Open AccessOxidation‐Reduction Potentials and Ionization States of Two Turnip Peroxidases
Author(s) -
Ricard Jacques,
Mazza Gilbert,
Williams Robert J. P.
Publication year - 1972
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1972.tb01945.x
Subject(s) - chemistry , peroxidase , metmyoglobin , ferric , ferrous , ionization , horseradish peroxidase , hemeprotein , titration , ionization energy , enzyme , photochemistry , heme , inorganic chemistry , myoglobin , ion , biochemistry , organic chemistry
In the present paper, differences in the environment of the iron atom of two turnip isoperoxidases, called P 1 , and P 7 , have been established. This study is performed by way of spectrophotometric titrations, and by the analysis of oxidation‐reduction potential variations with regard to hydrogen ion concentration. Turnip peroxidases P 1 and P 7 exhibit near infrared bands with intensities and energies almost identical to those of metmyoglobin and horseradish peroxidase. In the case of turnip hemoprotein P 1 , an ionization, with an apparent pK close to 10, has been shown only in the oxidized form of the enzyme. Two other ionizations, in the ferric and the ferrous forms, respectively, occur also. They possess the same apparent pK , equal to 4.3, and seem to correspond to the same group. Another ionization is seen, with an apparent pK equal to 6.3, which specifically occurs in the reduced peroxidase. These results are very close to those obtained by various workers on horseradish peroxidase. In the case of turnip hemoprotein P 7 , the situation is very different. An ionization, with a pK close to 6.5, occurs in the ferric form of the enzyme. Two other ionizations, one in the oxidized form, the other in the reduced form of the enzyme, occur also. They both possess the same apparent pK equal to 8.4. A further ionization pertaining to the ferrous form only, is also seen, with an apparent pK close to 5. The above results are in agreement with the existence of the same axial ligands, namely histidine and water, in both hemoproteins. A likely interpretation of the ionizations detected in the acidic pH range is to assume that the distal groups of the proteins, for both peroxidases P 1 and P 7 , are different. The occurrence, respectively, of a carboxyl group and a histidine residue would agree with the spectrophotometric and potentiometric titration data. The high value of the oxidation‐reduction potentials of peroxidase P 7 , and the unexpected stability of the oxygenated compound of its ferrous form, are also compatible with an oxygenase function of this hemoprotein.