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Related Experiment Videos

Substrate sites for the (Na+ + K+)-dependent ATPase.

J D Robinson

    Biochimica Et Biophysica Acta
    |May 13, 1976
    PubMed
    Summary

    Rat brain sodium-potassium ATPase exhibits distinct high- and low-affinity ATP binding sites. These sites display different kinetic properties, suggesting varied roles in enzymatic and transport functions.

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    Area of Science:

    • Biochemistry
    • Enzymology
    • Neuroscience

    Background:

    • The (Na+ + K+)-dependent ATPase (EC 3.6.1.3) is crucial for maintaining ion gradients in neuronal cells.
    • Understanding the kinetics of ATP binding is essential for elucidating enzyme mechanisms and function.

    Purpose of the Study:

    • To investigate the kinetic properties of ATP binding to rat brain (Na+ + K+)-dependent ATPase.
    • To characterize the distinct affinities and thermodynamic parameters of ATP interaction sites.

    Main Methods:

    • Kinetic studies using varying ATP concentrations to determine Michaelis constants (Km).
    • Photo-oxidation assays with methylene blue to assess ATP dissociation constants (Kd).
    • Analysis of temperature dependence to calculate entropy of binding.
    • Competitive inhibition studies with ATP analogs and phosphatase substrates.

    Main Results:

    • Identified two classes of ATP binding sites: high-affinity (Km ≈ 1 µM) and low-affinity (Km ≈ 0.5 mM).
    • Dissociation constant for low-affinity sites determined to be approximately 0.4 mM.
    • Significant differences in temperature dependence and entropy of binding were observed between the two sites (+27 cal/degree/mol for high-affinity, -20 cal/degree/mol for low-affinity).
    • ATP congeners and phosphatase substrates competed at the low-affinity sites, indicating cross-talk between ATPase and phosphatase activities.

    Conclusions:

    • The rat brain (Na+ + K+)-dependent ATPase possesses two distinct classes of ATP substrate sites.
    • These sites exhibit differential kinetic and thermodynamic properties, suggesting specialized roles.
    • A model involving two substrate sites on a dimeric enzyme is proposed to explain the observed diverse characteristics.

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