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 ¼ 25 s )1 and k in ¼ 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 ¼ 1.7 Torr at pH 7.4, accompanied with a remarkable Bohr effect (dH + ¼ )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.Keywords: Insect (midge) Hb; distal (E7) histidine; pH jump; swinging movement; heme oxidation.In previous papers, we reported that the hemoglobin from the 4th-instar larva of Tokunagayusurika akamusi, a common midge (Diptera) found in eutrophic lakes in Japan, is comprised of as many as 11 separable components (IA, IB, II, III, IV, V, VIA, VIB, VII, VIII and IX) on a DEAEcellulose column, and found that these can be classified into two groups on the basis of their presence or absence of the distal (E7) histidine residue. For instance, hemoglobin VII consists of 150 amino-acid residues and contains the usual distal histidine at position 64, whereas component V replaces it by an isoleucine at position 66 [1]. This is certainly one of the unique characters of T. akamusi hemoglobins, as all the Chironomus hemoglobins have a distal histidine residue. Among the T. akamusi hemoglobins, component VII was of particular interest. When its ferric met-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 hemoglobin being converted into a structure quite similar to that of Aplysia (sea hare) myoglobin lacking the usual distal histidine residue. The pH-dependent Soret magnetic circular dichroism (CD) spectra also revealed that hemoglobin VII is in an equilibrium between a hexacoordinate and a pentacoordinate structure for it...

Bishistidyl Heme Hexacoordination, a Key Structural Property in Drosophila melanogaster Hemoglobin

“…Such an observation would suggest that a bishistidyl heme species would be an evolutionary achievement, gained by specific globins, to cope with the need for a fast reduction process, such as that required in the pseudocatalytic activity associated to the scavenging of nitrogen-and oxygen-reactive species (72). In accord with this suggestion, a close correlation between the heme iron atom coordination state, the autoxidation rate, and the heme reactivity toward O 2 has been reported for midge larval Hbs (73).…”

Estudo espectroscópico do equilíbrio entre espécies hexacoordenada e pentacoordenada dos monômero d nativo e reconstituído da hemoglobina extracelular gigante de Glossoscolex paulistus em meio alcalino

“…Visca e colaboradores 49 empregam, inclusive, a determinação da razão entre as intensidades das bandas de Soret do composto estudado e de uma espécie atribuída como hexacoordenada para determinar a presença da espécie pentacoordenada, uma vez que este estado de coordenação tem uma absortividade molar muito menos intensa. De qualquer maneira, os dois fenômenos espectrais acima discutidos, ou seja, o deslocamento hipsocrômico e a diminuição de absortividade molar são fortes indicativos da formação de uma espécie pentacoordenada, conforme uma série de trabalhos envolvendo hemoproteínas, por meio de uma variada série de técnicas espectroscópicas 27,28,[50][51][52][53][54][55][56][57][58][59][60][61] . Ademais, surge uma banda de transferên-cia de carga do ligante para o metal (TCLM) em 620 nm, que também corresponde a uma espécie de spin alto, como é o caso do pentacoordenado.…”

Hemes férricos pentacoordenados e hexacoordenados dos monômeros d nativo e reconstituído da hemoglobina extracelular de Glossoscolex paulistus: estudos espectroscópicos em meio ácido

“…De fato, vários autores sugerem que, partindo-se de um heme hexacoordenado, a formação de um heme pentacoordenado implica em uma diminuição significativa da absortividade molar e um deslocamento hipsocrômico da banda de Soret, sendo que a grandeza deste deslocamento é peculiar para cada hemoglobina 37,[40][41][42][43][44][45][46] . Entretanto, o significativo deslocamento para 371 nm, encontrado no presente caso, indica não somente a formação de uma espécie pentacoordenada, mas demonstra que esse processo ocorre simultaneamente a um desenovelamento parcial da cadeia polipeptídica, isto é, a formação da espécie pentacoordenado estaria intrinsicamente relacionada a um processo inicial de desnaturação [41][42][43] . Consoante proposta estabelecida por Sage e colaboradores 19 , é possível que a formação da espécie pentacoordenada no meio áci-do esteja relacionada à protonação das histidinas que, inicialmente em meio neutro, estariam menos acessíveis ao solvente.…”