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Báo cáo khoa học: The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins

Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva ofToku-nagayusurika akamusi, a common midge found in Japan. In the polymorphic hemoglobin comprised of 11 separable components, hemoglobinVII (Ta-VIIHb) 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 theE7 position.As a result, the iron(III) was converted from a hexacoordinate to a pentacoordinate form by a concom-itant loss of the axial water ligand. . | Eur. J. Biochem. 270 1424-1433 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03498.x The swinging movement of the distal histidine residue and the autoxidation reaction for midge larval hemoglobins Satoshi Kamimura1 Ariki Matsuoka2 Kiyohiro Imai3 and Keiji Shikama1 4 1 Biological Institute Graduate School of Life Sciences Tohoku University Sendai Japan 2Fukushima Medical University Fukushima Japan 3Laboratory of Nanobiology Graduate School of Frontier Biosciences Osaka University Suita Osaka Japan 4PHP Laboratory for Molecular Biology Nakayama-Yoshinari Sendai Japan Some insects have a globin exclusively in their fast-growing larval stage. This is the case in the 4th-instar larva of Toku-nagayusurika 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 kout 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 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 ỖH -0.58 almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta-VII HbO2 in terms of the autoxi-dation 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 .