Structure of fully liganded Hb ζ2β2strapped in a tense conformation

Abstract

A variant Hb ζ2β2sthat is formed from sickle hemoglobin (Hb S; α2β2s) by exchanging adult α-globin with embryonic ζ-globin subunits shows promise as a therapeutic agent for sickle-cell disease (SCD). Hb ζ2β2sinhibits the polymerization of deoxygenated Hb Sin vitroand reverses characteristic features of SCDin vivoin mouse models of the disorder. When compared with either Hb S or with normal human adult Hb A (α2β2), Hb ζ2β2sexhibits 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 ζ2β2sthat was expressed in complex transgenic knockout mice and purified from their erythrocytes is presented. When fully liganded with carbon monoxide, Hb ζ2β2sdisplays a central water cavity, a ζ1–βs2 (or ζ2–βs1) 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 ζ1βs1 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 ζ2β2sand, moreover, decouples tertiary- and quaternary-structural events that are critical to Hb ligand binding and allosteric function.