Binding of inositol hexaphosphate to human methemoglobin

Area of Science:

• Biochemistry
• Structural Biology
• Spectroscopy

Background:

• Methemoglobin (MetHb) is a form of hemoglobin with oxidized iron, affecting oxygen transport.
• Inositol hexaphosphate (IP6) is a known allosteric effector of hemoglobin.
• Understanding ligand binding effects on hemoglobin structure is crucial for physiological studies.

Purpose of the Study:

• To characterize the spectral changes induced by inositol hexaphosphate (IP6) binding to methemoglobin (MetHb).
• To differentiate and interpret the fast and slow kinetic phases of these spectral transitions.
• To elucidate the conformational changes and their implications for hemoglobin structure and function.

Main Methods:

• Stopped-flow spectroscopy to measure absorbance changes.
• pH dependence studies of spectral transitions.
• Heme ligand binding studies to probe spin and reactivity changes.

Main Results:

• IP6 binding induces a rapid spectral transition (too fast for stopped-flow) and a slow transition (1-6s half-time).
• The rapid phase involves localized tertiary conformational changes in beta-chain hemes, increasing spin and reactivity.
• The slow phase involves a minor fraction of molecules, altering alpha-chain spectral properties and potentially forming an abortive deoxyhemoglobin A-like conformation.

Conclusions:

• The rapid spectral transition reflects immediate tertiary structural adjustments upon IP6 binding.
• The slow transition suggests an isomerization process, possibly an inhibited deoxyhemoglobin A-like conformation.
• Water molecules bound to heme iron may inhibit the rate and extent of this slow conformational change.