Figure 1:

The catalytic cycle of myeloperoxidase. Ground state MPO (MPO-Fe(III)) is oxidized by H₂O₂ to become Compound I (MPO-Fe(IV)^.π+), which can be reduced to the ground state MPO by either halogenation cycle, or peroxidation cycle, depending on the concentration of H₂O₂. In halogenation cycle, halide (Cl⁻, Br⁻, or I⁻) or pseudohalide (SCN⁻) is used to produce hypohalous acid (HOCl, HOBr, HOI, or HOSCN). In peroxidation cycle, Compound II (MPO-Fe(IV)) is formed as an intermediate. Ground state MPO can also be reduced to MPO-Fe(II) or Compound III (MPO-Fe(II)-O₂) by e⁻ or O₂^.-, respectively. MPO-Fe(II) can be transformed back to the ground state form, or to Compound III by binding to O₂. Compound III can also be converted back to the ground state form by O₂^.-, resulting in the formation of H₂O₂.