title: The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins.
abstract: Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobin VII (Ta-VII Hb) was of particular interest. When its ferric met-form was exposed to pH 5.0 from 7.2, the distal histidine was found to swing away from the E7 position. As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concomitant loss of the axial water ligand. The corresponding spectral changes in the Soret band were therefore followed by stopped-flow and rapid-scan techniques, and the observed first-order rate constants of k(out) = 25 s(-1) and kin = 128 s(-1) were obtained for the outward and inward movements, respectively, of the distal histidine residue in 0.1 m buffer at 25 degrees C. For O2 affinity, Ta-VII Hb showed a value of P50 = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (deltaH+ = -0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxidation rate over a wide range of pH from 4 to 11. The resulting pH-dependence curve was compared with those of another component Ta-V HbO2 and sperm whale MbO2, and described based on a nucleophilic displacement mechanism. In light of the O2 binding affinity, Bohr effect and considerable stability of the bound O2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O2 transport and storage as the major component in the larval hemolymph.
authors.affiliation: Biological Institute, Graduate School of Life Sciences, Tohoku University, Sendai, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Kamimura
authors.forename: Ariki
authors.initials: A
authors.lastname: Matsuoka
authors.forename: Kiyohiro
authors.initials: K
authors.lastname: Imai
authors.forename: Keiji
authors.initials: K
authors.lastname: Shikama
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002683:Chironomidae; D002942:Circular Dichroism; D017022:Flow Injection Analysis; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism; D006639:Histidine / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D019476:Insect Proteins / Q000737:chemistry* / Q000378:metabolism; D007814:Larva; D008024:Ligands; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry / Q000493:pharmacokinetics; D011485:Protein Binding / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D013057:Spectrum Analysis / Q000379:methods
chemical_list: D006454:Hemoglobins; D019476:Insect Proteins; D008024:Ligands; D006639:Histidine; D010100:Oxygen