GSBS Student Publications

Title

The 2.0 A crystal structure of Scapharca tetrameric hemoglobin: cooperative dimers within an allosteric tetramer

UMMS Affiliation

Graduate School of Biomedical Sciences; Department of Biochemistry and Molecular Biology

Date

10-13-1995

Document Type

Article

Medical Subject Headings

Allosteric Site; Amino Acid Sequence; Animals; Bivalvia; Carbon Monoxide; Crystallography, X-Ray; Hemoglobins; Humans; Models, Molecular; Molecular Sequence Data; *Protein Conformation; Water

Disciplines

Life Sciences | Medicine and Health Sciences

Abstract

The crystal structure of the allosteric tetrameric hemoglobin from Scapharca inaequivalvis (HbII) has been determined in the carbonmonoxy liganded state using a combination of anomalous scattering and molecular replacement. The molecular model has been refined at 2.0 A resolution to a conventional R-factor of 0.173 and a free R-factor of 0.244. The tetramer is formed from two identical heterodimers. Each heterodimer is assembled with intersubunit contacts involving the E and F helices and heme groups in a manner that is very similar to that of the cooperative Scapharca homodimeric hemoglobin. In addition, the ordered water structure observed in these dimeric interfaces is quite similar. These structural similarities strongly suggest that the dimers within the Scapharca tetramer are cooperative. Subunits assemble into a tetramer in a distinctly non-tetrahedral arrangement, with the pseudo 2-fold axes of the heterodimer oriented at an angle of 74.5 degrees relative to the molecular 2-fold. This arrangement requires that two subunit types have distinct locations and contacts, despite the very similar tertiary structures. HbII polymerizes to higher-order assemblages in a ligand, proton and anion dependent fashion. The lattice contacts in the HbII-CO crystal suggest possible modes for this association.

Rights and Permissions

Citation: J Mol Biol. 1995 Oct 13;253(1):168-86.

Related Resources

Link to Article in PubMed

Journal Title

Journal of molecular biology

PubMed ID

7473710