does hemoglobin Ypsilanti represent an allosteric intermediate?
Recent crystallographic studies on the mutant human hemoglobin Ypsilanti (beta 99 Asp–>Tyr) have revealed a previously unknown quaternary structure called “quaternary Y” and suggested that the new structure may represent an important intermediate in the cooperative oxygenation pathway of normal hemoglobin. Here we measure the oxygenation and subunit assembly properties of hemoglobin Ypsilanti and five additional beta 99 mutants (Asp beta 99–>Val, Gly, Asn, Ala, His) to test for consistency between their energetics and those of the intermediate species of normal hemoglobin. Overall regulation of oxygen affinity in hemoglobin Ypsilanti is found to originate entirely from 2.6 kcal of quaternary enhancement, such that the tetramer oxygenation affinity is 85-fold higher than for binding to the dissociated dimers. Equal partitioning of this regulatory energy among the four tetrameric binding steps (0.65 kcal per oxygen) leads to a noncooperative isotherm with extremely high affinity (pmedi
