Oxygen-evolving complex of photosystem II: correlating structure with spectroscopy

Area of Science:

• Photosynthesis research
• Biophysical chemistry
• Structural biology

Background:

• The oxygen-evolving complex (OEC) in photosystem II (PSII) cycles through redox states (S0-S4) during water oxidation.
• The S2 state exhibits two distinct spin isomers (ST=1/2 and ST=5/2), whose stabilization factors remain unclear.
• Previous studies utilized EPR and FTIR spectroscopy, alongside DFT modeling, to investigate the OEC's S1 to S2 transition.

Purpose of the Study:

• To elucidate the factors governing the stability of the two OEC spin isomers in the S2 state.
• To propose a mechanism for ammonia binding to the OEC and its influence on spin isomer stabilization.
• To explore the potential for spin isomers in the OEC's S1 state.

Main Methods:

• Analysis of structural and spectroscopic data for the OEC.
• Investigation of site-directed mutations, Ca(2+)/Cl(-) substitutions, and small molecule inhibitors.
• Characterization of ammonia binding to PSII centers under different temperature conditions.

Main Results:

• A hydrogen-bonding network involving D1-D61 and OEC-bound waters is proposed to stabilize specific spin isomers.
• Ammonia binding at 200 K favors the ST=5/2 form, while annealing at 273 K leads to an ammonia-altered ST=1/2 form.
• The proposed mechanism explains ammonia's specific binding over methylamine based on hydrogen-bonding requirements.

Conclusions:

• The hydrogen-bonding network is crucial for OEC spin isomer stabilization in the S2 state.
• Ammonia binding modulates OEC spin isomer populations through specific hydrogen-bonding interactions.
• The study suggests potential spin isomerism in the S1 state, analogous to the S2 state.