Conformational change and cooperative ligand binding in hemoglobin

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• The molecular mechanism of ligand binding cooperativity in hemoglobin remains incompletely understood despite extensive research.
• Existing models face experimental validation challenges, necessitating re-evaluation of current data.

Purpose of the Study:

• To re-examine experimental data on hemoglobin cooperativity to identify key molecular mechanisms.
• To investigate the role of inter-subunit strain in controlling ligand binding cooperativity.
• To provide insights for future studies on allosteric proteins.

Main Methods:

• Analysis of molecular structural differences between deoxy- and oxy-hemoglobin using atomic coordinates.
• Investigation of the energetic contributions of subunit interface segments to structural changes.
• Examination of temperature dependence of oxygen equilibrium constants and flash photolysis recombination curves.

Main Results:

• Experimental data are consistently explained by a model where strain at subunit interfaces governs ligand binding cooperativity.
• Structural analysis reveals specific subunit interface segments contributing significantly to deoxy- vs. oxy-hemoglobin structural differences.
• Oxygen equilibrium constants are well-explained by a sequence of conformational changes, with local interface changes influencing early Adair constants and subunit rearrangement affecting the fourth.

Conclusions:

• Strain transmission across subunit interfaces is a critical determinant of hemoglobin's ligand binding cooperativity.
• Conformational changes, initiated by local interface alterations and culminating in subunit rearrangement, dictate the cooperativity observed.
• Flash photolysis experiments under specific conditions can further elucidate the nature of this subunit rearrangement.