The thermodynamic efficiency of the Ca2+-Mg2+-ATPase is one hundred percent.

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.