Structure of fully liganded Hb ζ₂β₂^s trapped in a tense conformation

A variant Hb ζ₂β₂^s that is formed from sickle hemoglobin (Hb S; α₂β₂^s) by exchanging adult α-globin with embryonic ζ-globin subunits shows promise as a therapeutic agent for sickle-cell disease (SCD). Hb ζ₂β₂^s inhibits the polymerization of deoxy­genated Hb S in vitro and reverses characteristic features of SCD in vivo in mouse models of the disorder. When compared with either Hb S or with normal human adult Hb A (α₂β₂), Hb ζ₂β₂^s exhibits atypical properties that include a high oxygen affinity, reduced cooperativity, a weak Bohr effect and blunted 2,3-­diphosphoglycerate allostery. Here, the 1.95 Å resolution crystal structure of human Hb ζ₂β₂^s that was expressed in complex transgenic knockout mice and purified from their erythrocytes is presented. When fully liganded with carbon monoxide, Hb ζ₂β₂^s displays a central water cavity, a ζ1–β^s2 (or ζ2–β^s1) interface, intersubunit salt-bridge/hydrogen-bond interactions, C-terminal βHis146 salt-bridge interactions, and a β-cleft, that are highly unusual for a relaxed hemoglobin structure and are more typical of a tense conformation. These quaternary tense-like features contrast with the tertiary relaxed-like conformations of the ζ1β^s1 dimer and the CD and FG corners, as well as the overall structures of the heme cavities. This crystallographic study provides insights into the altered oxygen-transport properties of Hb ζ₂β₂^s and, moreover, decouples tertiary- and quaternary-structural events that are critical to Hb ligand binding and allosteric function.