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Neuroglobin and Other Hexacoordinated Hemoglobins Show a Weak Temperature Dependence of Oxygen Binding

Julien Uzan ^*, Sylvia Dewilde , Thorsten Burmester , Thomas Hankeln , Luc Moens , Djemel Hamdane ^*, Michael C. Marden ^* and Laurent Kiger ^*

^* INSERM U473, 94276 Le Kremlin-Bicêtre, France; Department of Biomedical Sciences, University of Antwerp, B-2610 Antwerp, Belgium; and Institute of Zoology and Institute of Molecular Genetics, Biosafety Research and Consulting, Johannes Gutenberg University of Mainz, D-55099 Mainz, Germany

Correspondence: Address reprint requests to Laurent Kiger, INSERM U473, 84 rue du General Leclerc, 94276 Le Kremlin-Bicetre Cedex, France. E-mail: kiger{at}kb.inserm.fr.

Mouse and human neuroglobins, as well as the hemoglobins from Drosophila melanogaster and Arabidopsis thaliana, were recombinantly expressed in Escherichia coli, and their ligand-binding properties were studied versus temperature. These globins have a common feature of being hexacoordinated (via the distal histidine) under deoxy conditions, as evidenced by a large amplitude for the alpha absorption band at 560 nm and the Soret band at 426 nm. The transition from the hexacoordinated form to the CO bound species is slow, as expected for a replacement reaction Fe-His Fe FeCO. The intrinsic binding rates would indicate a high oxygen affinity for the pentacoordinated form, due to rapid association and slow (100 ms–1 s) dissociation. However, the competing protein ligand results in a much lower affinity, on the order of magnitude of 1 torr. In addition to decreasing the affinity for external ligand, the competitive internal ligand leads to a weaker observed temperature dependence of the ligand affinity, since the difference in equilibrium energy for the two ligands is much lower than that of ligand binding to pentacoordinated hemoglobin. This effect could be of biological relevance for certain organisms, since it could provide a globin with an oxygen affinity that is nearly independent of temperature.

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