Protein-heme interaction in hemoglobin: evidence from Raman difference spectroscopy.

Area of Science:

• Biophysics
• Biochemistry
• Spectroscopy

Background:

• Human deoxyhemoglobin exists in two quaternary structures: R (relaxed) and T (tense).
• Hemoglobin cooperativity describes how oxygen binding to one heme site affects others.
• Understanding the electronic properties of hemoglobin is crucial for elucidating its function.

Purpose of the Study:

• To investigate the electronic differences between R and T quaternary structures of human deoxyhemoglobin.
• To explore the role of charge transfer in hemoglobin cooperativity.

Main Methods:

• Raman difference spectroscopy was used to measure native and chemically modified human deoxyhemoglobins.
• Spectroscopic data were analyzed to identify frequency shifts in oxidation state marker lines.

Main Results:

• Frequency differences in oxidation state marker lines were observed between R and T structures.
• The R structure showed an increased electron density in the porphyrin ring's antibonding pi* orbitals, attributed to charge transfer.
• The extent of charge transfer correlated with factors influencing quaternary structure energetics and cooperativity.

Conclusions:

• A charge transfer interaction between protein donor orbitals and porphyrin pi* orbitals contributes to hemoglobin structure and function.
• This electronic interaction may significantly impact the energetics of hemoglobin cooperativity.