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Structure-function relations in hemoglobin as determined by x-ray absorption spectroscopy.

P Eisenberger, R G Shulman, G S Brown

    Proceedings of the National Academy of Sciences of the United States of America
    |February 1, 1976
    PubMed
    Summary
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    Fluorescent X-ray absorption studies reveal minimal structural changes around the iron atom in ligated hemoglobins. These findings indicate that the increased oxygen affinity in hemoglobin variants is not due to localized strain at the heme iron.

    Area of Science:

    • Biophysics
    • Structural Biology
    • Biochemistry

    Background:

    • Hemoglobin's oxygen affinity is crucial for respiration.
    • Understanding structural changes in hemoglobin is key to explaining its function.
    • Previous studies suggested localized structural changes at the heme iron could influence oxygen binding.

    Purpose of the Study:

    • To determine the precise structural differences around the iron atom in oxy- and carbonmonoxyhemoglobin.
    • To investigate the role of strain energy at the heme iron in hemoglobin's varying oxygen affinity.
    • To compare structural parameters of ligated and unligated hemoglobin states.

    Main Methods:

    • Fluorescent X-ray absorption spectroscopy was employed.
    • A bis-imidazole heme complex with known structure served as a model.

    Related Experiment Videos

  • Spectra of oxyhemoglobin, carbonmonoxyhemoglobin, and cytochrome c were analyzed and compared.

    • Main Results:

    • Average iron-ligand bond distances were determined: 1.99 Å in oxyhemoglobin, 1.98 Å in carbonmonoxyhemoglobin, and 1.98 Å in cytochrome c (pH < 10.5).
    • Spectral changes between oxy- and deoxyhemoglobin were significantly larger than among ligated forms.
    • No significant structural differences (≤0.02 Å) were found between high and low oxygen affinity deoxyhemoglobin variants.

    Conclusions:

    • The strain energy at the iron atom in hemoglobin is negligible (≤4 x 10^-3 eV).
    • The significant difference in binding energy between high and low affinity forms (0.15 eV) is not localized at the heme iron.
    • Oxygen affinity modulation in hemoglobin is primarily governed by non-heme-localized factors.