Raman difference spectroscopy of heme-linked ionizations in cytochrome c peroxidase

The pH dependence of the oxidation-state marker line of hemoproteins is investigated in cytochrome c peroxidase with Raman difference spectroscopy. The frequency is sensitive to ionization of a group on the protein that regulates catalytic activity of the resting ferriheme enzyme. The oxidation-state marker line shows a transition with pK of 5.5 in good agreement with other spectroscopic measurements and kinetic measurements of binding of peroxide, and other ligands to the native enzyme. The shift of 0.8 cm-1 to higher frequency at pH 4.5 relative to the pH 6.4 value is interpreted in terms of a substantial decrease in pi-electron density in the porphyrin ring. Charge density in the pi-system is highest at maximal activity, as would be expected if donor-acceptor interactions with residues of the protein stabilize the oxidized Fe(IV) reaction intermediate. Evidence of additional heme-linked ionizations with pK values near 7.5 is found; this alkaline transition involves deprotonation of several groups of the protein, conversion of iron from high to low spin, and, possibly, denaturation of the protein.