The 1.8 Å structure of carbonmonoxy-β4 hemoglobin: Analysis of a homotetramer with the R quaternary structure of liganded α2β2 hemoglobin

Gloria E.O. Borgstahl, Paul H. Rogers, Arthur Arnone

Research output: Contribution to journalArticle

51 Scopus citations

Abstract

The β-chains isolated from the human hemoglobin α2β2 heterotetramer self-assemble to form a β4 homotetramer. We report the structure of the carbonmonoxy-β4 (COβ4) tetramer refined at a resolution of 1.8 Å. Compared to the three known quaternary structures of human hemoglobin, the T state, the R state and the R2 state, the quaternary structure of COβ4 most closely resembles the R state. While the degree of structural similarity between COβ4 and the R state of liganded α2β2 quite high, differences between the α and β-chain sequences result in interesting alternative packing arrangements at the subunit interfaces of COβ4. In particular, Arg40β and Asp99β interact across the COβ4 equivalent of the α1β2 interface to form two symmetry-related salt bridges that have no counterpart in either liganded or deoxyhemoglobin. Because these salt bridges are near a 2-fold symmetry axis, steric constraints prevent their simultaneous formation, and electron density images of Arg40β and Asp99β show equally populated dual conformations for the side-chains of both residues. Relative to the liganded α2β2 tetramer, the Arg40β ⋯ Asp99β salt bridges introduce ionic interactions that should strengthen the COβ4 tetramer. The COβ4 equivalent of the α1α2 and β1β2 interfaces strengthens the tetramer relative to the liganded α2β2 tetramer by tethering both ends of the central cavity. (The entrance to the central cavity is altered so that the N termini move closer together and the C termini further apart, forming an anion binding pocket that is absent in liganded α2β2 hemoglobin.) In contrast, analysis of the COβ4 counterpart of the α1β1 interface indicates that this interface is weakened in the COβ4 tetramer. These differences in interface stability provide a structural explanation for the published observation that the α2β2 tetramer assembles via a stable α1β1 dimer intermediate, whereas assembly of the COβ4 tetramer is characterized more accurately by a monomer-tetramer equilibrium.

Original languageEnglish (US)
Pages (from-to)817-830
Number of pages14
JournalJournal of Molecular Biology
Volume236
Issue number3
DOIs
StatePublished - 1994

Keywords

  • Conformational heterogeneity
  • Hemoglobin
  • Quaternary structure
  • R state
  • X-ray crystal structure

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology

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