Free-energy changes of the glutaminase reaction and the hydrolysis of the terminal pyrophosphate bond of adenosine triphosphate

Abstract

Reversal of the enzyme-catalyzed hydrolysis of the amide group of glutamine has been studied by following the heat changes with a sensitive calorimeter. At pH 5.5 and 25[degree], the apparent equilibrium constants were 1040, 885 or 765 moles/ 1000 g of water with 0,885, 0.444 or 0.223 M ammonium glutamate respectively. This trend disappeared after correcting for the activity coefficients. The respective thermodynamic equilibrium constants become 314,318 and 330 at pH 7 and 25[degree] and the free energy of hydrolysis is therefore - 3.42[plus or minus]0.06 kcal/mole. By the use of equilibrium data for the glutamine synthetase reaction (Levintow and Meister, 1954; Varner and Webster, 19551 the free energy of hydrolysis of MgATP[long dash] to MgADP[long dash] and HPO4[long dash] ions is - 7.0 kcal at pH 7 and 37[degree]. In the absence of Mg, the value of - 9.3-kcal (pH 7.5, 37[degree]) has been obtained for the free energy of hydrolysis of ATP to ADP" and HPO4.