Crystal structure of oxygen-evolving photosystem Ⅱ at a resolution of 1.9 Å

摘要

Photosystem Ⅱ is the site of photosynthetic water oxidation and contains 20 subunits with a total molecular mass of 350 kDa. The structure of photosystem Ⅱ has been reported at resolutions from 3.8 to 2.9 A. These resolutions have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic centre of water splitting. Here we report the crystal structure of photosystem Ⅱ at a resolution of 1.9 A. From our electron density map, we located all of the metal atoms of the Mn_4CaO_5 cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water molecules were bound to the Mn_4CaO_5 cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water molecules in each photosystem Ⅱ monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. The determination of the high-resolution structure of photosystem Ⅱ will allow us to analyse and understand its functions in great detail.