Quaternary structure of hemoglobin in solution.

Area of Science:

• Biochemistry and Molecular Biology
• Structural Biology
• Biophysics

Background:

• Allosteric regulation is crucial for protein function, involving conformational changes upon ligand binding.
• X-ray crystallography can be influenced by intermolecular contacts, potentially differing from solution structures.
• Understanding protein dynamics in solution is vital for comprehending physiological mechanisms.

Purpose of the Study:

• To determine the solution structure of tetrameric human adult carbonmonoxy-hemoglobin under near-physiological conditions.
• To investigate the dynamic nature of hemoglobin's quaternary structure in solution.
• To compare solution structures with existing crystallographic models (R and R2 states).

Main Methods:

• Utilized Nuclear Magnetic Resonance (NMR) spectroscopy.
• Measured (15)N-(1)H residual dipolar couplings in weakly oriented hemoglobin samples.
• Performed measurements under near-physiological pH, ionic strength, and temperature.

Main Results:

• Successfully determined the quaternary structure of carbonmonoxy-hemoglobin in solution.
• The solution structure represents a dynamic intermediate between the R and R2 crystallographic states.
• Observed exchange broadening at the subunit interface indicates rapid structural equilibrium.

Conclusions:

• Hemoglobin exists as a dynamic ensemble of structures in solution, not a single static conformation.
• NMR-based methods provide valuable insights into protein dynamics under physiological conditions.
• The findings reconcile crystallographic data with the solution behavior of hemoglobin.