of Signal IIslow, the QA-Fe2+ and the Fe3+ EPR signals, as well as the release of protein-bound manganese. It is concluded from the results that UV-B radiation affects photosystem II redox components at both the donor and acceptor side. The primary damage occurs at the water-oxidizing complex. Modification and/or inactivation of tyrosine-D, cytochrome b-559, and the QAFe2+ acceptor complex are subsequent events that coincide more closely with the UV-B-induced damage to the protein structure of the photosystem II reaction center.

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Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn(2+). Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Yz (+), the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Yz (+) reduction by benzidine was a linear function of benzidine concentration. The rate of Yz (+) reduction by Mn(2+) at pH 6 increased linearly at low Mn(2+) concentrations and reached a maximum at the Mn(2+) concentrations equal to several times the reaction center concentration. The rate was inhibited by K(+), Ca(2+) and Mg(2+). These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Yz (+) reduction at pH 7.5 was biphasic with a fast 400 μs phase that suggests binding of Mn(2+) near Yz (+) at a site that may be one of the native manganese binding sites.