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J. Biol. Chem., Vol. 280, Issue 29, 27222-27229, July 22, 2005

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

From the ^aDepartment of Physics, National Institute for the Physics of Matter (NFM), and Centre for Excellence in Biomedical Research, University of Genova, Genova I-16146, Italy, the ^bDepartment of Biomedical Sciences, University of Antwerp, Universiteitsplein 1, Antwerp B-2610, Belgium, ^dGlobal Phasing, Ltd., Sheraton House, Castle Park, Cambridge CB3 0AX, United Kingdom, the ^eDepartment of Biology and Interdepartmental Laboratory for Electron Microscopy, University Roma Tre, Viale Guglielmo Marconi 446, Roma I-00146, Italy, the ^fNational Institute for Infectious Diseases Lazzaro Spallanzani, Via Portuense 292, Roma I-00149, Italy, the ^gInstitute of Molecular Genetics, Johannes Gutenberg University of Mainz, Becherweg 32, Mainz D-55099, Germany, the ^hInstitute of Zoology, Johannes Gutenberg University of Mainz, Müllerweg 6, Mainz D-55099, Germany, the ⁱNational Cancer Institute Genova, Structural Biology Unit, Largo Rosanna Benzi 10, Genova I-16132, Italy, and the ^jDepartment of Biomolecular Sciences and Biotechnology, INFM, University of Milano, Via Celoria 26, Milano I-20131, Italy

Hemoglobins at high concentration have been isolated long ago from some insect larvae living in hypoxic environments. Conversely, a monomeric hemoglobin has been discovered recently in the fruit fly Drosophila melanogaster as intracellular protein expressed both in larvae and in the adult fly. Such a finding indicates that the oxygen supply in insects may be more complex than previously thought, relying not only on O₂ diffusion through the tubular tracheal system, but also on carrier-mediated transport and storage. We present here the crystal structure of recombinant D. melanogaster hemoglobin at 1.20 Å resolution. Spectroscopic data show that the protein displays a hexacoordinated heme, whose axial ligands are the proximal and distal His residues. Such bis-His ligation of the heme has sizable effects on the protein local structure. Three protein matrix cavities, comparable in size but not in topological locations with those of sperm whale myoglobin, are spread through the protein matrix; one of these can host a xenon atom. Additionally, D. melanogaster hemoglobin binds one molecule of 3-(cyclohexylamino)propanesulfonic acid (CAPS) buffer at a surface pocket, next to the EF hinge. Despite the high resolution achieved, no sequence/structure features specifically supporting the heme hexa- to pentacoordination transition required for diatomic ligand binding could be recognized.

Received for publication, April 8, 2005 , and in revised form, May 19, 2005.

The atomic coordinates and structure factors (codes 2BK9 and R2BK9) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported in part by a grant from the Italian National Research Council Project PS-Functional Genomics, Grant RBAU015B47_002 from the Ministry for University and Scientific Research, Grant Bu956/6-1 from the Deutsche Forschungsgemeinschaft, and a grant from the European Community Project QRTL-2001-01548. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^c Postdoctoral fellow of the Fund for Scientific Research Flanders.

^k Supported by the Istituto Giannina Gaslini (Genova, Italy) and the Fondazione Compagnia di San Paolo (Torino, Italy). To whom correspondence should be addressed. Tel.: 39-02-5031-4893; Fax: 39-02-5031-4895; E-mail: martino.bolognesi{at}unimi.it.

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