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Allosteric interactions in sipunculid and brachiopod hemerythrins.

D E Richardson, M Emad, R C Reem

    Biochemistry
    |February 24, 1987
    PubMed
    Summary

    Hemerythrins from sipunculids and brachiopods show different oxygen binding behaviors. Lingula reevii hemerythrin exhibits cooperativity and a Bohr effect, unlike Phascolopsis gouldii hemerythrin.

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    Area of Science:

    • Biochemistry
    • Structural Biology
    • Biophysics

    Background:

    • Hemerythrins (Hrs) are non-heme iron oxygen-binding proteins found in certain marine invertebrates.
    • Allosteric interactions play a crucial role in modulating ligand binding affinity and cooperativity in oxygen carriers.
    • Understanding these interactions is key to deciphering the functional diversity of respiratory proteins.

    Purpose of the Study:

    • To investigate the chemical and spectroscopic consequences of allosteric interactions in sipunculid (Phascolopsis gouldii) and brachiopod (Lingula reevii) hemerythrins.
    • To compare the oxygen binding properties and allosteric mechanisms of these two distinct hemerythrin types.
    • To elucidate the structural basis for differences in oxygen affinity and cooperativity.

    Main Methods:

    • Ligand binding assays to determine oxygen affinity and cooperativity.
    • Spectroscopic techniques, including Resonance Raman spectroscopy, to probe active site structure and conformational changes.
    • Analysis of oxygen equilibrium curves and fitting to conformational models.

    Main Results:

    • Phascolopsis gouldii hemerythrin showed reduced ligand affinity without cooperativity, while Lingula reevii hemerythrin exhibited cooperative O2 binding and a Bohr effect between pH 7-8.
    • Spectroscopic analysis revealed no significant differences in active site structures between the two hemerythrins.
    • Resonance Raman spectra indicated pH-dependent conformational changes in Lingula reevii oxyhemerythrin, linked to quaternary structural shifts, but not in Phascolopsis gouldii hemerythrin.
    • Ligand binding to deoxy and met forms was noncooperative for both hemerythrin types, contrasting with hemoglobins.

    Conclusions:

    • The observed differences in oxygen binding modulation are attributed to allosteric effects linking the active site to quaternary structural changes in the octameric hemerythrin structure.
    • A conformational model involving T (tensed), R (relaxed), and R-T hybrid states effectively describes oxygen equilibria.
    • Conformational changes are coupled to the oxidation state change of the binuclear iron site during oxygenation, with an allosteric energy of ~1.4 kcal/mol.
    • The findings provide insights into the mechanism of cooperativity in hemerythrins and offer a comparative perspective with hemocyanins.

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