METAL-LIGAND VIBRATIONS OF CYANOFERRIC MYELOPEROXIDASE AND CYANOFERRIC HORSERADISH-PEROXIDASE - EVIDENCE FOR A CONSTRAINED HEME POCKET IN MYELOPEROXIDASE

The low-frequency FeCN vibrations of cyanoferric myeloperoxidase (MPO) and horseradish peroxidase (HRP) have been measured by resonance Raman spectroscopy. The ordering of the frequencies of the predominantly FeC stretching and FeCN bending normal vibrational modes in the two peroxidases differs. These normal mode vibrations are identified by their wavenumber shifts upon isotopic substitution of the cyanide ligand. For MPO, the stretching mode v1 (361 cm−1) occurs at a lower frequency than the bending mode δ2 (454 cm−1). For HRP, the order is reversed as v1 (456 cm−1) is at a higher frequency than δ2 (404 cm−1). Normal coordinate analyses and model complexes have been used to address the origin of this behavior. The v1 stretching frequencies in cyanide complexes of iron porphyrin and iron chlorin model compounds are similar to one another and to that of HRP. Thus, the inverted order and altered frequencies of the v1 and δ2 vibrations in MPO, relative to those in HRP and the model compounds, are not inherent to the proposed iron chlorin prosthetic group in MPO but, rather, are attributed to distinct distal environmental effects in the MPO active site. The normal coordinate analyses for MPO and HRP showed that the vl and δ2 vibrational frequencies are not pure; the potential energy distributions for these modes respond not only to the geometry but also to the force constants of the v(FeC) and δ(FeCN) internal coordinates. By assuming a bent structure of the FeCN moiety in MPO relative to that in HRP, we were able to reproduce the reversed order of the v1 and δ2 frequencies in the leucocyte enzyme. The pH dependence of the Raman spectrum of cyanide-bound MPO supports the occurrence of a bent axial ligand structure. We invoke both distal and proximal effects to explain the FeCN vibrations of MPO. The amino acid residues responsible are judged to be those relevant to the catalytic properties of the enzymes. © 1990, American Chemical Society. All rights reserved.