title: Structural basis for the antipolymer activity of Hb ζ2βs2trapped in a tense conformation.
abstract: The phenotypical severity of sickle-cell disease (SCD) can be mitigated by modifying mutant hemoglobin S (Hb S, Hb α2βs2) to contain embryonic ζ-globin in place of adult α-globin subunits (Hb ζ2βs2). Crystallographical analyses of liganded Hb ζζ2βs2, though, demonstrate a tense (T-state) quaternary structure that paradoxically predicts its participation in--rather than its exclusion from--pathological deoxyHb S polymers. We resolved this structure-function conundrum by examining the effects of α→ζ exchange on the characteristics of specific amino acids that mediate sickle polymer assembly. Superposition analyses of the βssubunits of T-state deoxyHb α2βs2and T-state CO-liganded Hb ζ2βs2reveal significant displacements of both mutant βsVal6 and conserved β-chain contact residues, predicting weakening of corresponding polymer-stabilizing interactions. Similar comparisons of the α- and ζ-globin subunits implicate four amino acids that are either repositioned or undergo non-conservative substitution, abrogating critical polymer contacts. CO-Hb ζ2βs2additionally exhibits a unique trimer-of-heterotetramers crystal packing that is sustained by novel intermolecular interactions involving the pathological βsVal6, contrasting sharply with the classical double-stranded packing of deoxyHb S. Finally, the unusually large buried solvent-accessible surface area for CO-Hb ζ2βs2suggests that it does not co-assemble with deoxyHb Sin vivo. In sum, the antipolymer activities of Hb ζ2βs2appear to arise from both repositioning and replacement of specific α- and βs-chain residues, favoring an alternate T-state solution structure that is excluded from pathological deoxyHb S polymers. These data account for the antipolymer activity of Hb ζ2βs2, and recommend the utility of SCD therapeutics that capitalize on α-globin exchange strategies.
authors.affiliation: Institute for Structural Biology and Drug Discovery, and the Department of Medicinal Chemistry, School of Pharmacy, Virginia Commonwealth University, Richmond, VA 23298; msafo@vcu.edu.
authors.forename: Martin K
authors.initials: MK
authors.lastname: Safo
authors.affiliation: Institute of Biological Chemistry, Academia Sinica, Taipei 11529, Taiwan; kotping@gate.sinica.edu.tw.
authors.forename: Tzu-Ping
authors.initials: TP
authors.lastname: Ko
authors.affiliation: Howard Hughes Medical Institute, Janelia Farm Research Campus, Ashburn, VA 20147; eric.schreiter@gmail.com.
authors.forename: Eric R
authors.initials: ER
authors.lastname: Schreiter
authors.affiliation: Division of Hematology-Oncology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA 19104; jeruss@mail.med.upenn.edu.
authors.forename: J Eric
authors.initials: JE
authors.lastname: Russell
keywords: Hemoglobin; Mutation; Polymerization; Sickle Cell Disease; Structure; Tense State