Distance of P680 from the manganese complex in Photosystem II studied by time resolved EPR

Pulsed EPR spectroscopy: biological applications

Pulsed electron paramagnetic resonance (EPR) methods such as ESEEM, PELDOR, relaxation time measurements, transient EPR, high-field/high-frequency EPR, and pulsed ENDOR, have been used successfully to investigate the local structure and dynamics of paramagnetic centers in biological samples. These methods allow different contributions to the EPR spectra to be distinguished and can help unravel complicated EPR spectra consisting of overlapping resonance lines, as are often found in disordered protein samples. The basic principles, specific potentials, technical requirements, and limitations of these advanced EPR techniques will be reviewed together with recent applications to metal centers, organic radicals, and spin labels in proteins.

Determination of distances from tyrosine D to QA and chlorophyllZ in photosystem II studied by '2+1' pulsed EPR

A '2+1' pulse sequence electron spin echo (ESE) method was applied to measure the dipole interactions between the tyrosine YD+ and QA- in Photosystem II (PS II). In a CN--treated PS II, QA- EPR signal was observed at g=2.0045 position, because the non-heme Fe(II) was converted into a low-spin (S=0) state. The radical pair of YD+QA- was trapped by illumination for 8 min at 273 K, followed by dark adaptation for 3 min and freezing into 77 K. By using a proton matrix ENDOR, these trapped radicals were confirmed to be YD+ and QA-, respectively. The distance between the radical pair was estimated from the dipole interaction constant fitted to the observed '2+1' ESE time profile. The distance of YD+-QA- is determined to be 38.8+/-1.1 A. The magnetic dipole interaction between YD+ and ChlZ+ was determined in a Tris-treated PS II in which ChlZ+ was generated by illumination at 200 K for 10 min. The YD+-ChlZ+ distance was estimated to be 29.4+/-0.5 A. Copyright 1998 Elsevier Science B.V.

Spin-lattice relaxation of the phyllosemiquinone radical of photosystem I

The spin-lattice relaxation time (T1) of the phyllosemiquinone anion radical, A1-, of the photosystem I (PSI) reaction center, were measured between 4.5 and 85 K by electron spin-echo spectroscopy. The selective removal of the iron-sulfur centers, FA, FB, and FX, from PSI allowed the measurement of the intrinsic T1 of the A1- radical. The temperature dependence of the intrinsic (T1)-1 for A1- was found to be approximately T1.3 +/- 0.1. The spin-lattice relaxation of the reduced form of iron-sulfur center FX was also measured at low temperatures, in FA/FB-depleted PSI membranes. It was found that the fast-relaxing FX center enhances the spin-lattice relaxation of the phyllosemiquinone due to dipolar coupling. The effect of the reduced forms of FA/FB on the T1 of the phyllosemiquinone was minor compared to the effect of FX. By analyzing the data with a dipolar model in the light of limitations imposed by other information present in the literature, the distance between the phyllosemiquinone and FX in PSI is estimated to be 14.8 +/- 4 A.

Modeling of the D1/D2 proteins and cofactors of the photosystem II reaction center: implications for herbicide and bicarbonate binding

A three-dimensional model of the photosystem II (PSII) reaction center from the cyanobacterium Synechocystis sp. PCC 6803 was generated based on homology with the anoxygenic purple bacterial photosynthetic reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis, for which the X-ray crystallographic structures are available. The model was constructed with an alignment of D1 and D2 sequences with the L and M subunits of the bacterial reaction center, respectively, and by using as a scaffold the structurally conserved regions (SCRs) from bacterial templates. The structurally variant regions were built using a novel sequence-specific approach of searching for the best-matched protein segments in the Protein Data Bank with the "basic local alignment search tool" (Altschul SF, Gish W, Miller W, Myers EW, Lipman DJ, 1990, J Mol Biol 215:403-410), and imposing the matching conformational preference on the corresponding D1 and D2 regions. The structure thus obtained was refined by energy minimization. The modeled D1 and D2 proteins contain five transmembrane alpha-helices each, with cofactors (4 chlorophylls, 2 pheophytins, 2 plastoquinones, and a non-heme iron) essential for PSII primary photochemistry embedded in them. A beta-carotene, considered important for PSII photoprotection, was also included in the model. Four different possible conformations of the primary electron donor P680 chlorophylls were proposed, one based on the homology with the bacterial template and the other three on existing experimental suggestions in literature. The P680 conformation based on homology was preferred because it has the lowest energy. Redox active tyrosine residues important for P680+ reduction as well as residues important for PSII cofactor binding were analyzed. Residues involved in interprotein interactions in the model were also identified. Herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modeled in the plastoquinone QB binding niche using the structural information available from a DCMU-binding bacterial reaction center. A bicarbonate anion, known to play a role in PSII, but not in anoxygenic photosynthetic bacteria, was modeled in the non-heme iron site, providing a bidentate ligand to the iron. By modifying the previous hypothesis of Blubaugh and Govindjee (1988, Photosyn Res 19:85-128), we modeled a second bicarbonate and a water molecule in the QB site and we proposed a hypothesis to explain the mechanism of QB protonation mediated by bicarbonate and water. The bicarbonate, stabilized by D1-R257, donates a proton to QB2- through the intermediate of D1-H252; and a water molecule donates another proton to QB2-. Based on the discovery of a "water transport channel" in the bacterial reaction center, an analogous channel for transporting water and bicarbonate is proposed in our PSII model. The putative channel appears to be primarily positively charged near QB and the non-heme iron, in contrast to the polarity distribution in the bacterial water transport channel. The constructed model has been found to be consistent with most existing data.