Noncooperativity of the αβ Dimer in the Reaction of Hemoglobin with Oxygen

The theory that the alphabeta dimer is the functional unit of cooperativity in hemoglobin has been tested by determination of the oxygen equilibrium curve of stable deoxy dimers, obtained by the addition of 0.9 M MgCl(2) to human des-Arg 141alpha-hemoglobin. Cooperativity was absent in this medium, but was regained on transfer of the hemoglobin to a dilute phosphate buffer, where tetramers reformed. X-ray analysis of crystals of oxy- and deoxy-des-Arg hemoglobins showed that the removal of Arg 141alpha would leave the structure of alphabeta dimers unchanged. Nonreactivity of the sulfhydryl groups at 112beta proved that the subunits in deoxy dimers form the same contact as in oxy dimers, namely alpha(1)beta(1), and that no significant dissociation into free subunits occurs in 0.9 M MgCl(2). The absorption spectrum of the deoxy dimers corresponded to the sum of the spectra of the free deoxy alpha and beta subunits, and was different from that of the deoxy tetramer, showing the constraining salt bridges formed by the C-terminal residues in the tetramer to be necessary for the spectral changes normally observed on association of the deoxy subunits.