Abstract
The reduction potentials of the compound II/ferric and compound I/compound II couples have been studied, using potassium hexachloroiridate as a mediator titrant, by thin-layer spectroelectrochemistry. Compound I, which is 2 equiv more oxidized than the ferric (i.e., resting) form of the enzyme, was reversibly formed via a compound II intermediate; no evidence for a ferric porphyrin π-cation radical intermediate was obtained. At 25°C, E°′(compound I/compound II) = 897.9 ± 3 mV (NHE) and E°′-(compound II/ferric) = 869.1 ± 2 mV. Redox thermodynamic parameters, obtained from the temperature dependences of the reduction potentials of both couples, are reported. The reaction entropies (ΔS°rc) for the compound II/ferric and compound I/compound II couples are 19.8 ± 3.9 and 12.1 ± 3.7 eu, respectively. This result indicates that the reorganization energy for the macrocycle-centered couple is lower than that for the metal-centered one. Together with our observation that E°′ for the former is ca. 30 mV greater than that for the latter, these results suggest that compound I is more reactive toward outer-sphere reductants than compound II. In particular, the electron self-exchange rates for the compound I/compound II and compound II/ferric couples are estimated to be 4.4 × 10-1 and 4.9 × 10-4 M-1 s-1, respectively. Surprisingly, the formation of compound I from ferric HRP is accompanied by an almost zero standard entropy (ΔS°′) change.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 2866-2871 |
| Number of pages | 6 |
| Journal | Biochemistry |
| Volume | 34 |
| Issue number | 9 |
| State | Published - 1995 |
| Externally published | Yes |
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ASJC Scopus subject areas
- Biochemistry
Cite this
Variable-temperature spectroelectrochemical study of horseradish peroxidase. / Farhangrazi, Z. Shadi; Fossett, Martin E.; Powers, Linda S; Ellis, Walther R.
In: Biochemistry, Vol. 34, No. 9, 1995, p. 2866-2871.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Variable-temperature spectroelectrochemical study of horseradish peroxidase
AU - Farhangrazi, Z. Shadi
AU - Fossett, Martin E.
AU - Powers, Linda S
AU - Ellis, Walther R
PY - 1995
Y1 - 1995
N2 - The reduction potentials of the compound II/ferric and compound I/compound II couples have been studied, using potassium hexachloroiridate as a mediator titrant, by thin-layer spectroelectrochemistry. Compound I, which is 2 equiv more oxidized than the ferric (i.e., resting) form of the enzyme, was reversibly formed via a compound II intermediate; no evidence for a ferric porphyrin π-cation radical intermediate was obtained. At 25°C, E°′(compound I/compound II) = 897.9 ± 3 mV (NHE) and E°′-(compound II/ferric) = 869.1 ± 2 mV. Redox thermodynamic parameters, obtained from the temperature dependences of the reduction potentials of both couples, are reported. The reaction entropies (ΔS°rc) for the compound II/ferric and compound I/compound II couples are 19.8 ± 3.9 and 12.1 ± 3.7 eu, respectively. This result indicates that the reorganization energy for the macrocycle-centered couple is lower than that for the metal-centered one. Together with our observation that E°′ for the former is ca. 30 mV greater than that for the latter, these results suggest that compound I is more reactive toward outer-sphere reductants than compound II. In particular, the electron self-exchange rates for the compound I/compound II and compound II/ferric couples are estimated to be 4.4 × 10-1 and 4.9 × 10-4 M-1 s-1, respectively. Surprisingly, the formation of compound I from ferric HRP is accompanied by an almost zero standard entropy (ΔS°′) change.
AB - The reduction potentials of the compound II/ferric and compound I/compound II couples have been studied, using potassium hexachloroiridate as a mediator titrant, by thin-layer spectroelectrochemistry. Compound I, which is 2 equiv more oxidized than the ferric (i.e., resting) form of the enzyme, was reversibly formed via a compound II intermediate; no evidence for a ferric porphyrin π-cation radical intermediate was obtained. At 25°C, E°′(compound I/compound II) = 897.9 ± 3 mV (NHE) and E°′-(compound II/ferric) = 869.1 ± 2 mV. Redox thermodynamic parameters, obtained from the temperature dependences of the reduction potentials of both couples, are reported. The reaction entropies (ΔS°rc) for the compound II/ferric and compound I/compound II couples are 19.8 ± 3.9 and 12.1 ± 3.7 eu, respectively. This result indicates that the reorganization energy for the macrocycle-centered couple is lower than that for the metal-centered one. Together with our observation that E°′ for the former is ca. 30 mV greater than that for the latter, these results suggest that compound I is more reactive toward outer-sphere reductants than compound II. In particular, the electron self-exchange rates for the compound I/compound II and compound II/ferric couples are estimated to be 4.4 × 10-1 and 4.9 × 10-4 M-1 s-1, respectively. Surprisingly, the formation of compound I from ferric HRP is accompanied by an almost zero standard entropy (ΔS°′) change.
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UR - http://www.scopus.com/inward/citedby.url?scp=0028925951&partnerID=8YFLogxK
M3 - Article
C2 - 7893700
AN - SCOPUS:0028925951
VL - 34
SP - 2866
EP - 2871
JO - Biochemistry
JF - Biochemistry
SN - 0006-2960
IS - 9
ER -