Myeloperoxidase-catalyzed taurine chlorination: initial versus equilibrium rate

Area of Science:

• Biochemistry
• Enzymology
• Chemical Kinetics

Background:

• Myeloperoxidase (MPO) is crucial in innate immunity, producing hypochlorous acid (HOCl) from chloride ions.
• Taurine (Tau) acts as a scavenger of HOCl, forming monochlorotaurine [(N-Cl)-Tau], a stable oxidant involved in pathogen defense.
• Understanding the kinetics and mechanism of (N-Cl)-Tau formation is vital for elucidating MPO's role in biological systems.

Purpose of the Study:

• To investigate the initial and equilibrium rates of monochlorotaurine [(N-Cl)-Tau] formation mediated by myeloperoxidase (MPO).
• To explore the influence of pH and hydrogen peroxide (H2O2) concentration on MPO-catalyzed taurine chlorination.
• To elucidate the reaction mechanism, particularly the molecular step involving MPO intermediates and chloride.

Main Methods:

• Kinetic studies of (N-Cl)-Tau formation at varying pH (4.0-7.0) and H2O2 concentrations.
• Analysis of initial and equilibrium reaction rates to determine kinetic parameters.
• Mathematical modeling incorporating known acid-base and binding equilibria to describe the observed kinetics.

Main Results:

• No saturation of the MPO active site was observed under the tested H2O2 concentrations.
• The deceleration of Tau chlorination at equilibrium was quantitatively explained by the redox equilibrium between MPO compound I and compound II.
• Maximum chlorination rates were achieved around 0.4 mM H2O2 and pH 5.
• Kinetic data contradicted the previously accepted mechanism involving proton participation in the molecular step.

Conclusions:

• The findings support a mechanism involving a chlorinating MPO compound I-Cl(-) complex and/or hypochlorite (ClO-) formation.
• Hypochlorite (ClO-) likely diffuses out of the heme pocket before protonation to HOCl.
• Substrate accessibility to the chlorinating agent depends on substrate size and reaction site (enzyme active site vs. external solution).