Structures and energetics for O2 formation in photosystem II

Area of Science:

• Biochemistry and Biophysics
• Photosynthesis Research
• Computational Chemistry

Background:

• Water oxidation, catalyzed by photosystem II (PSII), is vital for life, producing oxygen from water and sunlight.
• The oxygen-evolving complex (OEC) within PSII, containing manganese and calcium, is responsible for O-O bond formation.
• Theoretical modeling, particularly density functional theory (DFT), is essential for understanding complex enzymatic mechanisms.

Purpose of the Study:

• To elucidate the mechanism of water oxidation in photosynthesis using theoretical modeling.
• To detail the O-O bond formation process within the OEC of PSII.
• To investigate the role of amino acid residues, like tyrosine (Tyr(Z)), in the catalytic cycle.

Main Methods:

• Development and refinement of DFT models of the OEC, increasing in size from 50 to 170 atoms.
• Optimization of theoretical models constrained by X-ray crystal structures of PSII.
• Analysis of structural and energetic changes throughout the catalytic cycle, including substrate binding and deprotonation.

Main Results:

• Models show the OEC forms a basin for water binding, with the second water molecule inducing reconstruction.
• A low-barrier mechanism for O-O bond formation is proposed, involving an oxygen radical reacting with a mu-oxo ligand within the Mn3Ca cube.
• The role of Tyr(Z) as an intermediate radical is explained, with its oxidation coupled to OEC oxidation and water deprotonation.

Conclusions:

• Theoretical studies provide detailed insights into the water oxidation mechanism in PSII.
• The findings clarify the O-O bond formation pathway and the involvement of key amino acid residues.
• This research advances our understanding of a fundamental process for life on Earth.