Figure 9.

Allosteric action in Scapharca dimeric hemoglobin HbI. (A) Difference electron density map corresponding to an early intermediate formed by 100ps inWT HbI is shown for subunit B (negative density is shown in red, positive in green). Notice the plume of density extending from the heme (orange) to both distal B and proximal F helices upon release of the ligand. Some key R-state water molecules at the dimer interface, like the two circled in blue, have been displaced. (B) Proposed mechanism for transmitting ligand-induced structural changes across the dimer interface. E–F helices are shown for the two subunits. When a ligand is dissociated from one subunit, the F helix gets pushed away from helix E by the out-of-heme-plane iron motion, observed already at 100ps. A clam-shell motion “hinge” in the middle of the E helix permits part of the E helix to move with the F helix as the E–F space opens. Due to the F–E’ “bolt” across the dimer interface, part of E’ helix moves along with the F helix, increasing the E’–F’ space in the second subunit. As the ligand in the second subunit gets released, the overall relative rotation of subunits completes the quaternary rotation in HbI during the R–T transition (Ren et al., 2012, see also movies in the supplemental information in the reference).