Cyanide binding to human plasma heme-hemopexin: a comparative study.

Area of Science:

• Biochemistry
• Biophysical Chemistry
• Metalloprotein Chemistry

Background:

• Hemopexin (HPX) is crucial for scavenging and delivering heme-iron.
• HPX-heme-iron complexes exhibit unique spectroscopic and reactivity properties.
• Understanding these properties is vital for heme metabolism research.

Purpose of the Study:

• To determine the kinetics and thermodynamics of cyanide binding to ferric and ferrous human plasma HPX-heme-Fe.
• To investigate the reduction kinetics of the ferric HPX-heme-Fe-cyanide complex.
• To elucidate the role of amino acid residues in modulating HPX-heme-Fe reactivity.

Main Methods:

• Spectroscopic analysis of cyanide binding to HHPX-heme-Fe(III) and HHPX-heme-Fe(II).
• Kinetic measurements for cyanide association and dissociation rates.
• Dithionite-mediated reduction assays for the HHPX-heme-Fe(III)-cyanide complex.

Main Results:

• Quantified thermodynamic and kinetic parameters for cyanide binding to both HHPX-heme-Fe(III) and HHPX-heme-Fe(II).
• Determined the rate constant for the dithionite-mediated reduction of HHPX-heme-Fe(III)-cyanide.
• Identified the involvement of amino acid residues like His236 in ligand protonation/deprotonation.

Conclusions:

• Cyanide binding kinetics and thermodynamics to HHPX-heme-Fe have been characterized.
• The reduction of HHPX-heme-Fe(III)-cyanide is a defined kinetic process.
• HPX-heme-Fe reactivity is influenced by specific amino acid residues facilitating ligand exchange.