Inhibitor binding to isolated chloroplast cytochrome bf complex

Area of Science:

• Biochemistry
• Photosynthesis Research
• Plant Molecular Biology

Background:

• The chloroplast cytochrome bf complex is crucial for electron transport during photosynthesis.
• Quinol oxidation is a key step in the electron transfer pathway mediated by the cytochrome bf complex.
• Specific inhibitors can probe the function and mechanisms of this complex.

Purpose of the Study:

• To investigate the effects of three quinol oxidation inhibitors (stigmatellin, tridecylstigmatellin, dibromothymoquinone) on the isolated cytochrome bf complex.
• To elucidate the mechanism of electron transfer between the Rieske center (FeS), cytochrome f, and plastocyanin (PC).
• To understand how inhibitors affect electron transfer rates, equilibrium coefficients, and complex formation.

Main Methods:

• Utilized an isolated system with Photosystem I (PSI) particles, plastocyanin (PC), and the cytochrome bf complex.
• Employed laser flash photolysis to induce oxidation of P700 and PC.
• Simulated reaction kinetics using rate coefficients with and without inhibitor binding to the bf complex.

Main Results:

• Inhibitors increased cytochrome f oxidation and slightly decreased PC oxidation after laser-induced P700 oxidation.
• Re-reduction of oxidized P700 was more complete in the presence of inhibitors.
• Inhibitors blocked electron transfer from the Rieske center (FeS) to cytochrome f and PC, increasing the cytochrome f/PC reaction equilibrium coefficient.

Conclusions:

• The Rieske center (FeS) donates electrons to cytochrome f and PC with a rate coefficient of 35 s(-1) in the absence of inhibitors.
• Inhibitors increase the equilibrium coefficient for the cytochrome f/PC reaction by decreasing the back reaction rate.
• Inhibitors promote the formation of transient complexes between P700, PC, and the bf complex, significantly altering redox potentials.