Arrested Substrate Binding Resolves Catalytic Intermediates in Higher-Plant Water Oxidation.

Area of Science:

• Biochemistry and Biophysics
• Photosynthesis Research
• Bioinorganic Chemistry

Background:

• The water-oxidizing Mn4CaO5 cluster in photosystem II (PSII) is crucial for photosynthesis.
• The S3 state is a key intermediate preceding oxygen evolution, binding the water substrate.
• Understanding the structural dynamics of the S3 state is vital for elucidating the water-oxidation mechanism.

Purpose of the Study:

• To investigate the structural heterogeneity of the S3 state in the water-oxidizing complex of PSII.
• To characterize the previously unobserved high-spin species within the S3 state.
• To provide insights into the S3 to S4 transition and oxygen-bond formation.

Main Methods:

• Utilized X- and Q-band Electron Paramagnetic Resonance (EPR) spectroscopy.
• Examined both native and methanol-treated PSII preparations from Spinacia oleracea.
• Analyzed intact photosynthetic membranes to confirm findings in a native context.

Main Results:

• Identified a previously uncharacterized high-spin (S=6) species in methanol-treated PSII preparations corresponding to the S3 state.
• Confirmed the presence of this high-spin S3 state as a major component in intact photosynthetic membranes, coexisting with the known intermediate-spin (S=3) form.
• Assigned the high-spin species to a water-unbound state involving a Mn4 subunit interacting with a coordinatively unsaturated Mn ion.

Conclusions:

• Resolved the structural heterogeneity of the S3 state in PSII, revealing both high-spin (water-unbound) and intermediate-spin conformations.
• Provided critical constraints on the S3 to S4 transition mechanism.
• Offered new perspectives on substrate binding, delivery pathways, and O-O bond formation in water oxidation.