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 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 k in  = 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 O 2 affinity, Ta‐VII Hb showed a value of P 50  = 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (δH +  = −0.58) almost equal to that of mammalian hemoglobins. We have also investigated the stability property of Ta‐VII HbO 2 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 HbO 2 and sperm whale MbO 2 , and described based on a nucleophilic displacement mechanism. In light of the O 2 binding affinity, Bohr effect and considerable stability of the bound O 2 against acidic autoxidation, we conclude that T. akamusi Hb VII can play an important role in O 2 transport and storage as the major component in the larval hemolymph.