Membrane charge affecting electron donation to PS II in chloroplasts

Area of Science:

• Photosynthesis research
• Plant biochemistry
• Thylakoid membrane biophysics

Background:

• Electron transport chain in photosynthesis is crucial for energy production.
• Thylakoid membranes contain essential protein complexes for light-dependent reactions.
• EDTA is known to chelate divalent cations like Mg(2+), potentially affecting membrane properties.

Purpose of the Study:

• To investigate the effect of EDTA pretreatment on chloroplast electron flow.
• To determine the role of cation valency and chloride ions in restoring electron flow.
• To analyze changes in charge density on the thylakoid membrane after EDTA treatment.

Main Methods:

• Chloroplasts were pretreated with EDTA (0.75 mM).
• Electron flow rates were measured at pH > 8.5.
• Inhibition reversal was studied using electron donors and various salt additions.
• Charge density in the Q region was calculated for control and treated chloroplasts.

Main Results:

• EDTA pretreatment significantly inhibited electron flow above pH 8.5.
• Restoration of electron flow by salts showed cation valency dependence: C(3+)>C(2+)>C(+).
• Maximal restoration required low chloride concentrations, essential for oxygen evolution.
• EDTA treatment increased negative charge density from -1.1 to -2.0 μC/cm(2) near Q.

Conclusions:

• EDTA treatment increases negative charge density on thylakoid membranes, likely by dissipating pH gradients and chelating Mg(2+).
• This increased negative charge density is closely related to the water donation site in photosystem II.
• Cations and chloride ions play critical roles in screening these negative charges and restoring electron transport.