2H ESE-ENDOR study of hydrogen bonding to the tyrosine radicals YD.bul. and YZ.bul. of photosystem II.

Force, Dee Ann; Randall, David; Britt, R. David; Tang, Xiao-Song; Diner, Bruce A.

doi:10.1021/ja00155a032

Cited by 94 publications

(123 citation statements)

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“…This is of particular interest because the hydrogen bond is believed to have a significant effect on redox potential and radical reactivity (48). The symmetry-related Y Z • radical in PS II is assumed to be in a more heterogeneous environment with a less well defined hydrogen bond (49). A comparative analysis of the binding situation of the tyrosine radicals, therefore, will help to understand the properties of Y Z…”

Section: Discussionmentioning

confidence: 99%

Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{_{D}^{{\bullet}}}}\end{equation}\end{document}

Hofbauer

Zouni

Bittl

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

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Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is used to study the dark-stable tyrosine radical Y D• in single crystals of photosystem II core complexes (cc) isolated from the thermophilic cyanobacterium Synechococcus elongatus. These complexes contain at least 17 subunits, including the water-oxidizing complex (WOC), and 32 chlorophyll a molecules͞PS II; they are active in light-induced electron transfer and water oxidation. I n oxygenic photosynthesis, two photosystems (PS I and PS II) function in sequence to convert light into energy-rich chemical compounds (1, 2). PS I uses energy from the absorption of a photon to reduce NADP ϩ to NADPH, which is required for CO 2 reduction. The electrons for this process are donated by PS II. On light excitation of PS II, an electron is transferred from the primary donor P 680 , a chlorophyll species, via an intermediate pheophytin Pheo a to the plastoquinone acceptors Q A and Q B . Two sequential univalent redox steps and concomitant protonation events lead to plastohydroquinol Q B H 2 , which leaves PS II and provides electrons to PS I via the cytochrome b 6 f complex (for review, see ref.2). The photooxidized cation radical P 680•ϩ has the highest oxidation potential of all cofactors known in nature (Նϩ1.1 V), which is sufficient for water oxidation. P 680•ϩ extracts an electron from a redox active tyrosine Y Z . The intermediate tyrosine radical Y Z• , in turn, oxidizes the water-oxidizing complex (WOC), a tetranuclear manganese cluster. The WOC passes through a cycle of four one-electron oxidation steps in which water is oxidized and protons and O 2 are released (for reviews, see refs. 3-7). The exact water-splitting mechanism is still unknown.The cofactors involved in the electron transfer chain of PS II are bound to two protein subunits, D 1 and D 2 . From amino acid sequence homology (8-10), two-dimensional electron crystallography (11), and computer modeling (12), D 1 and D 2 are assumed to be arranged analogously to the L and M subunits in the reaction center of purple bacteria. This analogy has been supported recently by x-ray crystallographic studies of the PS II single crystals (13). Whereas Y Z in D 1 connects P 680•ϩ to the WOC in the electron transfer chain, the homologous Y D in D 2 does not seem to take part in the charge separation process. However, Y D is also coupled to the WOC and, under illumination, forms a dark-stable radical Y D• (Fig. 1). The functional role of Y D is not understood in detail; it may be necessary for assembly of the PS II complex (14, 15). Recent results also suggest that it may play a role in preventing photoinhibition during activation of the PS II complex (16).In the light-induced single electron transfer process and in the water-splitting cycle, various paramagnetic species are formed (17) that have been studied by conventional X-band (9 GHz) electron paramagnetic resonance (EPR) techniques during the last decade. Most of these species can be observed only in the freeze-trapped state in frozen PS II solutions. ...

show abstract

Section: Discussionmentioning

confidence: 99%

Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: The tyrosine radical Y\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation}{\mathrm{_{D}^{{\bullet}}}}\end{equation}\end{document}

Hofbauer

Zouni

Bittl

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

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“…The origin for the stability of the Y D radical is thought to be caused by a deprotonation resulting in the more stable neutral radical Y D C, whereas such a deprotonation of Y Z C has not been clearly established yet. [7,8] First indications for the existence of a hydrogen bond between Y D C and other protein constituents stem from the time when the EPR signal from Y D C, then known as signal II, had not yet been identified to arise from a tyrosine radical. The signal appeared to be remarkably well-defined and some of the couplings seemed to disappear upon deuterium exchange.…”

Section: Introductionmentioning

confidence: 99%

High‐Field²H‐Mims‐ENDOR Spectroscopy on PSII Single Crystals: Hydrogen Bonding of Y_D^.

Keßen¹,

Teutloff²,

Kern³

et al. 2010

ChemPhysChem

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Hydrogen bonds are key determinants for protein structures and the fine-tuning of active-site properties. For example, they are responsible for the redox potential variability of protein-imbedded chromophores. By applying high-field (94 GHz) Mims-ENDOR spectroscopy on deuterium-exchanged frozen-solution samples and single crystals of photosystem II from Th. elongatus, we identified the hydrogen-bonding partner of the tyrosyl radical D2-Tyr160, Y(D)*, directly by the strength and orientation of the deuterium hyperfine coupling as D2-His189, that is, without relying on the disappearance of a hyperfine coupling interaction upon deletion of D2-His189. No indications for additional hydrogen bonds can be found in the spectra, thereby eliminating hypotheses about a water network as hydrogen-binding partner of Y(D)*.

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“…In the absence of Mn, TyrZ ⅐ is readily accessible from the lumen (22)(23)(24) and is thought to be in a more hydrophilic environment. In addition, TyrZ ⅐ exhibits a much more disordered H-bond, and the ring seems to be more mobile (25,26). The hydrogen bond to TyrZ ⅐ seems to involve D1 His-190 directly or indirectly (i.e., by means of one or more water molecules) (for discussion, see refs.…”

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confidence: 99%

Rapid formation of the stable tyrosyl radical in photosystem II

Faller

Debus

Brettel

et al. 2001

Proc. Natl. Acad. Sci. U.S.A.

118

145

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Two symmetrically positioned redox active tyrosine residues are present in the photosystem II (PSII) reaction center. One of them, TyrZ, is oxidized in the ns-s time scale by P680 ؉ and reduced rapidly (s to ms) by electrons from the Mn complex. The other one, TyrD, is stable in its oxidized form and seems to play no direct role in enzyme function. Here, we have studied electron donation from these tyrosines to the chlorophyll cation (P680 ؉ ) in Mndepleted PSII from plants and cyanobacteria. In particular, a mutant lacking TyrZ was used to investigate electron donation from TyrD. By using EPR and time-resolved absorption spectroscopy, we show that reduced TyrD is capable of donating an electron to P680 ؉ with t 1/2 Ϸ 190 ns at pH 8.5 in approximately half of the centers. This rate is Ϸ10 5 times faster than was previously thought and similar to the TyrZ donation rate in Mn-depleted wild-type PSII (pH 8.5). Some earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD (compared with that from TyrZ) can be reassessed. At pH 6.5, TyrZ (t 1/2 ‫؍‬ 2-10 s) donates much faster to P680 ؉ than does TyrD (t1/2 > 150 s). These different rates may reflect the different fates of the proton released from the respective tyrosines upon oxidation. The rapid rate of electron donation from TyrD requires at least partial localization of P680 ؉ on the chlorophyll (P D2) that is located on the D2 side of the reaction center.T yrosyl radicals play key roles in the mechanisms of a wide range of enzymes. Understanding these roles and how proteins are able to control these reactive species to carry out quite specific chemical reactions has been an important aim of researchers in this area (for review, see ref. 1). One of the earliest demonstrations of redox active tyrosines was in photosystem II (PSII), the water oxidizing enzyme, in which a tyrosyl radical, designated tyrosine Z ⅐ (TyrZ ⅐ ), is thought to play a key role in the active site, abstracting electrons and possibly protons from the substrate water that is bound to the highly oxidized Mn cluster (2-5).PSII contains a second tyrosyl radical, TyrD ⅐ , which is stable during enzyme function and which is located in a 2-fold rotationally symmetrical position to TyrZ on a subunit (D2) adjacent to that (D1) in which water oxidation takes place. TyrZ and TyrD are equally situated relative to the central chlorophyll (Chl) pair (P D1 and P D2 ) with the center-to-center distances for TyrZ-P D1 and TyrD-P D2 being Ϸ12.4 Å (6). It is this pair of Chls that bears the photogenerated cation (P680 ϩ ) that is considered to be the oxidant for the tyrosines (refs. 6-12; for recent reviews see refs. 2 and 3). The existing enzyme may have evolved from an ancestor in which the core was a homodimer with the two tyrosines having identical redox functions (13).Despite homologous positions in subunits D1 and D2, TyrZ and TyrD exhibit extremely different kinetics, different redox potentials, and play completely different functional roles in the enzyme. Thus, they con...

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2H ESE-ENDOR study of hydrogen bonding to the tyrosine radicals YD.bul. and YZ.bul. of photosystem II.

Cited by 94 publications

References 0 publications

High‐Field²H‐Mims‐ENDOR Spectroscopy on PSII Single Crystals: Hydrogen Bonding of Y_D^.

Rapid formation of the stable tyrosyl radical in photosystem II

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2H ESE-ENDOR study of hydrogen bonding to the tyrosine radicals YD.bul. and YZ.bul. of photosystem II.

Cited by 94 publications

References 0 publications

High‐Field2H‐Mims‐ENDOR Spectroscopy on PSII Single Crystals: Hydrogen Bonding of YD.

Rapid formation of the stable tyrosyl radical in photosystem II

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High‐Field²H‐Mims‐ENDOR Spectroscopy on PSII Single Crystals: Hydrogen Bonding of Y_D^.