Quaternary structure and spin-state transition in azide methemoglobin A

Area of Science:

• Biophysics
• Biochemistry
• Physical Chemistry

Background:

• Human hemoglobin exists in R (relaxed) and T (tense) quaternary states.
• Azide methemoglobin A (MetHbA) is a functional form of hemoglobin.
• Inositol hexaphosphate (IHP) is an allosteric effector that binds to hemoglobin.

Purpose of the Study:

• To investigate the temperature-dependent changes in human azide MetHbA.
• To elucidate the roles of spin and quaternary equilibria in MetHbA structure.
• To determine the thermodynamic parameters of quaternary transition.

Main Methods:

• Spectroscopic analysis of ultraviolet and visible absorption changes.
• Temperature-dependent measurements in the range of 4-35°C.
• Analysis of spin and quaternary equilibria.

Main Results:

• IHP-free hemoglobin showed a larger absorption change at 537 nm than IHP-bound hemoglobin.
• The T conformer exhibited higher high-spin content and was more populated at lower temperatures.
• Thermodynamic values for quaternary equilibrium were determined: ΔH = -13.3 kcal/mol and ΔS = -47.6 eu.

Conclusions:

• Quaternary transition energy is not localized at the heme moiety.
• The T state formation is not directly coupled with increased strain in the Fe-N(F8 His) linkage.
• Temperature-dependent structural changes in MetHbA are complex, involving coupled spin and quaternary equilibria.