Book of Abstracts: Albany 2003
June 17-21 2003
Spectroscopic Contributions to the Understanding of Hemoglobin Function: Implications for Structural Biology
Structural biology is based on the assumption that structural determinations will explain macromolecular function. One basis of these proposals has been the oft-quoted structure/functional connections in hemoglobin. Presently the Monod, Wyman, Changeux (MWC) model of hemoglobin function has great validity. In this model, ligand-binding affinities are linked to quaternary structure, and it has been shown that the model describes the function accurately to a high first approximation (1-5). To see how this understanding developed and the roles played by structural, spectroscopic and energetic results, we review two sets of experimental studies in 1970-71 that supported the applicability of MWC to hemoglobin. One set from NMR and ligand binding energetics, supported the quaternary-linked nature of binding required by the MWC model (1, 2, 4). In these studies the quaternary structures of half ligated Hb was switched by changing solution conditions and ligand affinity followed quaternary structure while the number of bound ligands remained constant. The other approach, by Perutz, (5) proposed a structural basis for MWC, by suggesting that in the deoxygenated quaternary structure the binding of oxygen broke a salt bridge that caused a lowered quaternary-linked affinity. However, experiments since that time, mostly by X-ray crystallography of hemoglobin in the deoxygenated quaternary structure have failed to show salt bridges breaking upon ligation, whereas affinities have remained low (6). This pattern of results shows that the small energies responsible for ligand-binding affinities and reaction rates have not been identified by discrete structural features. Rather, thermodynamic and energetic data from a variety of spectroscopic studies have played the central role in establishing the MWC model for hemoglobin. X-ray determinations of the quaternary structures continue to supply valuable correlations between those structures and affinity. However, the structural basis of the low affinity in the deoxy quaternary structure has not been determined despite many suggestions, presumably because the energies responsible are delocalized.
Robert G. Shulman
Department of Diagnostic Radiology