title: The thermodynamic efficiency of the Ca2+-Mg2+-ATPase is one hundred percent.
abstract: The thermodynamic efficiency of the Ca2+ -Mg2+ -ATPase of skeletal sarcoplasmic reticulum has been evaluated by comparing the Ca2+ gradient established with the ATP/(ADP*Pi) ratio. The evaluation was made at an external Ca2+ level (4.7 X 10(-8) M) which is below the Km value of 7 X 10(-8) M. The Mg-ATP and phosphate concentrations were held constant (0.1 mM) and the ADP concentration was varied. Maximal uptake to an internal free Ca2+ concentration of 17 mM was observed at infinite ATP/(ADP*Pi) ratio (absence of ADP). This corresponds to a [Ca2+] i/[Ca2+] 0 gradient of 3.6 X 10(5). A Ca2+ gradient one-half as large was observed at an ATP/(ADP*Pi) ratio of 3.5 X 10(3) M-1. The square of the Ca2+ gradient is shown to be proportional to the ATP/(ADP*Pi) ratio, for finite values of the latter. The proportionality constant is identical to the equilibrium constant for hydrolysis of ATP (9.02 X 10(6) M) under these conditions (0.1 mM Mg2+, 30 degrees C). The intrinsic thermodynamic efficiency of the pump is shown to be 100%, with a maximal uncertainty of 3%. The efficiency is lower under less optimal conditions, when the pump is inhibited and passive leak processes compete.
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004734:Energy Metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium