The pH-dependent swinging-out of the distal histidine residue in ferric hemoglobin of a midge larva (Tokunagayusurika akamusi)

Hemoglobin VII (TaVII) is a major component in the larval hemolymph of Tokunagayusurika akamusi, a common midge (Diptera) found in Japan. This protein contains 150 amino-acid residues including the usual distal histidine at position 64. When the aquomet-form was placed in acidic pH range, its Soret peak was considerably blue-shifted and accompanied by a marked decrease in intensity, indicative of the protein being converted into a structure quite similar to that of Aplysia myoglobin lacking the distal histidine residue. The pH-dependent magnetic circular dichroism spectra in the Soret region have also revealed that TaVII hemoglobin is in an equilibrium between a hexacoordinate and a pentacoordinate structure for its ferric heme iron. We attribute this to a transition from an iron-ligated water molecule that is hydrogen-bonded to the distal histidine, to a water-free iron with the histidine swung-out of the heme pocket. Furthermore, this process was described by the involvement of a single dissociable group with pK_a = 6.3 in 0.1 M KC1 at 25°C.