Identification and properties of an oxoferryl structure in myeloperoxidase compound II

Myeloperoxidase compound II has been characterized by using optical absorption and resonance Raman spectroscopies. Compared to compounds II in other peroxidases, the electronic and vibrational properties of this intermediate are strongly perturbed due to the unusual active-site iron chromophore that occurs in myeloperoxidase. Despite this difference in prosthetic group, however, other properties of myeloperoxidase compound II are similar to those observed for this intermediate in the more common peroxidases (horseradish peroxidase in particular). Two forms of the myeloperoxidase intermediate species, each with distinct absorption spectra, are recognized as a function of pH. We present evidence consistent with interconversion of these two forms via a heme-linked ionization of a distal amino acid residue with a pKa congruent to 9. From resonance Raman studies of isotopically labeled species at pH 10.7, we identify an iron-oxygen stretching frequency at 782 cm-1, indicating the presence of an oxoferryl (O = FeIV) group in myeloperoxidase compound II. We further conclude that the oxo ligand is not hydrogen bonded above the pKa but possibly exhibits oxygen exchange with the medium at pH values below the pKa due to hydrogen bonding of the oxo ligand to the distal protein group.