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The X-Ray Structure of Photosystem II Reveals a Novel Electron Transport

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The X-Ray Structure of Photosystem II Reveals a Novel Electron Transport FEBS 27259 FEBS Letters 543 (2003) 159^163 View metadata, citation and similar papers at core.ac.uk brought to you by CORE The X-ray structure of photosystem II reveals a novel electronprovided transport by Elsevier - Publisher Connector pathway between P680, cytochrome b559 and þ the energy-quenching cation, ChlZ Sergej Vasil’eva, Gary W. Brudvigb, Doug Brucea;Ã aDepartment of Biological Sciences, Brock University, St. Catharines, ON, Canada L2S 3A1 bDepartment of Chemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107, USA Received 13 March 2003; revised 7 April 2003; accepted 8 April 2003 First published online 1 May 2003 Edited by Richard Cogdell two core antenna Chl^protein complexes, CP43 and CP47. Abstract When water oxidation by photosystem II (PSII) is + The core complex of PSII contains about 35 Chl per RC. impaired, an oxidized chlorophyll (ChlZ) is formed that quenches excitation and may prevent photodamage. Both the Charge separation in the RC follows excitation of PSII and identi¢cation of this Chl+ and the mechanism of its oxidation arises from electron transfer from the primary electron donor and reduction are controversial. Using the available X-ray struc- (P680) to Pheo creating the primary radical pair, P680þ/ tures of PSII we calculated the e⁄ciency of two proposed Pheo3. P680þ is reduced by a tyrosine radical that subse- + + quenchers, ChlZ(D1) and ChlZ(D2). Of these two, only quently oxidizes the Mn cluster of the oxygen-evolving com- + ChlZ(D1) can quench to the degree observed experimentally. plex. When electron transport from the oxygen-evolving com- We also identify a chain of closely spaced pigments in the plex is impaired, alternative electron donors are oxidized by structure from Thermosynechococcus vulcanus that we propose P680þ. Illumination of PSII at low temperature results in ox- to form a novel electron transport pathway between ChlZ(D1), + idation of Cyt b559, or a Chl molecule if Cyt b559 is already L-carotene, P680 and cytochrome b559. ß 2003 Published by Elsevier Science B.V. on behalf of the oxidized [2]. It was proposed that this oxidizable Chl molecule Federation of European Biochemical Societies. and Cyt b559 form part of a low quantum yield photoprotec- tive electron transport pathway in PSII [3]. The oxidized Chl Key words: Photosynthesis; Non-photochemical quenching; was termed ChlZ and subsequently identi¢ed as one of the two Chlorophyll £uorescence; Excitation energy transfer; peripheral RC Chl molecules, ChlZ(D1) [4]. ChlZ appears to Electron transport form part of a Cyt b559-mediated cyclic electron transport pathway around PSII as it is ultimately oxidized by P680þ and reduced by Cyt b559 which in turn is reduced by plasto- 1. Introduction quinone [5]. þ The presence of fractional amounts of ChlZ per RC has Photosystem II (PSII) captures the energy of sunlight and been correlated with a dramatic quenching of £uorescence catalyzes the oxidation of water and reduction of plastoqui- emission from PSII [6,7]. As a strong quencher of excitation þ none in photosynthetic electron transport. These reactions energy, ChlZ may serve a photoprotective role when the do- involve a dangerous stepwise accumulation of redox potential nor side of PSII is impaired. that sensitizes PSII to photoinduced damage. PSII exhibits a Illumination of PSII at low temperature or in the presence number of mechanisms that serve either a protective or a of inhibitors of the oxidizing side of PSII results in the for- þ þ repair role in response to such light-induced damage [1]. mation of an oxidized carotenoid (Car ) as well as ChlZ [8,9]. þ þ The reaction center (RC) of PSII consists of the D1 and D2 The amounts of ChlZ and Car accumulated have been polypeptides, cytochrome b559 (Cyt b559) and the PsbI protein shown to vary with temperature and between PSII prepara- and contains six chlorophyll (Chl) and two pheophytin (Pheo) tions isolated from cyanobacteria and spinach [10]. It has molecules. Four Chl and two Pheo in the RC are arranged in been proposed that this Car participates in the alternative þ pseudo-C2-symmetrical fashion around a non-heme iron and electron transfer pathway involving P680 , ChlZ and Cyt constitute the presumed active and inactive electron transfer b559 [11]. The relative involvements and possibilities of linear, branches. Two additional RC Chls, ChlZ(D1) and ChlZ(D2), branched or parallel pathways involving Cyt b559, ChlZ and are located at the periphery of the RC. The light harvesting Car in electron donation to P680þ have been discussed [10^ capacity of the RC of PSII is increased by the association of 13]. In cyanobacteria, the ‘quenching’ ChlZ was identi¢ed, by a combination of site-directed mutagenesis and resonance Ram- an spectroscopy, as ChlZ(D1) liganded to D1-His118 [4]. *Corresponding author. Fax: (1)-905-688 1855. When the X-ray structure of PSII from Synechococcus elonga- E-mail address: [email protected] (D. Bruce). tus was resolved [14] one surprise was that Cyt b559 was lo- cated on the D2 side of the PSII complex, too far from Abbreviations: PS, photosystem; Chl, chlorophyll; P680, primary ChlZ(D1) to reduce it directly. Interestingly, in Chlamydomo- electron donor for photosystem II; ChlZ(D1) and ChlZ(D2), periph- eral reaction center chlorophylls of photosystem II; ChlD2, reaction nas reinhardtii, analogous site-directed mutations of histidines center chlorophyll of photosystem II liganded to both ChlZ(D1) and ChlZ(D2) led to the conclu- 0014-5793 / 03 / $22.00 ß 2003 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies. doi:10.1016/S0014-5793(03)00442-3 FEBS 27259 7-5-03 160 S. Vasil’ev et al./FEBS Letters 543(2003)159^163 sion that Chlþ(D2) was the quenching species [15], a result simulations were performed with PSII core dimer structures. We in- Z þ more consistent with the location of Cyt b . However, an- troduced one Chl a cation per core dimer (one Chl per two RCs) in 559 the simulation and £uorescence yield was compared to the yield of the other surprise from the X-ray structure was the relatively iso- same model without the cation. Maximum £uorescence yield was lated location of both ChlZ(D1) and ChlZ(D2) from other RC calculated with the rate of charge separation set to zero to facilitate chromophores and the antenna Chl of CP43 and CP47. Pre- comparison with the experimentally determined quenching by photo- þ liminary energy transfer calculations based on the X-ray accumulated ChlZ in frozen PSII RCs [6,7]. Unimolecular decay rates were 0.5 ns31, the index of refraction was 1.5 [21]. The Chl cation was structure showed that ChlZ(D1) and ChlZ(D2) were the two assumed to be a very e⁄cient quencher and was represented by a most weakly excitonically coupled pigments in PSII, neither unimolecular decay rate of 1000 ns31. Other model parameters were would serve well as an energy quencher [16]. the same as described previously [16]. The X-ray structure of PSII from Thermosynechococcus vul- canus includes two L-carotenes [17]. The two L-carotenes are 3. Results and discussion located on the D2 side of the RC near Cyt b559 and the RC ChlD2, consistent with a role mediating electron transport be- 3.1. Kinetic modeling of quenching of excitation energy by + tween them. The involvement of ChlZ(D1) and/or ChlZ(D2) Chl in PSII still remains di⁄cult to see, as both are distant from P680, the To simulate Chlþ quenching, we applied detailed kinetic carotenoids or Cyt b559. However, the T. vulcanus structure models for excited state dynamics in PSII based on the contains an ‘extra’ Chl in CP43 located between ChlZ(D1) and X-ray structures of S. elongatus and T. vulcanus. The models the antenna Chl of CP43. Could this Chl facilitate excitation were used to calculate the £uorescence yield of RCs in the þ energy transfer and electron transport between ChlZ(D1) and absence and presence of a Chl as a function of its location the rest of PSII? in PSII. We use energy transfer models for PSII, based on the X-ray Our calculations are summarized in Table 1. Locating the þ structures from S. elongatus and T. vulcanus, to calculate the Chl on either ChlZ(D1) or ChlZ(D2) in the model based on quenching e⁄ciency of a Chl cation located at a number of the structure of S. elongatus results in much less quenching of di¡erent sites, including ChlZ(D1) and ChlZ(D2). We propose £uorescence than on any other Chl. The quenching of approx- þ þ a mechanism for electron transport from ChlZ(D1) to P680 imately 30% of £uorescence in the presence of 0.5 ChlZ per which is dependent on the location of the L-carotenes and the RC for either ChlZ(D1) or ChlZ(D2) is much lower than the ‘extra’ Chl in CP43 found in the T. vulcanus structure. We experimentally reported quenching of 70% of £uorescence at þ suggest that a ‘chain’ of Chl molecules in CP43 are involved in 0.17 ChlZ per RC at 200 K [6] or approximately 80% of þ þ hole migration from P680 to ChlZ(D1) and that the quench- £uorescence at 0.6 ChlZ per RC at 85 K [7]. The low degree ing Chlþ is actually an equilibrium mixture of a number of of quenching is consistent with the poor energetic coupling of CP43 antenna Chl and ChlZ(D1). both ChlZ(D1) and ChlZ(D2) to RC and antenna pigments observed in the S. elongatus X-ray structure [16].
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