Architecture of the Photosynthetic Oxygen-Evolving Center

Abstract

Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII). We report the structure of PSII of the cyanobacterium Thermosynechococcus elongatus at 3.5 angstrom resolution. We have assigned most of the amino acid residues of this 650-kilodalton dimeric multisubunit complex and refined the structure to reveal its molecular architecture. Consequently, we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like Mn3CaO4 cluster linked to a fourth Mn by a mono-micro-oxo bridge. The details of the surrounding coordination sphere of the metal cluster and the implications for a possible oxygen-evolving mechanism are discussed.

Spectroscopic Evidence for Ca2+Involvement in the Assembly of the Mn4Ca Cluster in the Photosynthetic Water-Oxidizing Complex

“…EPR measurements were made in triplicate using a Bruker Elexsys 580 X-band spectrometer and Bruker ER 4116DM dual-mode EPR cavity. The EPR measurements Chart 1: Possible Structural Candidates for the Mn 4 Ca Core of the WOC in PSII Based on Single-Crystal Mn EXAFS (Structures 1-3) or XRD (Structure 4) Studies (9,13) apo-WOC-PSII { \}…”

“…However, the relative position of Ca 2+ within the Mn 4 O x cluster differs according to which type of data is emphasized and the assumptions of the models used to interpret the raw data. Spectroscopic and XRD data have constrained the possible core topologies to only a few types, with the structures given in Chart 1 proposed on the basis of one or more lines of evidence (9)(10)(11)13). These structural proposals have led to a number of mechanistic proposals for the role of calcium in O 2 evolution reviewed elsewhere (11,(14)(15)(16)(17).…”

The PSII calcium site revisited

“…Ferreira et al (2004) represented the structure of Ca 2+ as part of a cubane with three Mn atoms, and no protein ligands to the metal were shown. In contrast, Loll et al (2005b) proposes the structure of carboxylate oxo anion ligands to Ca 2+ from Glu189 and Ala 334, both of which provide oxo anion ligands to Mn atoms of the cluster (see Fig.…”

“…On the other hand, Loll et al (2005a) examined the effects of Ca 2+ binding on PsbO from T. elongatus and concluded that binding of the metal had inconsequential effects on structure. Murray and Barber (2006) analyzed data in Ferreira et al (2004) and speculate that a Ca 2+ binding site is present in T. elongatus PsbO that is near the lumenal side of PSII. At the same time, all of the currently available models based on crystallographic data (Loll et al 2005b) or on EXAFS results (Yachandra 2005;Yano et al 2006) place the functional Ca 2+ site in the OEC in close proximity to the Mn cluster, and do not predict ligation by amino acid side chain residues of PsbO.…”

Structural and Functional Differentiation of Three Groups of Tyrosine Residues by Acetylation of N-Acetylimidazole in Manganese Stabilizing Protein

“…It is proposed that MSP, located on the luminal surface within PSII, forms an eight-stranded -barrel. The whole protein takes on an elongated shape with two major domains (11). On the basis of this, homologous models for eukaryotic MSP were built by using bioinformatics analysis.…”