The oxidation of ferrocytochrome c in nonbinding buffer.

Area of Science:

• Biochemistry
• Physical Chemistry
• Electron Transfer Reactions

Background:

• Cytochrome c is a crucial protein in cellular respiration and electron transport.
• Understanding its redox reactions with external agents like ferro/ferricyanide is vital for biochemical studies.
• Previous studies have explored cytochrome c kinetics, but the impact of pre-exposure on its properties requires further investigation.

Purpose of the Study:

• To investigate the apparent equilibrium constant and oxidation rate of cytochrome c with iron hexacyanide.
• To determine the effect of ferricyanide pre-exposure on cytochrome c's properties.
• To analyze the ionic strength dependence of the oxidation reaction kinetics.

Main Methods:

• Studied horse heart ferricytochrome c and its reaction with ferricyanide.
• Utilized extensive dialysis and electrodialysis to prepare protein samples.
• Measured ionic strength dependence of equilibrium and rate constants using Tris-cacodylate or potassium phosphate buffers at pH 7.0.
• Compared kinetics of horse heart cytochrome c with Pseudomonas aeruginosa c551.

Main Results:

• Ferricytochrome c exposed to ferricyanide exhibited an altered apparent equilibrium constant, resistant to removal by dialysis.
• The bimolecular oxidation rate constant (kox) extrapolated to zero ionic strength was approximately 9 X 10(9) M-1 S-1, about 7% of the diffusion limit.
• An effective charge of +7.8 units was estimated for the oxidation reaction.
• Pseudomonas aeruginosa c551 showed significantly slower oxidation kinetics (kox at mu=0 ~ 6 X 10(3) M-1 S-1).

Conclusions:

• Ferricyanide pre-exposure induces a lasting change in horse heart cytochrome c's apparent equilibrium constant.
• Electron transfer in horse heart cytochrome c is highly efficient, occurring near every collision with the active site.
• The oxidation kinetics of different cytochrome c variants can vary significantly, impacting their functional roles.