Mechanism of ascorbic acid oxidation by cytochrome b(561).

Area of Science:

• Biochemistry
• Biophysics
• Metalloprotein Chemistry

Background:

• Cytochrome b(561) is a metalloprotein involved in redox reactions.
• Ascorbic acid (Vitamin C) is a key biological reducing agent.
• Understanding electron transfer mechanisms is vital in biochemistry.

Purpose of the Study:

• To elucidate the reaction mechanism between ascorbic acid and cytochrome b(561).
• To investigate the role of pH and specific residues in this redox process.
• To model the electron transfer dynamics and binding interactions.

Main Methods:

• Enzyme kinetics studies using ascorbic acid and cytochrome b(561).
• Inhibition studies with diethyl pyrocarbonate (DEPC).
• Redox mediation experiments with ferri/ferrocyanide.
• Molecular modeling of binding and reaction intermediates.

Main Results:

• DEPC modifies a histidine in the ascorbate-binding site, inhibiting direct electron transfer.
• Ferrocyanide can mediate reduction of DEPC-treated cytochrome b(561), distinguishing electron vs. hydrogen atom transfer.
• Kinetic parameters (K(m), V(max)) show pH-dependent activity, suggesting protonation states are critical.
• Molecular modeling supports a mechanism involving ascorbate monoanion binding to unprotonated histidine.

Conclusions:

• Cytochrome b(561) specifically binds the ascorbate monoanion via a histidine residue.
• The cytochrome facilitates electron transfer by complexing ascorbate, enabling reaction.
• The mechanism is reversible and pH-dependent, crucial for equilibrating ascorbate and semidehydroascorbate levels.