Interaction between tyrosineZ and substrate water in active photosystem II

Area of Science:

• Biochemistry
• Photosynthesis
• Biophysics

Background:

• The role of Tyrosine(Z) (Tyr(Z)) in photosynthetic water oxidation remains debated.
• It is unclear whether Tyr(Z) accepts electrons or hydrogen atoms from water.
• Previous studies often used inhibited Photosystem II (PSII), lacking direct evidence in active systems.

Purpose of the Study:

• To investigate the interaction between Tyr(Z) and substrate water in active Photosystem II (PSII).
• To clarify the mechanism of Tyr(Z) in water oxidation.
• To provide insights into the function of PSII.

Main Methods:

• Low-temperature Electron Paramagnetic Resonance (EPR) measurements.
• Quantum chemistry calculations.
• Studies on active PSII under various pH conditions.

Main Results:

• Tyr(Z) oxidation yield in S(0) and S(1) states was pH-independent around neutral pH at cryogenic temperatures.
• Observed pH-dependent changes were attributed to sample inactivation, not active PSII.
• Theoretical calculations showed direct water interaction hinders Tyr(Z) oxidation.
• Tyr(Z)(.) reduction kinetics were pH-independent between pH 4.5 and 8.

Conclusions:

• Tyr(Z) likely resides in a hydrophobic environment within active PSII, without direct contact with substrate water.
• pH changes in the bulk solution do not significantly affect Tyr(Z) oxidation/reduction in active PSII at cryogenic temperatures.
• Findings challenge the direct water interaction model and offer new perspectives on water oxidation mechanisms.