Hemoglobin-nitric oxide cooperativity: is NO the third respiratory ligand?

Area of Science:

• Biochemistry
• Physiology

Background:

• Hemoglobin's cooperative binding is key for oxygen and carbon dioxide transport.
• Nitric oxide (NO) interactions with hemoglobin are complex and debated.
• Understanding NO-hemoglobin dynamics is crucial for physiological studies.

Purpose of the Study:

• To investigate the cooperative binding mechanisms of nitric oxide (NO) with hemoglobin.
• To clarify whether NO binding to hemoglobin is cooperative.
• To determine the factors influencing NO transport by hemoglobin.

Main Methods:

• Kinetic analysis of nitric oxide association and dissociation rates with hemoglobin.
• Examination of hemoglobin oxidation reactions in the presence of nitric oxide.

Main Results:

• Nitric oxide (NO) binding to hemoglobin is not cooperative; its dissociation is cooperative.
• Cooperativity in NO-hemoglobin equilibrium binding is primarily observed in the dissociation rate.
• Slow NO dissociation and rapid hemoglobin oxidation to nitrate are significant barriers to NO transport by hemoglobin.

Conclusions:

• Hemoglobin's role in NO transport is limited by slow NO dissociation and rapid oxidation.
• While direct NO transport by hemoglobin is unlikely under normal conditions, its delivery via complex mechanisms warrants further investigation.
• Hemoglobin's interaction with NO differs significantly from its interaction with oxygen, with cooperativity primarily affecting dissociation.