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Electron paramagnetic resonance and optical evidence for interaction between siroheme and Fe4S4 prosthetic groups in complexes of Escherichia coli sulfite reductase hemoprotein with added ligands.

Publication ,  Journal Article
Janick, PA; Siegel, LM
Published in: Biochemistry
January 18, 1983

Janick & Siegel [Janick, P. A., & Siegel, L. M. (1982) Biochemistry 21, 3538-3547] showed that the EPR spectrum of the reduced Fe4S4 center (S = 1/2) in fully reduced native ("unligated") Escherichia coli NADPH-sulfite reductase hemoprotein subunit (SiR-HP) is perturbed by interaction with paramagnetic ferrous siroheme (S = 1 or 2) to yield several novel sets of EPR signals: one set with all g values between 2.0 and 2.8, termed "S = 1/2" type, and two sets with the lowest field g value between 4.7 and 5.4, termed "S = 3/2" type. The present study has shown that EPR spectra of fully reduced SiR-HP are nearly quantitatively converted to the classical "g = 1.94" type typical of S = 1/2 Fe4S4 clusters when the heme has been ligated by strong field ligands such as CO, CN-, S2-, and AsO2-, converting the ferroheme to S = 0. However, the exact line shapes and g values of the g = 1.94 differ markedly when different ligands are bound to the heme. Also, optical difference spectra taken between enzyme species in which the heme is kept in the same (Fe2+) oxidation state while the Fe4S4 center is reduced or oxidized show that the optical spectrum of the ligated siroheme is sensitive to the oxidation state of the Fe4S4 cluster. These results indicate that the heme-Fe4S4 interaction of native SiR-HP persists even when the heme Fe is bound to exogenous ligands. We have also found that the g values of the exchange-coupled S = 1/2 and S V 3/2 type signals of native reduced SiR-HP can be significantly shifted by addition of potential weak field heme ligands--halides and formate--or low concentrations of certain chaotropic agents--guanidinium salts and dimethyl sulfoxide--to the fully reduced enzyme. Such agents can also promote interconversion of the S = 1/2 and S = 3/2 type signals. These effects are reversed on removal of the agent. Treatment of reduced SiR-HP with relatively large concentrations of chaotropes, e.g., 60% dimethyl sulfoxide or 2 or 3 M urea, leads to abolition of the S = 1/2 and S = 3/2 EPR signals and their replacement by signals of the g = 1.94 type.

Duke Scholars

Published In

Biochemistry

DOI

ISSN

0006-2960

Publication Date

January 18, 1983

Volume

22

Issue

2

Start / End Page

504 / 515

Location

United States

Related Subject Headings

  • Urea
  • Sulfur
  • Sulfite Reductase (NADPH)
  • Sulfides
  • Spectrophotometry
  • Oxidoreductases Acting on Sulfur Group Donors
  • Oxidoreductases
  • Oxidation-Reduction
  • Ligands
  • Iron
 

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Journal cover image

Published In

Biochemistry

DOI

ISSN

0006-2960

Publication Date

January 18, 1983

Volume

22

Issue

2

Start / End Page

504 / 515

Location

United States

Related Subject Headings

  • Urea
  • Sulfur
  • Sulfite Reductase (NADPH)
  • Sulfides
  • Spectrophotometry
  • Oxidoreductases Acting on Sulfur Group Donors
  • Oxidoreductases
  • Oxidation-Reduction
  • Ligands
  • Iron