Neuroglobin and other hexacoordinated hemoglobins show a weak temperature dependence of oxygen binding

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.