Bicarbonate is a key regulator but not a substrate for O₂ evolution in Photosystem II

Area of Science:

• Biochemistry
• Photosynthesis
• Structural Biology

Background:

• Photosystem II (PSII) is crucial for photosynthesis, splitting water to produce oxygen (O2) and reducing plastoquinone.
• While water is the accepted source of O2, alternative hypotheses involving bicarbonate persist.
• Understanding the O2 evolution mechanism is vital for comprehending photosynthesis.

Purpose of the Study:

• To review the regulatory roles of bicarbonate in PSII activity and assembly.
• To critically evaluate the hypothesis that bicarbonate is a substrate for O2 evolution.
• To present evidence challenging the bicarbonate-based O2 evolution theory.

Main Methods:

• Review of existing literature on Photosystem II and bicarbonate.
• Analysis of biochemical, spectroscopic, and structural biology data.
• Thermodynamic evaluation of oxygen-centered bicarbonate oxidation.

Main Results:

• Bicarbonate plays regulatory roles in PSII activity and assembly.
• Biochemistry, spectroscopy, and structural biology experiments have not detected bicarbonate at the O2 evolution active site.
• Thermodynamic arguments for bicarbonate oxidation exist but do not confirm it as a substrate.

Conclusions:

• The prevailing evidence supports water as the sole source of O2 in photosynthesis.
• The hypothesis of bicarbonate as a substrate for O2 evolution is not supported by experimental data.
• Further research should focus on the established water-splitting mechanism in Photosystem II.