A non-equilibrium thermodynamics model of reconstituted Ca2+-ATPase |
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| Authors: | A. R. Waldeck Karel van Dam Jan Berden P. W. Kuchel |
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| Affiliation: | (1) Department of Biochemistry, The University of Sydney, Sydney 2006, N.S.W., Australia (e-mail: P.Kuchel@biochem.usyd.edu.au), AU;(2) E.C. Slater Institute for Biochemical Research, The University of Amsterdam, Plantage Muidergracht 12, 1081 TV Amsterdam, The Netherlands, NL |
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| Abstract: | A non-equilibrium thermodynamics (NET) model describing the action of completely coupled or `slipping' reconstituted Ca2+-ATPase is presented. Variation of the coupling stoichiometries with the magnitude of the electrochemical gradients, as the ATPase hydrolyzes ATP, is an indication of molecular slip. However, the Ca2+ and H+ membrane-leak conductances may also be a function of their respective gradients. Such non-ohmic leak typically yields `flow-force' relationships that are similar to those that are obtained when the pump slips; hence, caution needs to be exercised when interpreting data of Ca2+-ATPase-mediated fluxes that display a non-linear dependence on the electrochemical proton (Δμ˜H) and/or calcium gradients (Δμ˜Ca). To address this issue, three experimentally verifiable relationships differentiating between membrane leak and enzymic slip were derived. First, by measuring Δμ˜H as a function of the rate of ATP hydrolysis by the enzyme. Second, by measuring the overall `efficiency' of the pump as a function of Δμ˜H. Third, by measuring the proton ejection rate by the pump as a function of its ATP hydrolysis rate. Received: 19 June 1997 / Accepted: 3 December 1997 |
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| Keywords: | Coupling stoichiometry Electrogenicity Slip Calcium pumping Proton ejection |
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