Fine Sampling of the R→T Quaternary-Structure Transition of a Tetrameric Hemoglobin.

Although the end points of the functional transitions of tetrameric hemoglobins (Hbs) have been well characterized, atomic-resolution data on R-T intermediate states are extremely limited. Herein, the X-ray structures of two independent tetramers of the fully ligated carbomonoxy form of Trematomus newnesi hemoglobin (Hb1Tn) within the same crystal are described. These structures show peculiar features in the heme pocket, EF corner, and tertiary/quaternary structure. Distal histidine side chains have a propensity to swing out of the heme pocket and thus allow compression of the EF corner. In this rotameric state, the distal His group does not interact with the CO ligand, consistent with FTIR spectra recorded in solution. At the quaternary-structure level, one tetramer is an intermediate R-T state, whereas the other assumes a T-like structure. Altogether, the structures of these tetramers provide the best available atomic-level picture of the R→T transition of vertebrate Hbs.