Heme binding by hemopexin: evidence for multiple modes of binding and functional implications

Area of Science:

• Biochemistry
• Spectroscopy
• Protein-heme interactions

Background:

• Hemopexin is a plasma protein crucial for heme transport.
• Heme binding influences protein conformation and function.
• Understanding heme-protein interactions is vital for biological processes.

Purpose of the Study:

• To investigate the heme environment in hemopexin complexes using circular dichroism (CD) spectroscopy.
• To analyze the effects of different heme types and redox states on hemopexin conformation.
• To explore the role of hemopexin domains in heme binding and spectral properties.

Main Methods:

• Circular dichroism (CD) spectroscopy was employed.
• Ferri-, ferro-, and CO-ferro complexes of hemopexin were studied.
• Four types of iron tetrapyrroles and three species of hemopexin were utilized.

Main Results:

• Ferri-heme-hemopexin complexes generally showed positive ellipticity near the Soret maximum.
• A notable exception was the human ferri-protoheme hemopexin complex with a bisignate spectrum.
• Redox and ligand changes induced conformational shifts, altering the heme environment.
• Rabbit mesoheme-N-domain complexes showed distinct CD spectra compared to intact hemopexin.
• The addition of the C-domain to the N-domain restored CD characteristics.

Conclusions:

• Hemopexin exhibits conformational flexibility in response to heme redox state and ligand binding.
• Specific heme-hemopexin interactions can lead to unique spectral signatures.
• Both N- and C-domains of hemopexin contribute to the overall heme-bound conformation and spectral properties.