The Ligand-binding Properties of Hemoglobin Hiroshima (α2β2146 asp)

Hemoglobin Hiroshima is a β chain variant of human hemoglobin in which aspartate is substituted for histidine at position 146. From chemical and crystallographic studies, it has been postulated that the histidine normally found at this position stabilizes the deoxy conformation by both inter- and intra-chain salt linkages and releases Bohr protons during ligand binding. Previous equilibrium studies had shown that hemoglobin Hiroshima does exhibit an increased oxygen affinity and a reduced alkaline Bohr effect. Kinetic studies of the carbon monoxide and n-butyl isocyanide binding reaction show that deoxyhemoglobin Hiroshima in low ionic strength buffers exhibits behavior intermediate between that of the isolated chains and that of deoxyhemoglobin A at neutral pH. At pH 7.0, the time course of both of these reactions exhibited little or no wave length dependence, suggesting that the α and β chains within deoxyhemoglobin Hiroshima react with ligands at similar apparent association rates. This result is in contrast to previous studies both with isolated chains and with deoxyhemoglobin A at this pH. However, similar behavior has been observed for deoxyhemoglobin A at pH 9.1 and des(his 146 β)deoxyhemoglobin at pH 7.0. Further analysis of the available kinetic and spectral data indicates that the intermediate ligand binding behavior of deoxyhemoglobin Hiroshima is not a result of an equilibrium distribution of deoxy A-like and liganded A-like conformations, but rather represents the properties of a new and distinct protein conformation which results from the disruption of the salt linkages at the COOH terminus of the β chains by either deprotonating, physically removing, or substituting aspartate for histidine 146 β.