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 pKa = 6.3 in 0.1 M KCl at 25 degrees C.
Akiyama
K
K
Biological Institute, Tohoku University, Sendai, Japan.
Fukuda
M
M
Kobayashi
N
N
Matsuoka
A
A
Shikama
K
K
D000818:Animals; D002942:Circular Dichroism; D004175:Diptera / Q000737:chemistry*; D006454:Hemoglobins / Q000302:isolation & purification; D006458:Hemolymph / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007814:Larva / Q000737:chemistry; D015394:Molecular Structure
D006454:Hemoglobins; D006639:Histidine