Ligation of the iron in the heme-heme oxygenase complex: X-ray absorption, electronic absorption and magnetic circular dichroism studies

Barton K. Hawkins, Angela Wilks, Linda S. Powers, Paul R. Ortiz De Montellano, John H. Dawson

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

Heme oxygenase (HO) catalyzes the first steps in the breakdown of heme to biliverdin and carbon monoxide. It is a membrane-bound protein that has been shown to exist in two isoforms, HO-1 and HO-2. Recently, a soluble, truncated form of rat HO-1 (rHO) lacking the 23 amino-acid membrane anchor has been expressed in E. coli. Extended X-ray absorption fine structure (EXAFS) data on ferric rHO and its fluoride derivative support assignment of the axial iron ligands as oxygen and/or nitrogen donors having distances similar to ferric myoglobin. The electronic absorption and magnetic circular dichroism (MCD) spectra of the ferric and ferrous protoheme complexes of rHO as well as various ligand adducts are very similar to the corresponding spectra of myoglobin. The present study is the first investigation of the heme-heme oxygenase complex with EXAFS and MCD spectroscopy and establishes that the proximal ligand to the heme in rHO is histidine. Furthermore, the close similarity between the electronic absorption and MCD spectra of ferric rHO and myoglobin over the pH range 6 to 10 is consistent with distal heme ligation of ferric rHO as a water molecule or hydroxide ion, depending on pH. Taken together and in conjunction with the results of earlier studies, EXAFS, electronic absorption, and MCD spectroscopy solidly establish that the ligands to the heme in rHO are identical to those in myoglobin, namely, histidine/H2O at low pH and histidine/OH at high pH.

Original languageEnglish (US)
Pages (from-to)165-173
Number of pages9
JournalBiochimica et Biophysica Acta - Protein Structure and Molecular Enzymology
Volume1295
Issue number2
DOIs
StatePublished - Jul 18 1996
Externally publishedYes

Keywords

  • Distal water/hydroxide
  • Extended X-ray absorption fine structure spectroscopy
  • Heme protein
  • Magnetic circular dichroism spectroscopy
  • Oxygenase
  • Proximal histidine

ASJC Scopus subject areas

  • Biophysics
  • Structural Biology
  • Biochemistry
  • Molecular Biology

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