Noncooperativity of the ac3 Dimer in the R (human/dissociation/equilibrium/sulfhydryl/absorption/

The theory that the af3 dimer is the func- tional 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 MgCa1 to human des-Arg 141a-hemoglobin. Cooperativity was ab- sent 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 141a would leave the structure of af3 dimers unchanged. Non- reactivity of the sulfhydryl groups at 112/3 proved that the subunits in deoxy dimers form the same contact as in oxy dimers, namely all, and that no significant dissociation into free subunits occurs in 0.9 M MgC12. The absorption spectrum of the deoxy dimers corresponded to the sum of the spectra of the free deoxy a and ,3 subunits, and was different from that of the deoxy tetramer, showing the con- straining 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 suburnits.