Neuroglobin ligand binding kinetics

Neuroglobin, cytoglobin, and hemoglobins from Drosophila melanogaster and Arabidopsis thaliana were studied for their ligand binding properties versus temperature. These globins have a common feature of being hexacoordinated (via the distal histidine) under deoxy conditions, displaying and enhanced amplitude for the alpha absorption band at 560 nm. External ligands can bind, but the transition from the hexacoordinated form to the ligand (L) bound species is slow, as expected for a replacement reaction Fe-His <--> Fe <--> Fe-L. Histidine binding is on the order of 1 ms; dissociation times are variable, and may be as long as 1 s for the highest histidine affinities. Oxygen binds rapidly but dissociates slowly, requiring as much as 1 s. These rates would correspond to a very high affinity for the pentacoordinated form; however, competition with the distal histidine leads decreases the affinity for the external ligand. The observed oxygen affinity remains in the range of 1 to 10 mm Hg. The low oxygen dissociation indicates a stabilization via H-bonds as for certain globins from parasites (Ascaris, the trematodes). Other ligands such as CO, or CN for the ferric form, show a decreased affinity, since only the competition with the E7 histidine, but not the stabilizing H-bond, plays a role. In addition, 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 would lead to an oxygen affinity that is nearly independent of temperature.