Allosteric free energy changes at the alpha(1)beta(2) interface of human hemoglobin probed by proton exchange of Trp beta 37

The energetic changes that occur on ligand binding in human hemoglobin have been investigated by measurements of the exchange rates of the indole prot on of Trp beta 37(C3). The Trp beta 37 residues are located in helices C of the beta -subunits and are involved in contacts with the segments FG of th e alpha -subunits at the interdimeric alpha (1)beta (2) and alpha (2)beta ( 1) interfaces of the hemoglobin tetramer, In the quaternary structure chang e that accompanies ligand binding to hemoglobin, these contacts undergo min imal changes in relative orientation and in packing, thereby acting as hing es, or flexible joints. The exchange rates of the indole proton of Trp beta 37(CS) were measured by nuclear magnetic resonance spectroscopy, in both d eoxygenated and ligated hemoglobin. The results indicate that, at 15 degree sC, the exchange rate is increased from 9.0 10(-6) to 3.3 10(-4) s(-1) upon ligand binding to hemoglobin, This change suggests that the structural uni ts at the hinge regions of the alpha (1)beta (2)/alpha (2)beta (1) interfac es containing Trp beta 37(C3) are specifically stabilized in unligated hemo globin, and experience a change in structural free energy of similar to4 kc al/(mol tetramer) upon ligand binding. Therefore, the hinge regions of the alpha (1)beta (2)/alpha (2)beta (1) interfaces could play a role in the tra nsmission of free energy through the hemoglobin molecule during its alloste ric transition. (C) 2001 Wiley-Liss, Inc.