The Journal of Membrane Biology

, Volume 74, Issue 2, pp 139–153

Electrical and biochemical properties of an enzyme model of the sodium pump

  • J. Brian Chapman
  • Edward A. Johnson
  • J. Mailen Kootsey
Articles

DOI: 10.1007/BF01870503

Cite this article as:
Chapman, J.B., Johnson, E.A. & Kootsey, J.M. J. Membrain Biol. (1983) 74: 139. doi:10.1007/BF01870503

Summary

The electrochemical properties of a widely accepted six-step reaction scheme for the Na+, K+-ATPase have been studied by computer simulation. Rate coefficients were chosen to fit the nonvectorial biochemical data for the isolated enzyme and a current-voltage (I–V) relation consistent with physiological observations was obtained with voltage dependence restricted to one (but not both) of the two translocational steps. The vectorial properties resulting from these choices were consistent with physiological activation of the electrogenic sodium pump by intracellular and extracellular sodium (Na+) and potassium (K+) ions. The model exhibited K+/K+ exchange but little Na+/Na+ exchange unless the energy available from the splitting of adenosine triphosphate (ATP) was reduced, mimicking the behavior seen in squid giant axon. The vectorial ionic activation curves were voltage dependent, resulting in large shifts in apparent Km's with depolarization. At potentials more negative than the equilibrium or reversal potential transport was greatly diminished unless the free energy of ATP splitting was reduced. While the pump reversal potential is at least 100 mV hyperpolarized relative to the resting potential of most cells, the voltage-dependent distribution of intermediate forms of the enzyme allows the possibility of considerable slope conductance of the pumpI–V relation in the physiological range of membrane potentials. Some of the vectorial properties of an electrogenic sodium pump appear to be inescapable consequences of the nonvectorial properties of the isolated enzyme. Future application of this approach should allow rigorous quantitative testing of interpretative ideas concerning the mechanism and stoichiometry of the sodium pump.

Key Words

Na+, K+-ATPase sodium pump electrogenic computer simulation enzyme kinetics thermodynamics 

Copyright information

© Springer-Verlag New York Inc. 1983

Authors and Affiliations

  • J. Brian Chapman
    • 1
    • 2
  • Edward A. Johnson
    • 1
    • 2
  • J. Mailen Kootsey
    • 1
    • 2
  1. 1.Departments of PhysiologyMonash UniversityClaytonAustralia
  2. 2.Duke University Medical CenterDurham

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