Reduction of oxidized cytochrome c by ascorbate ion.

Area of Science:

• Biochemistry
• Electron Transfer Reactions
• Protein Kinetics

Background:

• Cytochrome c is a crucial protein in cellular respiration and electron transport.
• Ascorbate (vitamin C) is a known reducing agent with potential biological roles.
• Previous studies proposed complex mechanisms for ascorbate reduction of cytochrome c.

Purpose of the Study:

• To elucidate the kinetics and mechanism of oxidized cytochrome c reduction by ascorbate.
• To differentiate between simple electron transfer and protein-bound electron transfer mechanisms.
• To determine the rate constant for the electron transfer step.

Main Methods:

• Investigated reaction kinetics in various ionic media: potassium nitrate, KMes, potassium sulfate, and potassium ascorbate.
• Measured reaction rates at 25°C with an ionic strength of 0.1.
• Analyzed data to determine the order of the reaction and rate constants.

Main Results:

• Results strongly support a simple second-order electron transfer mechanism between ascorbate dianion and cytochrome c.
• A rate constant of 8 x 10^5 M^-1 s^-1 was determined for the electron transfer step.
• The rate constant was found to be largely independent of the specific ions used to control ionic strength.

Conclusions:

• The reduction of cytochrome c by ascorbate proceeds via direct, simple second-order electron transfer.
• This study refutes recent proposals suggesting electron transfer from an ascorbate dianion bound to cytochrome c.
• The determined rate constant provides a quantitative measure for this fundamental biochemical reaction.