Three-state combinatorial switching in hemoglobin tetramers: comparison between functional energetics and molecular structures.

In a previous study on cyanomethemoglobin the 10 tetrameric species (each with a unique combination of ligated and unligated subunits) were found to exhibit three distinct free energies of cooperative interaction. The distribution of these free energies among the partially ligated species is incompatible with a two-state mechanism of molecular switching and requires a minimum of three molecular structures with distinctly different free energies of heme-heme interaction. Ligand-linked transitions between the three cooperativity states were found to be "combinatorial"--i.e., dependent upon changes in both the number and specific configuration of bound ligands. Here we present results from two other chemical systems that mimic intermediate oxygenation states. In these systems the heme iron is replaced by manganese in certain of the subunits. We find the same distribution of cooperative free energies as reported for the cyanomethemoglobin system. These results demonstrate that the three-state combinatorial nature of cooperative switching is neither a special feature of the cyanomet reactions nor of the substitution of manganese for iron, but reflects a fundamental property of hemoglobin. These findings are compared with crystallographic structural results on partially ligated hemoglobins.