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Related Experiment Videos

Structure and stability of human thyroxine-binding globulin.

M C Gershengorn, R E Lippoldt, H Edelhoch

    The Journal of Biological Chemistry
    |December 10, 1977
    PubMed
    Summary
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    Human plasma thyroxine-binding globulin (TBG) is a compact molecule whose structure is altered by thyroxine binding. Changes in TBG structure at low pH can irreversibly affect hormone binding.

    Area of Science:

    • Biochemistry
    • Molecular Biology
    • Structural Biology

    Background:

    • Thyroxine-binding globulin (TBG) is the primary carrier protein for thyroid hormones in human plasma.
    • Understanding TBG's structure is crucial for comprehending thyroid hormone transport and regulation.

    Purpose of the Study:

    • To investigate the secondary and tertiary structure of human plasma TBG.
    • To determine the effects of thyroxine binding and pH on TBG structure and function.

    Main Methods:

    • Circular dichroism (CD) spectroscopy (far and near UV) was employed.
    • Fluorescence properties, specifically tryptophanyl fluorescence, were analyzed.
    • Molecular stability was assessed at varying temperatures and pH levels.

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    Main Results:

    • TBG exhibits a compact, symmetric molecular structure with a mix of alpha-helical and beta-sheet secondary structures.
    • Thyroxine binding induced modifications in the near-UV CD spectrum, indicating tertiary structure changes.
    • TBG demonstrated stability below 50°C at pH 9 and below 35°C at pH 10.5.
    • A pH-dependent molecular transition below pH 5 led to irreversible loss of hormone binding, involving a conversion of alpha-helical to beta-sheet structures.

    Conclusions:

    • Human plasma TBG possesses a defined secondary and tertiary structure that is sensitive to ligand binding and pH.
    • The structural integrity of TBG is essential for its hormone-binding capacity, with low pH conditions causing irreversible functional impairment.