Positions of Q(A)and Chl(Z)Relative to Tyrosine Y(Z)and Y(D)in Photosystem II Studied by Pulsed EPR

Pulsed EPR Dipolar Spectroscopy on Spin Pairs with one Highly Anisotropic Spin Center: The Low-Spin FeIII Case

Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling constants and thus the distance between electron spin centers. Up to now, PDS measurements have been mostly applied to spin centers whose g-anisotropies are moderate and therefore have a negligible effect on the dipolar coupling constants. In contrast, spin centers with large g-anisotropy yield dipolar coupling constants that depend on the g-values. In this case, the usual methods of extracting distances from the raw PDS data cannot be applied. Here, the effect of the g-anisotropy on PDS data is studied in detail on the example of the low-spin Fe³⁺ ion. First, this effect is described theoretically, using the work of Bedilo and Maryasov (Appl. Magn. Reson. 2006, 30, 683-702) as a basis. Then, two known Fe³⁺ /nitroxide compounds and one new Fe³⁺ /trityl compound were synthesized and PDS measurements were carried out on them using a method called relaxation induced dipolar modulation enhancement (RIDME). Based on the theoretical results, a RIDME data analysis procedure was developed, which facilitated the extraction of the inter-spin distance and the orientation of the inter-spin vector relative to the Fe³⁺ g-tensor frame from the RIDME data. The accuracy of the determined distances and orientations was confirmed by comparison with MD simulations. This method can thus be applied to the highly relevant class of metalloproteins with, for example, low-spin Fe³⁺ ions.

Pulsed EPR Dipolar Spectroscopy under the Breakdown of the High-Field Approximation: The High-Spin Iron(III) Case

Pulsed EPR dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling and thus the distance between electron-spin centers. To date, PDS measurements to metal centers were limited to ions that adhere to the high-field approximation. Here, the PDS methodology is extended to cases where the high-field approximation breaks down on the example of the high-spin Fe³⁺ /nitroxide spin-pair. First, the theory developed by Maryasov et al. (Appl. Magn. Reson. 2006, 30, 683-702) was adapted to derive equations for the dipolar coupling constant, which revealed that the dipolar spectrum does not only depend on the length and orientation of the interspin distance vector with respect to the applied magnetic field but also on its orientation to the effective g-tensor of the Fe³⁺ ion. Then, it is shown on a model system and a heme protein that a PDS method called relaxation-induced dipolar modulation enhancement (RIDME) is well-suited to measuring such spectra and that the experimentally obtained dipolar spectra are in full agreement with the derived equations. Finally, a RIDME data analysis procedure was developed, which facilitates the determination of distance and angular distributions from the RIDME data. Thus, this study enables the application of PDS to for example, the highly relevant class of high-spin Fe³⁺ heme proteins.

Cytochrome b₅₅₉ and cyclic electron transfer within photosystem II

Cytochrome b₅₅₉ (Cyt b₅₅₉), β-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PSII), which are believed to protect PSII from photodamage under conditions in which the primary electron-donation pathway leading to water oxidation is inhibited. Among these cofactors, Cyt b₅₅₉ is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b₅₅₉ is preoxidized, the photooxidation of several of the 11 Car and 35 Chl molecules present per PSII is observed. In this review, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are discussed. This article is part of a Special Issue entitled: Photosystem II.

The differences in microenvironments and functions of tyrosine radicals YZ and YD in photosystem II studied by EPR

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were performed to investigate the difference in microenvironments and functions between tyrosine Z (Y(Z)) and tyrosine D (Y(D)). Mn-depletion or Ca(2+)-depletion causes extension of the lifetime of tyrosine radical Y(Z)(*), which can be trapped by rapid freezing after illumination at about 250 K. Above pH 6.5, Y(Z)(*) radical in Mn-depleted PS II shows similar EPR and ENDOR spectra similar to that of Y(D)(*) radical, which are ascribed to a typical neutral tyrosine radical. Below pH 6.5, Y(Z)(*) radical shows quite different EPR and ENDOR spectra. ENDOR spectra show the spin density distribution of the low-pH form of Y(Z)(*) that has been quite different from the high-pH form of Y(Z)(*). The spin density distribution of the low-pH Y(Z)(*) can be explained by a cation radical or the neutral radical induced by strong electrostatic interaction. The pH dependence of the activation energy of the recombination rate between Y(Z)(*) and Q(A)(-) shows a gap of 4.4 kJ/mol at pH 6.0-6.5. In the Ca(2+)-depleted PS II, Y(Z)(*) signal was the mixture of the cation-like and normal neutral radicals, and the pH dependence of Y(Z)(*) spectrum in Ca(2+)-depleted PS II is considerably different from the neutral radical found in Mn-depleted PS II. Based on the recent structure data of cyanobacterial PS II, the pH dependence of Y(Z)(*) could be ascribed to the modification of the local structure and hydrogen-bonding network induced by the dissociation of ASP170 near Y(Z).

Long-range distance determinations in biomacromolecules by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy provides a variety of tools to study structures and structural changes of large biomolecules or complexes thereof. In order to unravel secondary structure elements, domain arrangements or complex formation, continuous wave and pulsed EPR methods capable of measuring the magnetic dipole coupling between two unpaired electrons can be used to obtain long-range distance constraints on the nanometer scale. Such methods yield reliably and precisely distances of up to 80 A, can be applied to biomolecules in aqueous buffer solutions or membranes, and are not size limited. They can be applied either at cryogenic or physiological temperatures and down to amounts of a few nanomoles. Spin centers may be metal ions, metal clusters, cofactor radicals, amino acid radicals, or spin labels. In this review, we discuss the advantages and limitations of the different EPR spectroscopic methods, briefly describe their theoretical background, and summarize important biological applications. The main focus of this article will be on pulsed EPR methods like pulsed electron-electron double resonance (PELDOR) and their applications to spin-labeled biosystems.

Structure of the Mn4-Ca cluster as derived from X-ray diffraction

The catalytic centre for light-induced water oxidation in photosystem II (PSII) is a multinuclear metal cluster containing four manganese and one calcium cations. Knowing the structure of this biological catalyst is of utmost importance for unravelling the mechanism of water oxidation in photosynthesis. In this review we describe the current state of the X-ray structure determination at 3.0 A resolution of the water oxidation complex (WOC) of PSII. The arrangement of metal cations in the cluster, their coordination and protein surroundings are discussed with regard to spectroscopic and mutagenesis studies. Limitations of the presently available structural data are pointed out and possible perspectives for the future are outlined, including the combination of X-ray diffraction and X-ray spectroscopy on single crystals.

The functional sites of chlorophylls in D1 and D2 subunits of photosystem II identified by pulsed EPR

The functional site of ChlZ, an auxiliary electron donor to P680+, was determined by pulsed ELDOR applied to a radical pair of YD * and Chlz+ in oriented PS II membranes from spinach. The radical-radical distance was determined to be 29.5 A and its direction was 50 degrees from the membrane normal, indicating that a chlorophyll on the D2 protein is responsible for the EPR Chlz+ signal. Spin polarized ESEEM (Electronin Spin Echo Envelop Modulation) of a 3Chl and QA - radical pair induced by a laser flash was observed in reaction center D1D2Cytb559 complex, in which QA was functionally reconstituted with DBMIB and reduced chemically. QA -ESEEM showed a characteristic oscillating time profile due to dipolar coupling with 3Chl. By fitting with the dipolar interaction parameters, the distance between 3Chl and QA - was determined to be 25.9 A, indicating that the accessory chlorophyll on the D1 protein is responsible for the 3Chl signal.