Factors regulating the slow electrogenic phase in green algae and higher plants

Area of Science:

• Photosynthesis research
• Bioenergetics
• Plant physiology

Background:

• The electrochromic rise in the millisecond range is a key phenomenon in understanding photosynthetic electron transport.
• Previous studies have investigated its components and dependencies, but a clear understanding of the rate-limiting steps and controlling factors remained elusive.

Purpose of the Study:

• To elucidate the rate-limiting step and controlling factors of the electrochromic rise in the millisecond range in Chlorella cells and spinach chloroplasts.
• To investigate the influence of redox states, pH, transmembrane electric field, and pH gradients on this process.

Main Methods:

• Studied the electrochromic rise in Chlorella cells and spinach chloroplasts.
• Analyzed the half-time and amplitude of the electrochromic rise under varying conditions.
• Correlated kinetic behavior with proposed reaction mechanisms involving specific proteins and quinones.

Main Results:

• The half-time of the electrochromic rise is independent of redox states but dependent on transmembrane electric field and pH gradients.
• The limiting step of the overall process is the electrogenic reaction itself.
• Amplitude is controlled by reactant redox states and free energy, influenced by delta pH and electric field.

Conclusions:

• The electrogenic reaction is the rate-limiting step in the studied process.
• Photosystem II can rapidly reduce cytochrome f+ even when the electrogenic reaction is energetically hindered.
• A proposed reaction mechanism involving a quinone (UH2), Rieske protein (FeS), and a new protein (X2) explains the kinetic behavior.