Abstact

The crystal structure of the deoxygenated form of the human hemoglobin beta 4 tetramer (deoxy beta 4) has been determined and refined at a resolution of 1.9 A. A detailed comparison of the quaternary structures of carbonmonoxy-beta 4 (CO beta 4) and deoxy beta 4 shows that ligand binding to the beta 4 tetramer produces only slight movements of the subunits relative to each other. Therefore, unlike the hemoglobin alpha 2 beta 2 tetramer, where the transition from an unliganded T state tetramer to a liganded R state tetramer results in a large change in quaternary structure, beta 4 is locked in a quaternary structure that very closely resembles the R state. By comparing the high-resolution structures of T state deoxy alpha 2 beta 2, R state deoxy beta 4 and R state CO beta 4, it is possible to partition the changes in beta subunit tertiary structure into those that arise from changes in quaternary structure and those that result solely from ligand binding. Specifically, when viewed from the heme reference frame, comparison of the structures of T state deoxy alpha 2 beta 2 and R state deoxy beta 4 shows that the T-to-R quaternary structure transition induces changes in beta subunit tertiary structure that are approximately halfway toward the tertiary structure observed in liganded beta 4 and liganded alpha 2 beta 2. When viewed from the reference frame of the globin backbone atoms, the T-to-R quaternary structure transition induces a small rotation of the heme group and a shift of the "allosteric core" (the end of the F helix, the FG corner, the beginning of the G helix, and the heme group) away from the E helix. These movements open the ligand binding pocket and place the heme in a more symmetric position relative to the proximal histidine residue. Together, these effects work in unison to give the subunits of deoxy beta 4 a tertiary structure that has high ligand affinity.