Concerted proton-electron transfer between ascorbic acid and cytochrome b561

Area of Science:

• Biochemistry
• Molecular Biology
• Redox Biology

Background:

• Ascorbic acid (vitamin C) is a vital biological reductant.
• Its reducing power is paradoxical at physiological pH due to the unfavorable oxidation of its predominant monoanion form.
• The ascorbate dianion is a potent electron donor but is present at insufficient concentrations.

Purpose of the Study:

• To rationalize the rapid reduction of cytochrome b561 by ascorbate in adrenal medullary chromaffin vesicles.
• To propose a general mechanism for biological ascorbic acid utilization.

Main Methods:

• Theoretical analysis of ascorbate redox chemistry at physiological pH.
• Consideration of reaction mechanisms involving electron transfer and concerted proton-electron transfer.
• Comparison with known biological reduction processes, such as cytochrome b561 reduction.

Main Results:

• The abundant ascorbate monoanion is a poor electron donor due to unfavorable radical formation.
• Concerted proton-electron transfer offers a mechanism to circumvent unfavorable intermediates.
• This mechanism explains the rapid reduction of cytochrome b561 by the ascorbate monoanion.

Conclusions:

• Biological ascorbic acid utilization may commonly involve concerted proton-electron transfer.
• This mechanism allows efficient electron donation from the ascorbate monoanion.
• It provides a unified explanation for rapid biological redox reactions involving vitamin C.