Cyanide binding to ferrous and ferric microperoxidase-11

Area of Science:

• Biochemistry
• Chemical Kinetics
• Protein Chemistry

Background:

• Microperoxidase-11 (MP11) is an undecapeptide derived from horse heart cytochrome c.
• MP11 features a solvent-exposed heme group with heme-Fe axially coordinated by a histidyl residue.
• Pentacoordinated MP11 shows significantly higher reactivity towards cyanide compared to hexacoordinated cytochrome c.

Purpose of the Study:

• To investigate the kinetic and thermodynamic reactions of ferrous and ferric MP11 with cyanide.
• To elucidate the binding and dissociation mechanisms of cyanide with MP11.
• To compare the reactivity of MP11 with other heme proteins regarding cyanide interaction.

Main Methods:

• Kinetic analysis of cyanide binding and dissociation rates.
• Thermodynamic evaluation of cyanide equilibrium constants.
• pH-dependence studies of cyanide binding to MP11-Fe(III).

Main Results:

• Second-order rate constants for cyanide binding to MP11-Fe(II) and MP11-Fe(III) were determined.
• First-order rate constants for cyanide dissociation from MP11-Fe(II) and MP11-Fe(III) were measured.
• Dissociation equilibrium constants were consistent with kinetic parameters, indicating no intermediate species.

Conclusions:

• MP11's pentacoordinated heme iron exhibits high reactivity with cyanide.
• Ligand accessibility and cyanide ionization influence heme-Fe-cyanide complex formation.
• Heme pocket polarity and hydrogen bonding affect cyanide dissociation from the protein.