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Multiple structures and functions of cytochrome oxidase.

L Powers, B Chance

    Journal of Inorganic Biochemistry
    |March 1, 1985
    PubMed
    Summary

    Cytochrome c oxidase, a key respiratory enzyme, features a unique sulfur bridge in its resting oxidized state. This enzyme also exhibits peroxidase activity, potentially protecting its primary oxidase function.

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    Area of Science:

    • Biochemistry
    • Biophysics
    • Enzyme kinetics

    Background:

    • Cytochrome c oxidase is the terminal enzyme in the respiratory chain, crucial for cellular respiration.
    • Understanding its structure and function is vital for comprehending energy production.
    • Previous studies have investigated its redox centers but lacked detailed structural insights into functional intermediates.

    Purpose of the Study:

    • To investigate the structure of the four redox centers (2Fe, 2Cu) in cytochrome c oxidase.
    • To characterize the resting oxidized form and functional intermediates using X-ray absorption spectroscopy.
    • To elucidate the role of the active site structure in the enzyme's dual oxidase and peroxidase activities.

    Main Methods:

    • X-ray absorption studies, including Extended X-ray Absorption Fine Structure (EXAFS).
    • X-ray fluorescence detection optimized for low-concentration samples.
    • Low-temperature cryostats and simultaneous optical monitoring for sample integrity.
    • Comparison of iron and copper EXAFS data, and EXAFS of copper-depleted enzyme.

    Main Results:

    • The resting oxidized form exhibits a sulfur bridge between active site copper and iron (~3.8 A separation).
    • Reduced state shows CO/O2 bound to active site iron, distinct from the sulfur-bridged copper.
    • An oxidized form lacking the sulfur bridge displays peroxidase activity, forming stable intermediates with peroxides.
    • The active site copper does not participate in the peroxidatic reaction.

    Conclusions:

    • The sulfur-bridged active site is characteristic of the resting oxidized state and is isolable by specific methods.
    • The enzyme possesses a unique peroxidase activity, possibly a protective mechanism for its oxidase function.
    • Structural insights reveal distinct states of the active site during redox cycling and substrate interaction.

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