A non-equilibrium thermodynamics model of reconstituted Ca(2+)-ATPase.
A non-equilibrium thermodynamics (NET) model describing the action of completely coupled or 'slipping' reconstituted Ca(2+)-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 Ca(2+)-ATPase-mediated fluxes that display a non-linear dependence on the electrochemical proton (delta mu H) and/or calcium gradients (delta mu Ca). To address this issue, three experimentally verifiable relationships differentiating between membrane leak and enzymic slip were derived. First, by measuring delta mu 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 H. Third, by measuring the proton ejection rate by the pump as a function of its ATP hydrolysis rate.
Waldeck
A R
AR
Department of Biochemistry, University of Sydney, N.S.W., Australia.
van Dam
K
K
Berden
J
J
Kuchel
P W
PW
D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000378:metabolism; D002462:Cell Membrane / Q000201:enzymology; D008081:Liposomes; D008432:Mathematical Computing; D008954:Models, Biological; D008956:Models, Chemical; D012519:Sarcoplasmic Reticulum / Q000201:enzymology; D013816:Thermodynamics
D008081:Liposomes; D000252:Calcium-Transporting ATPases