Coordination and spin state equilibria as a function of pH, ionic strength, and protein concentration in oxidized dimeric Scapharca inaequivalvis hemoglobin

Carla Spagnuolo, Francesca De Martino, Alberto Boffi, Denis L. Rousseau, Emilia Chiancone

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

The oxidized homodimeric Scapharca inaequivalvis hemoglobin undergoes changes in coordination and spin state as a function of pH, ionic strength, and protein concentration which have been monitored by optical absorption spectroscopy. Three species contribute to the spectra between pH 5.8 and 8.7: (i) a hexacoordinate high spin aquomet derivative, whose concentration is essentially constant over the whole pH range analyzed; (ii) a pentacoordinate high spin component which prevails at alkaline pH values, and (iii) a hexacoordinate low spin hemichrome, which is formed at acid pH. The contribution of each of the components to the observed spectra was calculated with the singular value decomposition procedure and has been described quantitatively in terms of a linkage scheme which accounts for the change in heme coordination and for the observation that the high spin to low spin transition entails dissociation into monomers. An important feature of the linkage scheme is the cooperative binding of protons to aquomet dimers. Stopped flow experiments to study the kinetics indicate that dissociation into monomers is the rate-limiting process. The unusually strong tendency of oxidized HbI to loose the heme-bound water molecule is discussed in terms of strain in the iron-proximal histidine bond.

Original languageEnglish (US)
Pages (from-to)20441-20445
Number of pages5
JournalJournal of Biological Chemistry
Volume269
Issue number32
StatePublished - Aug 12 1994
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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