Hydroxide rather than histidine is coordinated to the heme in five- coordinate ferric Scapharca inaequivalvis hemoglobin

Tapan Kanti Das, Alberto Boffi, Emilia Chiancone, Denis L. Rousseau

Research output: Contribution to journalArticle

27 Citations (Scopus)

Abstract

The ferric form of the homodimeric Scapharca hemoglobin undergoes a pH- dependent spin transition of the heme iron. The transition can also be modulated by the presence of salt. From our earlier studies it was shown that three distinct species are populated in the pH range 6-9. At acidic pH, a low-spin six-coordinate structure predominates. At neutral and at alkaline pHs, in addition to a small population of a hexacoordinate high-spin species, a pentacoordinate species is significantly populated. Isotope difference spectra clearly show that the heme group in the latter species has a hydroxide ligand and thereby is not coordinated by the proximal histidine. The stretching frequency of the Fe-OH moiety is 578 cm-1 and shifts to 553 cm-1 in H2 18O, as would be expected for a Fe-OH unit. On the other hand, the ferrous form of the protein shows substantial stability over a wide pH range. These observations suggest that Scapharca hemoglobin has a unique heme structure that undergoes substantial redox-dependent rearrangements that stabilize the Fe-proximal histidine bond in the functional deoxy form of the protein but not in the ferric form.

Original languageEnglish (US)
Pages (from-to)2916-2919
Number of pages4
JournalJournal of Biological Chemistry
Volume274
Issue number5
DOIs
StatePublished - Jan 29 1999

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Scapharca
Heme
Histidine
Hemoglobins
Isotopes
Stretching
Proteins
Iron
Salts
Ligands
Oxidation-Reduction
hydroxide ion
Population

ASJC Scopus subject areas

  • Biochemistry

Cite this

Hydroxide rather than histidine is coordinated to the heme in five- coordinate ferric Scapharca inaequivalvis hemoglobin. / Das, Tapan Kanti; Boffi, Alberto; Chiancone, Emilia; Rousseau, Denis L.

In: Journal of Biological Chemistry, Vol. 274, No. 5, 29.01.1999, p. 2916-2919.

Research output: Contribution to journalArticle

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AU - Chiancone, Emilia

AU - Rousseau, Denis L.

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N2 - The ferric form of the homodimeric Scapharca hemoglobin undergoes a pH- dependent spin transition of the heme iron. The transition can also be modulated by the presence of salt. From our earlier studies it was shown that three distinct species are populated in the pH range 6-9. At acidic pH, a low-spin six-coordinate structure predominates. At neutral and at alkaline pHs, in addition to a small population of a hexacoordinate high-spin species, a pentacoordinate species is significantly populated. Isotope difference spectra clearly show that the heme group in the latter species has a hydroxide ligand and thereby is not coordinated by the proximal histidine. The stretching frequency of the Fe-OH moiety is 578 cm-1 and shifts to 553 cm-1 in H2 18O, as would be expected for a Fe-OH unit. On the other hand, the ferrous form of the protein shows substantial stability over a wide pH range. These observations suggest that Scapharca hemoglobin has a unique heme structure that undergoes substantial redox-dependent rearrangements that stabilize the Fe-proximal histidine bond in the functional deoxy form of the protein but not in the ferric form.

AB - The ferric form of the homodimeric Scapharca hemoglobin undergoes a pH- dependent spin transition of the heme iron. The transition can also be modulated by the presence of salt. From our earlier studies it was shown that three distinct species are populated in the pH range 6-9. At acidic pH, a low-spin six-coordinate structure predominates. At neutral and at alkaline pHs, in addition to a small population of a hexacoordinate high-spin species, a pentacoordinate species is significantly populated. Isotope difference spectra clearly show that the heme group in the latter species has a hydroxide ligand and thereby is not coordinated by the proximal histidine. The stretching frequency of the Fe-OH moiety is 578 cm-1 and shifts to 553 cm-1 in H2 18O, as would be expected for a Fe-OH unit. On the other hand, the ferrous form of the protein shows substantial stability over a wide pH range. These observations suggest that Scapharca hemoglobin has a unique heme structure that undergoes substantial redox-dependent rearrangements that stabilize the Fe-proximal histidine bond in the functional deoxy form of the protein but not in the ferric form.

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