Oxidation-reduction potential measurements on chloroperoxidase and its complexes

Abstract

The oxidation-reduction potential of chloroperoxidase, an enzyme which catalyzes peroxidative chlorination, bromination, and iodination reactions, has been investigated. In addition to catalyzing biological halogenation reactions, chloroperoxidase is unusual in that the carbon monoxide complex of ferrous chloroperoxidase shows the typical long wavelength Soret absorption associated with P-450 hemoproteins. The pH dependence of the chloroperoxidase oxidation-reduction potential shows a discontinuity around pH 4.7. Similarly, measurements of the affinity of ferrous chloroperoxidase for carbon monoxide monitored both by spectroscopic and potentiometric titration exhibit a discontinuity in the pH 4.7 region. Oxidation-reduction potential measurements on chloroperoxidase in a CO atmosphere also show a discontinuous pH profile. These results suggest that ferrous chloroperoxidase undergoes reversible modification at low pH and that these changes are reflected in the oxidation-reduction potential. The oxidation-reduction potential of chloroperoxidase at pH 6.9 is - 140 mV, close to that measured for cytochrome P-450cam in the presence of substrate. The oxidation-reduction potential of chloroperoxidase at pH 2.7, the pH optimum for enzymatic chlorination, is +150 mV. The oxidation-reduction potentials of the halide complexes of chloroperoxidase (chloride, bromide, and iodide) are essentially identical with the potential measurements on the native enzyme. These observations suggest that, although halide anions bind to the enzyme, they probably do not bind as an axial ligand to the heme ferric iron.

Direct Electron Transfer Between Ligninolytic Redox Enzymes and Electrodes

“…The 1 e À -redox potentials for the ferrous/ferric transitions in a number of LiP isozymes determined by potentiometric titration with mediators are in the range of À 142 to À 127 mV, and for MnP in the range of À 88 to À 93 mV (vs. NHE), pH 7.0 [88] compared to À 278 mV determined for HRP [89,90]. Cyclic voltammetry of LiP and MnP at pyrolytic graphite electrodes demonstrated similar values for the redox potentials of the ferrous/ferric transitions in LiP, MnP, and HRP, À 126, À 122, and À 136 mV (vs. NHE), pH 7.0, respectively [26,91], see Fig 2. The overall 1 e À reaction was coupled to the transfer of one proton to the ferric enzyme, in acidic media, which was interpreted as the presence of a heme-linked ionization of an amino acid in the reduced forms of the LiP and MnP similar to that of other plant peroxidases [26,88].…”

Metalloporphyrines as Active Site AnaloguesLessons from Enzymes and Enzyme Models

“…37,38 There are several possibilities for designing catalytic enzyme models having E 0 values close to those of P450 cam or CPO (E 0 ) -140 mV). 39 The synthetically most convenient way seems to be the preparation of electrondeficient porphyrins, such as 14 and 15 (Figure 3), which contain two meso-dichlorophenyl and two meso-pentafluorophenyl substituents, respectively. Porphyrin 14 exhibits E 0 ) -460 mV 40 and 15 a redox potential of E 0 ) -134 mV, 41 well within the range of E 0 values of CPO and P450.…”

Direct Peroxidase Bioelectrocatalysis on a Variety of Electrode Materials

“…By this means ferriperoxidase can be directly reduced at the electrode surface to its ferrous state. The formal reduction potential E8' for the Fe(III/II) couple of HRP is À 467 mV (Ag/AgCl) at pH 7.0 and À 417 mV at pH 6.0, respectively [32,33]. Similar to other plant peroxidases, the values of E8' for the couple Fe(III/II) of HRP are pH-dependent [10] since the redox process involves a transfer of one proton.…”