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Modulation of oxygen binding to insect hemoglobins: The structure of hemoglobin from the botfly Gasterophilus intestinalis

¹ Department of Physics, CNR-INFM, and Center for Excellence in Biomedical Research, University of Genova, I-16146 Genova, Italy
² Department of Biomolecular Sciences and Biotechnology, CNR-INFM, University of Milano, I-20131 Milano, Italy
³ Department of Biomedical Sciences, University of Antwerp, B-2610 Antwerp, Belgium
⁴ Department of Biology, University of Ghent, B-9000 Ghent, Belgium
⁵ Department of Biology and Interdepartmental Laboratory for Electron Microscopy, University "Roma Tre", I-00146 Roma, Italy

(RECEIVED July 30, 2005; FINAL REVISION September 13, 2005; ACCEPTED September 16, 2005)

Hemoglobins (Hbs) reversibly bind gaseous diatomic ligands (e.g., O₂) as the sixth heme axial ligand of the penta-coordinate deoxygenated form. Selected members of the Hb superfamily, however, display a functionally relevant hexa-coordinate heme Fe atom in their deoxygenated state. Endogenous heme hexa-coordination is generally provided in these Hbs by the E7 residue (often His), which thus modulates accessibility to the heme distal pocket and reactivity of the heme toward exogenous ligands. Such a pivotal role of the E7 residue is prominently shown by analysis of the functional and structural properties of insect Hbs. Here, we report the 2.6 Å crystal structure of oxygenated Gasterophilus intestinalis Hb1, a Hb known to display a penta-coordinate heme in the deoxygenated form. The structure is analyzed in comparison with those of Drosophila melanogaster Hb, exhibiting a hexa-coordinate heme in its deoxygenated derivative, and of Chironomus thummi thummi HbIII, which displays a penta-coordinate heme in the deoxygenated form. Despite evident structural differences in the heme distal pockets, the distinct molecular mechanisms regulating O₂ binding to the three insect Hbs result in similar O₂ affinities (P₅₀ values ranging between 0.12 torr and 0.46 torr).

Keywords: Gasterophilus intestinalis hemoglobin; parasitic botfly hemoglobin; insect hemoglobin; heme hexa-/penta-coordination; oxygen recognition; protein structure

Abbreviations: Hb, hemoglobin • Mb, myoglobin • RMSD, root mean square deviation • CttHbIII, Chironomus thummi thummi HbIII • DmHb, Drosophila melanogaster Hb • GiHb1, Gasterophilus intestinalis Hb1 • amino acid residues have been labeled according to their sequence number (in parentheses) and their topological position within the globin fold (Perutz 1989).

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051742605.

Reprint requests to: Martino Bolognesi, Department of Biomolecular Sciences and Biotechnology, University of Milano, Via Celoria 26, I-20131 Milano, Italy; e-mail: martino.bolognesi{at}unimi.it; fax: +39-02-503-14895.

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