Takahashi, K.; Lee, Y.; Nishizawa, T.; Tame, J. R. H.

The long-standing debate on the preferred conformation of liganded hemoglobin (Hb) in solution has yet to be completely resolved. While some studies have used lyophilized human hemoglobin for structural studies by cryo-EM, we recently presented the first cryo-EM analysis of freshly prepared human and crocodilian Hbs. Further three-dimensional (3D) classification analysis of these datasets reveals distinct structural characteristics. CO-bound adult human Hb (CO-HbA) shows a mixture of conformations, with the R2 conformation most populated, R strongly represented, and other intermediate states present in sufficient quantity to produce maps. CO-bound crocodile Hb showed the R conformation and, unexpectedly, a smaller population of molecules in a T-like conformation. The amino acid substitution Glu {beta}39 [->] Arg, unique to crocodilian Hbs, appears to favour the R conformation over R2.