Oxygenic photosynthesis in plants, algae and cyanobacteria is initiated at photosystem II, a homodimeric multisubunit protein-cofactor complex embedded in the thylakoid membrane. Photosystem II captures sunlight and powers the unique photo-induced oxidation of water to atmospheric oxygen. Crystallographic investigations of cyanobacterial photosystem II have provided several medium-resolution structures (3.8 to 3.2 A) that explain the general arrangement of the protein matrix and cofactors, but do not give a full picture of the complex. Here we describe the most complete cyanobacterial photosystem II structure obtained so far, showing locations of and interactions between 20 protein subunits and 77 cofactors per monomer. Assignment of 11 beta-carotenes yields insights into electron and energy transfer and photo-protection mechanisms in the reaction centre and antenna subunits. The high number of 14 integrally bound lipids reflects the structural and functional importance of these molecules for flexibility within and assembly of photosystem II. A lipophilic pathway is proposed for the diffusion of secondary plastoquinone that transfers redox equivalents from photosystem II to the photosynthetic chain. The structure provides information about the Mn4Ca cluster, where oxidation of water takes place. Our study uncovers near-atomic details necessary to understand the processes that convert light to chemical energy.

The oxidation of water to dioxygen is catalyzed within photosystem II (PSII) by a Mn 4 Ca cluster, the structure of which remains elusive. Polarized extended x-ray absorption fine structure (EXAFS) measurements on PSII single crystals constrain the Mn 4 Ca cluster geometry to a set of three similar high-resolution structures. Combining polarized EXAFS and x-ray diffraction data, the cluster was placed within PSII, taking into account the overall trend of the electron density of the metal site and the putative ligands. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 angstrom-resolution x-ray structures or other previously proposed models.Oxygen, which makes up about 20% of Earth's atmosphere, comes mostly from photosynthesis that occurs in cyanobacteria, green algae, and higher plants (1). These organisms have within photosystem II (PSII) an oxygen-evolving complex (OEC), in which the energy of sunlight is used to oxidize water to molecular oxygen. The heart of the OEC is a cluster of four Mn atoms and one Ca atom (Mn 4 Ca) connected by mono-μ-oxo, di-μ-oxo, and/or hydroxo bridges. The specific protein environment and one chloride ion are also essential for the water-splitting activity (1). During the oxidation of water, the OEC cycles through five different oxidation states, which are known as S i states (where i ranges from 0 to 4), that couple the one-electron photochemistry of the PSII reaction center with the fourelectron chemistry of water oxidation (2).The structure of the Mn 4 Ca cluster and its role in the mechanism of water oxidation have been investigated with the use of spectroscopic methods (1), especially electron ‡ To whom correspondence should be addressed. messinger@mpi-muelheim. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript paramagnetic resonance and electron nuclear double-resonance spectroscopy (3-9), x-ray spectroscopy (10), and Fourier transform infrared (FTIR) spectroscopy (11). In addition, recent x-ray diffraction (XRD) studies of single crystals of PSII provide critical information about its structure at 3.8 to 3.0 Å resolution (12-16). However, even XRD data of the highest resolution presently available are insufficient to accurately determine the positions of Mn, Ca, and the bridging and terminal ligands. This is reflected by the differences in the placement of the metal ions and putative ligands in the 3.0 (16) and 3.5 Å (14) structures. Furthermore, at the x-ray dose and temperature used in the XRD studies, the geometry of the Mn 4 Ca cluster is disrupted, initiated by the rapid reduction of Mn(III) and Mn(IV) present in the dark-stable S 1 state to Mn(II), as shown by Mn x-ray absorption near-edge structure (XANES) studies and Mn x-ray absorption fine structure (EXAFS) studies of PSII single crystals (17).EXAFS experiments with PSII require a substantially lower x-ray dose than XRD measurements (17), and the onset of radiation damage can be precisely determined and controlled by monitoring the Mn K-edge positio...