Absence of cooperative energy at the heme in liganded hemoglobins

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Abstract
Using resonance Raman and IR absorption spectroscopies, there are no energetically significant structural changes at the heme upon the quaternary structure transition in [carp] 6-coordinate Hb. These observations are at variance with the presently accepted mechanism for cooperativity, which postulates severe strain in the T quaternary structure of liganded Hb. By consideration of the present results, and studies on deoxyhemoglobins and photodissociated Hb, a view of the distribution of the free energy of cooperativity emerges. In 5-coordinate deoxyhemoglobins the Fe histidine bond is able to respond to the protein structure, thereby accounting for a wide variation (40 cm-1) in its frequency. When a 6th ligand is present and the Fe is pulled into plane, the histidine-heme-ligand complex becomes structurally rigid, thereby preventing protein-induced changes at the heme. Instead, in liganded Hb 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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