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Excited-State Dynamics in Photosystem II: Insights from the X-Ray Crystal Structure

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Excited-State Dynamics in Photosystem II: Insights from the X-Ray Crystal Structure Excited-state dynamics in photosystem II: Insights from the x-ray crystal structure Sergej Vasil’ev*†, Peter Orth‡, Athina Zouni§, Thomas G. Owens¶, and Doug Bruce* *Department of Biological Sciences, Brock University, St. Catharines, ON, Canada L2S 3A1; ‡Institut fu¨r Chemie, Kristallographie, Freie Universita¨t Berlin, Takustrasse 6, D-14195 Berlin, Germany; §Max-Volmer-Institut fu¨r Biophysikalische Chemie und Biochemie, Technische Universita¨t Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany; and ¶Department of Plant Biology, Cornell University, Ithaca, NY 14853 Communicated by Elisabeth Gantt, University of Maryland, College Park, MD, May 14, 2001 (received for review February 9, 2001) The heart of oxygenic photosynthesis is photosystem II (PSII), a intrinsic photophysical and photochemical processes is essential multisubunit protein complex that uses solar energy to drive the to the construction of a comprehensive kinetic model of PS II splitting of water and production of molecular oxygen. The effec- and thus to a fundamental understanding of biological energy tiveness of the photochemical reaction center of PSII depends on transduction in photosynthesis. the efficient transfer of excitation energy from the surrounding Previously, kinetic models applied to the analysis of excited- antenna chlorophylls. A kinetic model for PSII, based on the x-ray state processes in PSII have been based on a concept of rapid crystal structure coordinates of 37 antenna and reaction center spectral equilibration—that is, the excited state reaches a ther- pigment molecules, allows us to map the major energy transfer mal equilibrium among the various spectral forms of Chl in the routes from the antenna chlorophylls to the reaction center chro- antenna and RC on a time scale that is short compared with the mophores. The model shows that energy transfer to the reaction photochemically limited lifetime of the excited state (5–7). A center is slow compared with the rate of primary electron transport model for PSII based on these assumptions, known also as the and depends on a few bridging chlorophyll molecules. This unex- Reversible Radical Pair (RRP) model, was introduced by Schatz pected energetic isolation of the reaction center in PSII is similar to and Holzwarth (5) in 1988. Simulations based on the Fo¨rster that found in the bacterial photosystem, conflicts with the estab- inductive resonance mechanism (8) with randomly assigned lished view of the photophysics of PSII, and may be a functional positions and orientations of chromophores restricted by vol- requirement for primary photochemistry in photosynthesis. In umes of pigment–protein complexes also supported the concept addition, the model predicts a value for the intrinsic photochemical of fast spectral equilibration (9). However, a detailed kinetic rate constant that is 4 times that found in bacterial reaction centers. description of excited-state dynamics within and between pig- ment–protein complexes of PS II is impossible without knowing the actual positions and orientations of all Chls. This same s the site of water splitting and oxygen production, photo- information is required if one is to extract the intrinsic rate system II (PSII) is essential for oxygenic photosynthesis. A constant for photochemical charge separation (k ) by using the This multisubunit protein complex consists of at least 17 poly- p constraints of experimentally determined lifetimes. peptides and catalyzes the oxidation of water and the reduction The availability of high-resolution x-ray crystallographic data of plastoquinone (1). The PSII holocomplex of higher plants and for the PSII core complex (10) permits a quantitative analysis of green algae contains 200–300 chlorophyll (Chl) molecules and our commonly held assumptions about excited-state dynamics in various carotenoids that are noncovalently bound to a variety of PSII. The rationale for questioning the concept of rapid equil- PSII polypeptides (2). The minimal functionally active PSII ibration of the excited state is immediately obvious on inspection complex contains the reaction center (RC) polypeptides (D1, of the PSII x-ray structure (Fig. 1). Here one observes a large gap D2, cytochrome b559), the Chla core antenna polypeptides (CP43 between the electron transfer pigments and the antenna Chl that and CP47), and the polypeptides of the oxygen-evolving com- may act as a physical barrier to the fast spectral equilibration so plex. The total number of Chls in this PSII core complex is less commonly assumed in PSII. In this paper, we apply this recently than 40 per RC (3). Light energy absorbed by any PSII Chl acquired structural knowledge to critically evaluate our under- generates an excited state, which is ultimately transferred to the standing of excited-state transfer and primary electron transport primary electron donor in photosystem II, the RC photoactive in PSII. We present a detailed kinetic model of excitation pigment P680. Within the excited-state lifetime, primary charge ϩ Ϫ Ϫ dynamics in PSII based on the coordinates of the 35 Chls and 2 separation [formation of P680 and pheophytin (Pheo )] and Pheos in the PSII x-ray structure. Comparison of our simulations charge stabilization (reduction of the primary quinone electron Ϫ with the available experimental data clearly shows that there is acceptor, QA, by Pheo ) occur with greater than 90% efficiency. no fast spectral equilibration among the antenna and the RC The kinetics of excited-state decay in PSII are highly depen- pigments. A critical consequence of slower energy transfer steps dent on the redox state of the RC (4). Accordingly, these kinetics in PSII is that the kp for primary charge separation is much larger contain valuable information about rates and mechanisms of than previously assumed. excited-state energy transfer, primary charge separation, and stabilization reactions in the RC complex. Unfortunately, it is Materials and Methods very difficult to measure directly these rates by using time- Plant Material and Thylakoid Membrane Isolation. A PSI-less strain resolved spectroscopic techniques because of the complications of the cyanobacterium Synechocystis 6803 (a generous gift of W. of excited-state transfer processes that precede the electron transfer steps. In addition, transient absorption measurements in Abbreviations: PSII, photosystem II; Chl, chlorophyll; RC, (photosynthetic) reaction center; the Qy band of Chl are complicated by spectral congestion and also by competing absorption, bleaching, and stimulated emis- P680, primary electron donor in photosystem II; Pheo, pheophytin; RRP, Reversible Radical Pair; ChlZ, peripheral reaction center chlorophylls. sion. Although the Qx band of Pheo is relatively narrow and Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, isolated from chlorophyll absorption, transient absorption mea- www.rcsb.org (PDB ID code 1ILX). surements in this band cannot distinguish between the loss of †To whom reprint requests should be addressed. E-mail: [email protected]. Pheo ground-state population caused by the production of an ion The publication costs of this article were defrayed in part by page charge payment. This pair or by the formation of electronically excited Pheo. Never- article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. theless, determination of the rate constants describing these §1734 solely to indicate this fact. 8602–8607 ͉ PNAS ͉ July 17, 2001 ͉ vol. 98 ͉ no. 15 www.pnas.org͞cgi͞doi͞10.1073͞pnas.141239598 Downloaded by guest on September 26, 2021 BIOPHYSICS Fig. 1. X-ray structure of PSII showing all chromophores. Yellow and green, Chls efficiently transferring excitation to the RC (yellow, most efficient); blue, peripheral Chls not coupled directly to the RC; dark pink, RC Chls; pale pink, RC Pheos; red, long-wavelength Chl of CP47 coordinated by His-114. Chls numbered from 9 to 21 are associated with the CP43 polypeptide; Chls numbered from 22 to 37 are associated with the CP47 polypeptide. (Upper) View from the stromal side of the membrane. (Lower) Side view (stromal surface above). Vermaas, Arizona State University, Phoenix, AZ) was grown at 1.3 was assumed for pairs with Ͼ15-Å spacing and 1.0 for Ͻ15-Å Ϫ 30°C in BG-11 growth media in the presence of 15 mM glucose spacing (16). The Stokes shift was set to 127 cm 1. Distances Ϫ ϭ ជ ⅐ ជ Ϫ at a light intensity of 5 ␮E(Eϭ einstein, mol of photons) m 2 between chromophores and orientation factors (kij ei ej Ϫ ជ ⅐ ជ ជ ⅐ ជ ជ ជ 1 3[ej rij][ej rij], where ei and ej are unit vectors in the direction s (11). Thylakoid membranes were prepared from the cells ជ according to the method of Tang and Diner (12). Immediately of the Qy transition dipole moment and rij is the unit vector in the direction of the line connecting the centers of the two after isolation, the thylakoid membranes were frozen in liquid Ϫ molecules) were calculated from the x-ray structural data (10). nitrogen and stored at 80°C until use. Excited-state relaxation kinetics were obtained by using a nu- merical solution to the Pauli master equation (17, 18). To Picosecond Fluorescence-Decay Kinetics and Global Lifetime Analysis. account for reversible charge separation and charge stabiliza- Fluorescence-decay kinetics were measured with the time- tion, we introduced two additional states corresponding to ϩ Ϫ ϩ Ϫ correlated single-photon counting apparatus as described (13). [P680 Pheo ] and [P680 QA]. We assumed a rate constant for Ϫ Ϫ1 Details of measurement conditions and the global lifetime electron transfer from Pheo to QA of (0.48 ns) (5, 19) and analysis used to extract the PSII-associated decay components used the rate constants for charge separation and charge re- are as described (14). PSII kinetic parameters that we measured combination as parameters to fit the excited-state decay kinetics for Synechocystis 6803 were very close to the same parameters predicted by the model to experimental fluorescence decay kinetics.
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