Mutations of the betaN102 residue of HbA not only inhibit the ligand-linked T to Re state transition, but also profoundly affect the properties of the T state itself.

The properties of three HbA variants with different mutations at the beta 102 position, beta N102Q, beta N102T, and beta N102A, have been examined. All three are inhibited in their ligand-linked transition from the low affinity T quaternary state to the high affinity R-e quaternary state. In the presence of inositol hexaphosphate, IHP, none of them exhibits cooperativity in the binding of oxygen. This is consistent with the destabilization of the R-e state as a result of the disruption of the hydrogen bond that normally forms between the beta 102 asparagine residue and the alpha 94 aspartate residue in the R-e state. However, these three substitutions also alter the properties of the T state of the hemoglobin tetramer. In the presence of IHP, the first two substitutions result in large increases in the ligand affinities of the beta-subunits within the T state structure. The beta N102A variant, however, greatly reduces the pH dependencies of the affinities of the alpha and beta subunits, K-1(alpha) and K-1(beta), respectively, for the binding of the first oxygen molecule in the absence of IHP. In the presence of IHP, the T state of this variant is strikingly similar to that of HbA under the same conditions. For both hemoglobins, K-1(alpha) and K-1(beta) exhibit only small Bohr effects. In the absence of IHP, the affinities of the alpha and beta subunits of HbA for the first oxygen are increased, and both exhibit greatly increased Bohr effects. However, in contrast to the behavior of HbA, the ligand-binding properties of the T state tetramer of the beta N102A variant are little affected by the addition or removal of IHP. It appears that along with its effect on the stability of the liganded R-e state, this mutation has an effect on the T state that mimics the effect of adding IHP to HbA. It inhibits the set of conformational changes, which are coupled to the K-1 Bohr effects and normally accompany the binding of the first ligand to the HbA tetramer in the absence of organic phosphates.