Resonance raman spectra of bovine liver catalase compound II. Similarity of the heme environment to horseradish peroxidase compound II

Resonance Raman spectroscopy has been used to investigate the structure and environment of the heme group in bovine liver catalase compound II. Both Soret- and Q-band excitation have been employed to observe and assign the skeletal stretching frequencies of the porphyrin ring. The oxidation state marker band ν₄ increases in frequency from 1373 cm^-1 in ferricatalase to 1375 cm^-1 in compound II, consistent with oxidation of the iron atom to the Fe(IV) state. Oxidation of five-coordinate, high-spin ferricatalase to compound II is accompanied by a marked increase of the porphyrin core marker frequencies that is consistent with a six-coordinate low-spin state with a contracted core. An Fe(IV) = O stretching band is observed at 775 cm^-1 for compound II at neutral pH, indicating that there is an oxo ligand at the sixth site. At alkaline pH, the Fe(IV) = O stretching band shifts to 786 cm^-1 in response to a heme-linked ionization that is attributed to the distal His-74 residue. Experiments carried out in H₂¹⁸O show that the oxo ligand of compound II exchanges with bulk water at neutral pH, but not at alkaline pH. This is essentially the same behavior exhibited by horse-radish peroxidase compound II and the exchange reaction at neutral pH for both enzymes is attributed to acid/base catalysis by a distal His residue that is believed to be hydrogen-bonded to the oxo ligand. Thus, the structure and environment of the heme group of the compound II species of catalase and horseradish peroxidase are very similar. This indicates that the marked differences in their reactivities as oxidants are probably due to the manner in which the protein controls access of substrates to the heme group.