Structural analysis for photosystem II in the intermediate Si-state by using X-ray free electron laser pulses

2018 Fiscal Year Final Research Report

• Project/Area Number: 16H06162
• Research Category: Grant-in-Aid for Young Scientists (A)
• Allocation Type: Single-year Grants
• Research Field: Structural biochemistry
• Research Institution: Okayama University
• Principal Investigator: Suga Michihiro 岡山大学, 異分野基礎科学研究所, 准教授 (60634920)
• Research Collaborator: Shen Jian-Ren
• Project Period (FY): 2016-04-01 – 2019-03-31
• Keywords: 光化学系 / 光合成 / X線自由電子レーザー / 動的構造 / ダイナミクス / 膜タンパク質 / 人工光合成

Outline of Final Research Achievements

Photosystem II (PSII) catalyzes photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been analyzed at resolutions higher than 2.0A by using X-ray as well as XFEL. The mechanism of O=O bond formation, however, remains obscure owing to the lack of intermediate-state structures determined at atomic resolution. We prepared PSII in the S3 state by providing two-flash illumination into micro-sized crystals at room temperature and determined the structure at 2.35A resolution by using an XFEL. An isomorphous difference Fourier map between the two-flash-illuminated and dark-adapted states revealed apparent structural changes. Among them, the insertion of a new oxygen atom O6 close to the putative substrate oxo-bridge O5 was observed. We proposed the mechanism of O=O bond formation between O5 and O6. The findings provide a structural basis for the mechanism of oxygen evolution.

Free Research Field

動的構造生物学

Academic Significance and Societal Importance of the Research Achievements

光化学系IIは可視光による水分子の酸化反応を触媒することのできる唯一の天然触媒である。今回X線自由電子レーザーを利用した反応中間体の構造解析によりO6とよばれる水分子が触媒部分に取り入れられることで水分解反応が起こるということを実験的に観察し、触媒反応の基本原理を世界で初めて実証した。この知見は再生可能でクリーンなエネルギーを作り出す人工光合成研究への応用も期待できる。