2,3-DPG-Hb complex: a hypothesis for an asymmetric binding

This study was undertaken to test the symmetry of 2,3-diphosphoglycerate (2 ,3-DPG) binding site in hemoglobin (Hb). From Arnone's study [A. Arnone, Na ture (London) 237 (1972) 146] the 2,3-DPG binding site is located at the to p of the cavity, that runs through the center of the deoxy-Hb molecule. How ever, it is possible that this symmetry reported by Arnone, for crystals of 2,3-DPG-Hb complex, might not be conserved in solution. In this paper, we report the P-31 nuclear magnetic resonances of the 2,3-DPG interaction with Hb. The 2,3-DPG chemical shifts of the P-2 and P-3 resonance are both pH- and hemoglobin-dependent [protein from man, polar bear (Ursus maritimus), A rctic fox (Alopex lagopus) and bovine]. 2,3-DPG binds tightly to deoxyhemog lobin and weakly, nevertheless significantly, to oxyhemoglobin. In particul ar, our results suggest similar spatial position of the binding site of 2,3 -DPG in both forms of Hb in solutions. However, the most unexpected result was the apparent loss of symmetry in the binding site, which might correlat e with the ability of the hemoglobin to modulate its functional behavior. T he different interactions of the phosphate groups indicate small difference s in the quaternary structure of the different deoxy forms of hemoglobin. G iven the above structural perturbation an asymmetric binding in the complex could justify, at least in part, different physiological properties of Hb. Regardless, functionally relevant effects of 2,3-DPG seem to be measured a nd best elucidated through solution studies. (C) 2000 Elsevier Science B.V. All rights reserved.