Properties of inactive Photosystem II centers

Area of Science:

• Photosynthesis research
• Plant biochemistry
• Photochemistry

Background:

• A subset of Photosystem II (PS II) reaction centers exhibit impaired electron transfer from QA to QB.
• These inactive centers are unable to perform oxygen evolution due to a predominant back reaction after charge separation.
• Understanding these inactive centers is crucial for elucidating the complete mechanism of photosynthesis.

Purpose of the Study:

• To characterize the properties of inactive PS II centers using fluorescence and absorption spectroscopy.
• To investigate the role of inactive centers in the overall fluorescence induction kinetics.
• To determine the effect of dichlorobenzoquinone on electron transfer in inactive centers.

Main Methods:

• Analysis of fluorescence induction kinetics (Fo-Fpl phase) under weak illumination.
• Absorption change experiments to quantify inactive and active centers.
• Comparison of fluorescence yield dependence on closed centers.
• Testing the effect of dichlorobenzoquinone on electron transfer.

Main Results:

• Inactive centers contribute significantly (estimated 65%) to the Fo-Fpl fluorescence rise, while S-states modulate it (35%).
• The faster kinetics of the Fo-Fpl phase suggest inactive centers share light-harvesting antennae with active centers.
• Dichlorobenzoquinone does not restore electron transfer in inactive centers but enhances steady-state electron flow by relieving plastoquinone reoxidation bottlenecks.

Conclusions:

• Inactive PS II centers possess distinct properties influencing fluorescence kinetics and electron transfer pathways.
• Excitation energy transfer likely occurs between antennas of inactive and active centers.
• Discrepancies in literature may stem from differentiating single-turnover inactive centers from steady-state blocked centers.