© 1980 by The Society for Integrative and Comparative Biology
Anionic Control of Function in Vertebrate Hemoglobins1
Marine Biomedical Center Duke University Marine Laboratory Beaufort North Carolina 28516
Point mutations in the amino acid sequence of normal human hemoglobin have provided a powerful means of probing structure-function relationships in this respiratory protein. Through studies of specific hemoglobin variants it has been possible to gain a better understanding of how electrostatic interactions exercise control over the functional properties of hemoglobin. Human hemoglobin variants of particular interest in this respect are those with alterations of amino or carboxyl terminal residues and alterations at or near the binding site for the physiologically important cofactor 2,3-diphosphoglycerate. In deoxy hemoglobin it has been established that salt bridges formed by the terminal residues constrain the tetramer in a low affinity conformation. From the information presently available, it appears that the charge cluster of the 2,3 diphosphoglycerate binding site is an important part of the innerspring mechanism that tends to destabilize the deoxy conformation. When anions bind to these residues the repulsive interactions between the positively charged residues of this region are decreased. This provides a direct means by which anionic interactions with hemoglobin can shift the conformational equilibrium toward the low affinity state. Accordingly anion and pH effects are decreased in a number of hemoglobin variants whose substitutions reduce the positive charge density in the region of the binding site for polyphosphates. The presence of the charge cluster provides for a fine tuning of hemoglobin s functional properties that is responsive to the concentration of metabolic effectors in vivo. The degree to which this is possible varies in the vertebrate hemoglobins which have been examined. In human hemoglobin eight positively charged residues contribute to the charge cluster and anionic modulation of oxygen affinity is effective. Susceptibility to anionic modulation is decreased in hemoglobins where the charge cluster is less developed and is completely absent in some vertebrate hemoglobins. Anionic modulation, which occurs via an effect on the equilibrium between conformational states of high and low oxygen affinity is possible even in systems which do not show cooperative interactions in oxygen binding. This is well established by studies on isolated chains of human hemoglobin and by studies on enzymatically modified tetramers of Amphiuma hemoglobin.