Using resonance Raman and infrared absorption spectroscopies, we show that there are no energetically significant structural changes at the heme upon the quaternary structure transition in six-coordinate hemoglobins. These observations are at variance with the presently accepted mechanism for cooperativity, which postulates severe strain in the T quaternary structure of liganded hemoglobin. By consideration of the present results, and studies on deoxyhemoglobins and photodissociated hemoglobins, a view of the distribution of the free energy of cooperativity emerges. In five-coordinate deoxyhemoglobins the iron-histidine bond is able to respond to the protein structure, thereby accounting for a wide variation (40 cm-1) in its frequency. In contrast, when a sixth ligand is present and the iron is pulled into plane, the histidine-heme-ligand complex becomes structurally rigid, thereby preventing protein-induced changes at the heme. Instead, in liganded hemoglobin the changes in structure that occur at the subunit interface upon the quaternary structure transition are accommodated away from the heme by relatively weak bonds in the protein.
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