Michael Marden | PubMed Health

PubMed2002

Importance of helices A and H in oxygen binding differences between bovine and human hemoglobins.

Human and bovine hemoglobins (Hbs) exhibit several functional differences. They have a similar oxygen affinity in the presence of 2,3-diphosphoglycerate (2,3-DPG); however, bovine Hb has a greatly diminished 2,3-DPG effect, which itself is chloride dependent. The question is to determine whether these differences have a common structural origin, or whether they evolved in an independent fashion. The decreased 2,3-DPG effect can be partially reproduced by mutations at the effector binding sites, substituting the betaNA1 valine-NA2 histidine present in human Hb with a methionine. While changes of human Hb at these sites could provoke the bovine characteristic of the lower 2,3-DPG effect, the oxygen affinities of these mutated Hbs were not as low as that of the bovine Hb. Modifications responsible for tertiary structural modifications of helix A in human Hb might help shift the N-terminal methionine position, thereby locking helix A in place. We replaced the residues proline beta5(A2), arginine beta104(G6), and tyrosine beta130(H8) of human Hb by the residues present in bovine beta-globin, namely alanine, lysine, and phenylalanine, respectively. These mutations did not allow us to obtain a low oxygen affinity recombinant Hb (rHb). This indicates that other factors also influence oxygen binding and the effects are only partially coupled.

Adult↗

PubMedDate not supplied

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.

Animals↗