Structure of Sr-substituted photosystem II at 2.1 Å resolution and its implications in the mechanism of water oxidation

Abstract

Oxygen-evolving complex of photosystem II (PSII) is a tetramanganese calcium penta-oxygenic cluster (Mn 4 CaO 5 ) catalyzing light-induced water oxidation through several intermediate states (S-states) by a mechanism that is not fully understood. To elucidate the roles of Ca 2+ in this cluster and the possible location of water substrates in this process, we crystallized Sr 2+ -substituted PSII from Thermosynechococcus vulcanus, analyzed its crystal structure at a resolution of 2.1 Å, and compared it with the 1.9 Å structure of native PSII. Our analysis showed that the position of Sr was moved toward the outside of the cubane structure of the Mn 4 CaO 5 -cluster relative to that of Ca 2+ , resulting in a general elongation of the bond distances between Sr and its surrounding atoms compared with the corresponding distances in the Ca-containing cluster. In particular, we identified an apparent elongation in the bond distance between Sr and one of the two terminal water ligands of Ca 2+ , W3, whereas that of the Sr-W4 distance was not much changed. This result may contribute to the decrease of oxygen evolution upon Sr 2+ -substitution, and suggests a weak binding and rather mobile nature of this particular water molecule (W3), which in turn implies the possible involvement of this water molecule as a substrate in the O-O bond formation. In addition, the PsbY subunit, which was absent in the 1.9 Å structure of native PSII, was found in the Sr-PSII structure.membrane protein | water-splitting | structural changes | cyanobacteria | artificial photosynthesis P hotosynthetic light-induced water-splitting produces electrons, protons, and molecular oxygen from water; the latter product maintains the oxygenic atmosphere indispensable for sustaining oxygenic life on the earth. This process takes place in photosystem II (PSII), a multisubunit membrane protein complex containing 20 subunits with an overall molecular mass of 350 kDa (1, 2). In PSII, light is absorbed by the reaction center chlorophyll a molecules (P680), which initiates a series of electron transfer reactions leading to the formation of the charge separated state P 680 + /Q A − (the first quinone acceptor of PSII). The oxidized reaction center chlorophyll is rapidly reduced by a redox active tyrosine (Y Z ), which is the Tyr161 residue of the D1 protein. Subsequently, the oxidized Y Z abstracts an electron from an oxygen-evolving complex (OEC), which is located close to Y Z . Upon abstraction of four electrons from OEC, two water molecules are split into protons and molecular oxygen. Thus, the structure of OEC cycles through five distinct states termed S i (where i = 0-4), with the S 0 -state being the most reduced one, and the S 4 -state a transit one. Among these S-states, the S 1 -state is dark stable, and the molecular oxygen is produced in the transition of S 3 -(S 4 )-S 0 (3).To understand the mechanism of photo-induced water-splitting, extensive studies have been carried out to reveal the structure of OEC (4-10). From these studies, it has been clear th...

Proton Matrix ENDOR Studies on Ca2+-depleted and Sr2+-substituted Manganese Cluster in Photosystem II

“…The proton matrix ENDOR spectra of the Sr 2ϩ -substituted PSII were also very similar with those from the Ca 2ϩ -containing manganese cluster, indicating that the structure of the manganese cluster and their surrounding environment are similar in the S 2 state. The crystal structure analysis has shown that the structures of Ca 2ϩ -and Sr 2ϩ -containing manganese cluster are very similar in the S 1 state (4,21). Although the crystal structure in the S 2 state has not been reported yet, our results indicated that the structure of Ca 2ϩ -containing PSII in the S 2 state is well conserved in the Sr 2ϩ -substituted PSII.…”

“…Sr 2ϩ -substituted cyanobacterial PSII was prepared as described previously (21). The samples were suspended in a buffer containing 5% glycerol, 20 mM NaCl, 3 mM CaCl 2 (for native PSII) or 3 mM SrCl 2 (for Sr 2ϩ -substituted PSII), and 20 mM Mes/NaOH (pH 6.1).…”

“…X-ray crystal structure analysis showed that the overall structure of the manganese cluster and the position of the manganese ions were not changed in the Sr 2ϩ -substituted PSII, whereas the Sr 2ϩ ion was located toward the outside of the cubane relative to the Ca 2ϩ ion (21). It is notable that the distance between W3 and Ca 2ϩ /Sr 2ϩ was elongated by the Sr 2ϩ substitution, whereas the distance between W4 and Sr 2ϩ was similar to that between W4 and Ca 2ϩ (21). EPR spectroscopy has been used to characterize the manganese cluster (23)(24)(25)(26) extensively, and a typical, S 2 state multiline signal centered at g ϭ 2 has been well characterized.…”

“…Extended x-ray absorption fine structure spectroscopy showed that the distances between manganese ion pairs were not changed by Ca 2ϩ depletion (22). X-ray crystal structure analysis showed that the overall structure of the manganese cluster and the position of the manganese ions were not changed in the Sr 2ϩ -substituted PSII, whereas the Sr 2ϩ ion was located toward the outside of the cubane relative to the Ca 2ϩ ion (21). It is notable that the distance between W3 and Ca 2ϩ /Sr 2ϩ was elongated by the Sr 2ϩ substitution, whereas the distance between W4 and Sr 2ϩ was similar to that between W4 and Ca 2ϩ (21).…”

“…6 -8). Ca 2ϩ in higher plant PSII can be depleted by biochemical treatments, resulting in the inhibition of oxygen evolution (9 -19), which can be recovered by the reconstitution of Ca 2ϩ or Sr 2ϩ (10,14,16,20,21 (20). Although Sr 2ϩ substitution does not affect the turnover numbers of the catalyst, it does lower its efficiency (TOF), leading to a lower activity (20).…”