Ferric microperoxidase-11 catalyzes peroxynitrite isomerization

Area of Science:

• Biochemistry
• Chemical Kinetics

Background:

• Microperoxidase-11 (MP11), an undecapeptide from horse heart cytochrome c, provides a model for studying heme group reactivity.
• Cytochrome c derivatives are implicated in various biological processes, including redox reactions.

Purpose of the Study:

• To investigate the catalytic activity of ferric MP11 (MP11-Fe(III)) in the isomerization of peroxynitrite to nitrate (NO3-).
• To elucidate the role of the heme-Fe(III) coordination state in modulating cytochrome c reactivity.

Main Methods:

• Kinetic studies of peroxynitrite isomerization catalyzed by MP11-Fe(III) across a pH range (3.6–8.1) at 20.0°C.
• Investigating the effect of cyanide binding to the heme-Fe atom on the catalytic activity.
• Comparing MP11-Fe(III) activity with other cytochrome c derivatives.

Main Results:

• The second-order rate constant (kon) for peroxynitrite isomerization by MP11-Fe(III) significantly decreases with increasing pH (from 1.1×10^5 M^-1s^-1 at pH 3.6 to 6.1×10^3 M^-1s^-1 at pH 8.1).
• A pKa of 6.9 suggests preferential reaction with peroxynitrous acid.
• Cyanide binding, which occupies the sixth coordination site of heme-Fe, completely inhibits the catalytic activity, indicating the necessity of a penta-coordinated heme-Fe.
• The rate constants observed for MP11-Fe(III) are comparable to those of penta-coordinated cardiolipin-cytochrome c complexes.

Conclusions:

• The penta-coordinated heme-Fe atom in MP11-Fe(III) is essential for catalyzing peroxynitrite isomerization to nitrate.
• Heme-Fe(III) coordination state plays a critical role in determining the reactivity of cytochrome c derivatives.
• MP11 serves as a valuable tool for dissecting the mechanistic aspects of heme protein function.