Structure of photosystem II and substrate binding at room temperature

Area of Science:

• Biochemistry and Biophysics
• Photosynthesis Research
• Structural Biology

Background:

• Photosystem II (PS II) drives oxygen production via water oxidation, crucial for Earth's atmosphere.
• The oxygen-evolving complex (OEC) within PS II contains a Mn4CaO5 cluster that cycles through S-states (S0-S4) during water oxidation.
• Understanding the O-O bond formation mechanism and substrate water binding in the OEC is essential but challenging.

Purpose of the Study:

• To obtain damage-free, room-temperature structures of PS II in functional states (S1, S3-enriched).
• To investigate substrate water binding sites within the OEC using ammonia as a water analogue.
• To differentiate between proposed O-O bond formation mechanisms by comparing native and ammonia-bound states.

Main Methods:

• Utilized femtosecond X-ray free electron laser (XFEL) pulses for X-ray diffraction.
• Collected damage-free, room-temperature structural data of PS II.
• Employed ammonia binding to the Mn4CaO5 cluster to probe water-binding sites and OEC states (S1, S3-enriched).

Main Results:

• Reported room-temperature, damage-free structures of dark-adapted (S1) and two-flash illuminated (S3-enriched) PS II.
• Identified ammonia binding sites, indicating they are not substrate water sites in the S2 and S3 states.
• Provided structural basis for discriminating between various O-O bond formation hypotheses.

Conclusions:

• Room-temperature XFEL structures provide crucial insights into the functional states of the OEC.
• Ammonia binding serves as a valuable tool to map water interactions and active site dynamics.
• The study significantly advances the mechanistic understanding of water oxidation and oxygen evolution in photosynthesis.