A non equilibrium thermodynamics model of reconstituted Ca2+-ATPase

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 (Delta<(mu)over tilde>(H)) and/or calcium gradients (Delta<(mu)over tilde>(Ca)). To address this issue, three experimentally verifiable relationships differentiating between membrane leak and enzymic slip were derived. First, by measuring Delta<(mu)over tilde>(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 Delta<(mu)over tilde>(H). Third, by measuring the proton ejection rate by the pump as a function of its ATP hydrolysis rate.