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

The ferric form of the homodimeric Scapharca hemoglobin undergoes a pH-depe ndent spin transition of the heme iron. The transition can also be modulate d 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-sp in six-coordinate structure predominates. At neutral and at alkaline pHs, i n addition to a small population of a hexacoordinate high-spin species, a p entacoordinate species is significantly populated. Isotope difference spect ra clearly show that the heme group in the latter species has a hydroxide l igand and thereby is not coordinated by the proximal histidine, The stretch ing frequency of the Fe-OH moiety is 578 cm(-1) and shifts to 553 cm(-1) in (H2O)-O-18, 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 rang e. These observations suggest that Scapharca hemoglobin has a unique heme s tructure that undergoes substantial redox-dependent rearrangements that sta bilize the Fe-proximal histidine bond in the functional deoxy form of the p rotein but not in the ferric form.