The stereochemical mechanism of the cooperative effects in hemoglobin revisited

In 1970, Perutz tried to put the allosteric mechanism of hemoglobin, proposed by Monod, Wyman and Changeux in 1965, on a stereochemical basis. He interpreted their two-state model in terms of an equilibrium between two alternative structures, a tense one (T) with low oxygen affinity, constrained by salt-bridges between the C-termini of the four subunits, and a relaxed one (R) lacking these bridges. The equilibrium was thought to be governed primarily by the positions of the iron atoms relative to the porphyrin: out-of-plane in five-coordinated, high-spin deoxyhemoglobin, and in-plane in six-coordinated, low-spin oxyhemoglobin. The tension exercised by the salt-bridges in the IT-structure was to be transmitted to the heme-linked histidines and to restrain the movement of the iron atoms into the porphyrin plane that is necessary for oxygen binding. At the B-hemes, the distal valine and histidine block the oxygen-combining site in the T-structure; its tension was thought to strengthen that blockage. Finally, Perutz attributed the linearity of proton release with early oxygen uptake to the sequential rupture of salt-bridges in the T-structure and to the accompanying drop in pKa of the weak bases that form part of them. Almost every feature of this mechanism has been disputed, but evidence that has come to light more than 25 years later now shows it to have been substantially correct. That new evidence is reviewed below.