Partial restoration of normal functional properties in carboxypeptidase A-digested hemoglobin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Chemistry

Background:

• Human hemoglobin A (HbA) undergoes functional modifications upon digestion with carboxypeptidase A, removing specific beta-chain terminal residues.
• Modified HbA exhibits high ligand affinity, a diminished Bohr effect, and lacks cooperativity in the absence of organic phosphates.

Purpose of the Study:

• To investigate the functional impact of inositol hexaphosphate (IHP) on carboxypeptidase A-digested human hemoglobin A.
• To elucidate the role of IHP in modulating ligand-binding kinetics and allosteric properties of modified HbA.

Main Methods:

• Enzymatic digestion of HbA with carboxypeptidase A.
• Ligand-binding studies (O2 and CO) with modified HbA in the presence and absence of IHP.
• Spectroscopic analysis (extinction coefficient) and kinetic measurements (flash photolysis).

Main Results:

• IHP addition to digested HbA restores a significant Bohr effect and introduces heme-heme interaction.
• IHP causes a 5-fold decrease in ligand affinity and slows CO binding kinetics.
• CO rebinding after flash photolysis becomes biphasic in the presence of IHP, resembling unmodified HbA.

Conclusions:

• Inositol hexaphosphate acts as an allosteric effector, shifting the equilibrium between high- and low-affinity conformational states of digested HbA.
• IHP reintroduces cooperativity and Bohr effect by stabilizing a low-affinity, T-like state.
• The findings support the model of IHP allosterically regulating hemoglobin conformation and function.