Flash-induced redox changes in oxygen-evolving spinach Photosystem II core particles

Area of Science:

• Biochemistry
• Photosynthesis research
• Spectroscopy

Background:

• Photosystem II (PS II) is crucial for oxygenic photosynthesis.
• Understanding the S-state cycle and redox reactions within PS II is key to elucidating its mechanism.
• Previous studies on PS II membranes provide a basis for comparison.

Purpose of the Study:

• To investigate flash-induced redox reactions in spinach PS II core particles.
• To characterize the kinetics of S-state transitions and tyrosine D oxidation.
• To compare findings with those from PS II membrane fragments.

Main Methods:

• Absorbance difference spectroscopy in the UV-region.
• Electron Paramagnetic Resonance (EPR) spectroscopy.
• Utilized flash-induced redox reactions with and without artificial electron acceptors.

Main Results:

• Period-four oscillations in UV absorbance linked to S-state cycle were restored by electron acceptors, but period-two oscillations were not.
• All active centers were in state S1 after dark adaptation, unlike in PS II membranes.
• S-state transition kinetics (Z(+)S1→ZS2, Z(+)S2→ZS3, Z(+)S3→ZS0) and Z(+)S0→ZS1 transition were characterized.
• Deactivation of higher S-states and oxidation of S0 to S1 occurred rapidly (seconds to minutes).
• Tyrosine D(+) exhibited similar fast kinetics, appearing in inactive and active centers.

Conclusions:

• Spinach PS II core particles exhibit distinct S-state cycling behavior compared to PS II membranes.
• The kinetics of S-state transitions and tyrosine D oxidation are rapid in PS II core particles.
• Electron transport limitations and the role of electron acceptors influence PS II redox dynamics.