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J Biol Chem, Vol. 274, Issue 5, 2916-2919, January 29, 1999

Hydroxide Rather Than Histidine Is Coordinated to the Heme in Five-coordinate Ferric Scapharca inaequivalvis Hemoglobin

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

From the  Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461 and the ^§ CNR Center of Molecular Biology, c/o Department of Biochemical Sciences, University "La Sapienza," 00185 Rome, Italy

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 cm¹ and shifts to 553 cm¹ in H₂¹⁸O, 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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