The Monod-Wyman-Changeux allosteric model describes haemoglobin oxygenation with only one adjustable parameter

The Monod-Wyman-Changeux allosteric model describes the oxygen equilibrium of haemoglobin in terms of three parameters: the oxygen association constant for the T state, KT, that for the R state, KR, and the allosteric constant, L0, where Li = [Ti]/[Ri] and Ti and Ri are the T and R states, respectively, combined with i oxygen molecules (i = 0 to 4). This model predicts that heterotropic effects exerted by various non-haem ligands are due exclusively to displacement of the allosteric equilibrium (T0 in equilibrium R0) and neither the KT nor the KR values depend upon the concentration of non-haem ligands (solution conditions), whereas the experimental data indicated that not only L0, but also KT, varies significantly with changes in solution conditions. On the basis of accurate oxygen equilibrium data, which were obtained recently under a variety of solution conditions, I examined correlations between the Monod-Wyman-Changeux parameters and found the following: as long as the constraints imposed on the haemoglobin molecule are not very strong, both KR and L4 are essentially constant and, further, log L0 + 4 log KT = log L4 + 4 log KR is constant, irrespective of solution conditions. As a consequence, the haemoglobin oxygenation can virtually be described in terms of only one parameter, either KT or L0. Implications of this phenomenon are also described.