Pseudomonas cytochrome c peroxidase. XIII. pH-denaturation of the enzyme

The effect of pH on the oxidized Pseudomonas cytochrome c peroxidase molecule was studied by measuring the peroxidatic activity, the sedimentation velocity, the circular dichroic spectra in the far UV and Soret regions, and the optical absorption spectra of the enzyme in the pH range 2.5-13.0 at a constant ionic strength (micron = 0.1). The enzyme was stable in a narrow pH region, pH 6.0 - 7.4. In the low pH range the gross tertiary structure was observed to change quite simultaneously with the enzymatic activity and secondary structure. The optical absorption spectra indicated that there were no coordinated internal protein liqands in the 6th coordination positions of the heme prosthetic groups at the lowest pH studied. In the high pH range the secondary structure and the protein environment of hemes were observed to remain stable after the tertiary structure had changed and the activity had decreased. According to the optical absorption spectra the 6th internal protein ligands of hemes were retained at the highest pH studied.