Cyanide binding to human plasma heme–hemopexin: A comparative study

Hemopexin (HPX) displays a pivotal role in heme scavenging and delivery to the liver. In turn, heme–Fe–hemopexin (HPX–heme–Fe) displays heme-based spectroscopic and reactivity properties. Here, kinetics and thermodynamics of cyanide binding to ferric and ferrous hexa-coordinate human plasma HPX–heme–Fe (HHPX–heme–Fe(III) and HHPX–heme–Fe(II), respectively), and for the dithionite-mediated reduction of the HHPX–heme–Fe(III)–cyanide complex, at pH 7.4 and 20.0 °C, are reported. Values of thermodynamic and kinetic parameters for cyanide binding to HHPX–heme–Fe(III) and HHPX–heme–Fe(II) are K = (4.1 ± 0.4) × 10−6 M, kon = (6.9 ± 0.5) × 101 M−1 s−1, and koff = 2.8 × 10−4 s−1; and H = (6 ± 1) × 10−1 M, hon = 1.2 × 10−1 M−1 s−1, and hoff = (7.1 ± 0.8) × 10−2 s−1, respectively. The value of the rate constant for the dithionite-mediated reduction of the HHPX–heme–Fe(III)–cyanide complex is l = 8.9 ± 0.8 M−1/2 s−1. HHPX–heme–Fe reactivity is modulated by proton acceptor/donor amino acid residue(s) (e.g., His236) assisting the deprotonation and protonation of the incoming and outgoing ligand, respectively.

► Cyanide binding to ferric HHPX–heme–Fe.
► Cyanide binding to ferrous HHPX–heme–Fe.
► Dithionite-mediated reduction of ferric HHPX–heme–Fe–cyanide.
► Cyanide binding to HHPX–heme–Fe is limited by ligand deprotonation.
► Cyanide dissociation from HHPX–heme–Fe–cyanide is limited by ligand protonation.