The oxygen-evolving complex requires chloride to prevent hydrogen peroxide formation

Area of Science:

• Biochemistry
• Photosynthesis research
• Plant physiology

Background:

• Photosystem II (PSII) is crucial for oxygenic photosynthesis.
• Illumination of PSII can lead to side reactions, including peroxide production.
• The role of chloride ions and the oxygen-evolving complex (OEC) in these reactions is not fully understood.

Purpose of the Study:

• To investigate the mechanism of hydrogen peroxide (H2O2) production in PSII core preparations.
• To determine the involvement of the oxygen-evolving complex (OEC) and chloride ions (Cl-) in H2O2 generation.
• To elucidate the function of Cl- in regulating water oxidation in PSII.

Main Methods:

• Measuring H2O2 production rates in isolated PSII core preparations under varying pH, sucrose, and Cl- concentrations.
• Assessing the effect of inhibitors like NH2OH and NH3 on H2O2 production and water oxidation.
• Analyzing the relationship between H2O2 production and water oxidation rates.

Main Results:

• H2O2 production rates reached up to 60 mumol H2O2 (mg Chl.h)-1, maximal at pH 7.2 with low sucrose and 1.5-3.0 mM Cl-.
• H2O2 production increased with pH (6.8-7.2) and was inversely proportional to water oxidation (pH 6.8-7.5).
• NH2OH abolished H2O2 production, while NH3 inhibited it, implicating the OEC. EDTA did not affect production.

Conclusions:

• The oxygen-evolving complex (OEC) is responsible for H2O2 production in illuminated PSII preparations.
• A proposed mechanism involves OH- displacing bound Cl- in the S2 state, followed by oxidation to H2O2.
• Chloride ions (Cl-) are essential for the OEC to prevent premature active site access, ensuring efficient water oxidation to O2.