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A pH-dependent aquomet-to-hemichrome transition in crystalline horse methemoglobin.
Abstract: In 1947, Perutz and co-workers reported that crystalline horse methemoglobin undergoes a large lattice transition as the pH is decreased from 7.1 to 5.4. We have determined the pH 7.1 and 5.4 crystal structures of horse methemoglobin at 1.6 and 2.1 A resolution, respectively, and find that this lattice transition involves a 23 A translation of adjacent hemoglobin tetramers as well as changes in alpha heme ligation and the tertiary structure of the alpha subunits. Specifically, when the pH is lowered from 7.1 to 5.4, the Fe(3+) alpha heme groups (but not the beta heme groups) are converted from the aquomet form, in which the proximal histidine [His87(F8)alpha] and a water molecule are the axial heme ligands, to the hemichrome (bishistidine) form, in which the proximal histidine and the distal histidine [His58(E7)alpha] are the axial heme ligands. Hemichrome formation is coupled to a large tertiary structure transition in the eight-residue segment Pro44(CD2)alpha-Gly51(D7)alpha that converts from an extended loop structure at pH 7.1 to a pi-like helix at pH 5.4. The formation of the pi helix forces Phe46(CD4)alpha out of the alpha heme pocket and into the interface between adjacent hemoglobin tetramers where it participates in crystal lattice contacts unique to the pH 5.4 structure. In addition, the transition from aquomet alpha subunits to bishistidine alpha subunits is accompanied by an approximately 1.2 A movement of the alpha heme groups to a more solvent-exposed position as well as the creation of a solvent channel from the interior of the alpha heme pocket to the outside of the tetramer. These changes and the extensive rearrangement of the crystal lattice structure allow the alpha heme group of one tetramer to make direct contact with an alpha heme group on an adjacent tetramer. These results suggest possible functional roles for hemichrome formation in vivo.
Publication Date: 2003-08-27 PubMed ID: 12939139DOI: 10.1021/bi030059tGoogle Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Comparative Study
- Journal Article
- Research Support
- Non-U.S. Gov't
- Research Support
- U.S. Gov't
- P.H.S.
Summary
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The research paper discusses the pH-dependent change in the structure of horse methemoglobin from aquomet to hemichrome form when the pH is reduced from 7.1 to 5.4.
Research Method and Findings
The research was carried out based on an earlier study reported by Perutz and his team in 1947. This initial research showed that crystalline horse methemoglobin experienced significant changes in its lattice structure when the pH was reduced from 7.1 to 5.4. In the current research, the molecular structures at these two pH levels were determined – at a resolution of 1.6 and 2.1 A respectively – discovering that:
- The lattice transition involves a 23 A translation of adjacent hemoglobin tetramers, changes in alpha heme ligation, and alterations in the tertiary structure of the alpha subunits.
The researchers found that when the pH is lowered from 7.1 to 5.4:
- The Fe(3+) alpha heme groups, but not the beta heme groups, are converted from an aquomet form, where the axial heme ligands are the proximal histidine and water molecule, to a hemichrome form with the axial heme ligands being the proximal and distal histidine.
- Hemichrome creation is associated with a significant tertiary structure transition in the eight-residue segment that transitions from an extended loop structure at pH 7.1 to a pi-like helix at pH 5.4.
- The creation of the pi-helix forces Phe46(CD4)alpha out of the alpha heme pocket and into the interface amid adjacent hemoglobin tetramers, where it participates in crystal lattice contacts unique to the pH 5.4 structure.
- The transition from aquomet alpha subunits to bishistidine alpha subunits involves an approximately 1.2 A movement of the alpha heme groups to a more solvent-exposed position as well as the creation of a solvent channel from the interior of the alpha heme pocket to the outside of the tetramer.
Implication of Findings
The changes and the extensive rearrangement of the crystal lattice structure allow an alpha heme group of one tetramer to make direct contact with an alpha heme group on an adjacent tetramer. This suggests potential functional roles for hemichrome formation in natural biological processes.
Cite This Article
APA
Robinson VL, Smith BB, Arnone A.
(2003).
A pH-dependent aquomet-to-hemichrome transition in crystalline horse methemoglobin.
Biochemistry, 42(34), 10113-10125.
https://doi.org/10.1021/bi030059t Publication
Researcher Affiliations
- Department of Biochemistry, College of Medicine, The University of Iowa, Iowa City, Iowa 52242, USA.
MeSH Terms
- Animals
- Crystallography, X-Ray
- Dimerization
- Ferric Compounds / chemistry
- Heme / chemistry
- Hemeproteins / chemistry
- Histidine / chemistry
- Horses
- Hydrogen Bonding
- Hydrogen-Ion Concentration
- Ligands
- Methemoglobin / chemistry
- Models, Molecular
- Protein Structure, Quaternary
- Protein Structure, Tertiary
- Protein Subunits / chemistry
- Static Electricity
- Water / chemistry
Grant Funding
- GM-58890 / NIGMS NIH HHS
Citations
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- Lecomte JTJ. Hemoglobin: Some (Dis)Assembly Required. Biophys J 2020 Mar 24;118(6):1235-1237.
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- Ioannou A, Varotsis C. Modifications of hemoglobin and myoglobin by Maillard reaction products (MRPs). PLoS One 2017;12(11):e0188095.
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- Shibayama N. Effects of allosteric effectors on oxygen binding to crystals of hemoglobin in the R-quaternary structure. Protein Sci 2025 Jan;34(1):e5232.
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