Spectroscopic contributions to the understanding of hemoglobin function: Implications for structural biology

Structural biology is based on the assumption that structural determination s will explain macromolecular function. To examine the basis of these propo sals, the structure/function connections in hemoglobin have been examined. Presently the Monod, Wyman, Changeux (MWC) model of hemoglobin function has great validity. In this model, ligand-binding affinities are linked to qua ternary structure, and it has been shown that the model describes the funct ion accurately to a high first approximation. To see how this understanding developed, we review two sets of experimental studies in 1970-71 that supp orted the applicability of MWC to hemoglobin oxygen binding. One set of dat a from NMR and ligan binding kinetics supported the quaternary-linked natur e of binding required by the MWC model. The other approach, by Perutz, prop osed a structural basis for MWC, by suggesting that in one quaternary struc ture the binding of oxygen broke a salt bridge that caused a lowered quater nary-linked affinity. However, experiments since that time, mostly by X-ray crystallography of deoxygenated hemoglobin, have failed to show salt bridg es breaking upon ligation, whereas affinities have remained low. This patte rn of results shows that the small energies responsible for ligand-binding affinities and reaction rates have not been identified by discrete structur al features. Rather, thermodynamic and kinetic data from a variety of spect roscopic studies have played the central role in establishing the MWC model for hemoglobin.