Abstract
Abstract The observed standard free energy change (ΔG0obs) for the ATP-pyrophosphorylase reaction (EC 3.6.1.8) has been calculated for near physiological conditions of temperature, ionic strength, and free magnesium concentration. The observed equilibrium constant (Kobs) for the acetyl-CoA synthetase reaction (EC 6.2.1.1) has been determined at both 25 and 38°, pH 7.0, ionic strength 0.25, and varying free [Mg2+]. The Kobs of this reaction reflects the difference between the ΔG0obs for the hydrolysis of acetyl-CoA and the ΔG0obs for the hydrolysis of ATP to AMP and inorganic PPi. Using Σ and square brackets to indicate total concentrations of all the ionic species present: [see PDF for equation] The observed value of this constant varies with the free [Mg2+] being 10.7 (38°) and 15.1 (25°) at free [Mg2+] = 0; 9.17 (38°) and 12.9 (25°) at free [Mg2+] = 10-4 m, and 9.88 (38°) and 11.4 (25°) at free [Mg2+] = 10-3 m. The values of ΔH0obs for this reaction are also a function of the free [Mg2+], being 4.9 Cal per mole (20.5 kJ per mole) at free [Mg2+] = 0; 4.8 Cal per mole (20.0 kJ per mole) at free [Mg2+] = 10-4 m; and 3.6 Cal per mole (15.3 kJ per mole) at free [Mg2+] = 10-3 m. The ΔG0obs for the hydrolysis of acetyl-CoA is virtually unaffected by the free [Mg2+] and has been previously determined to be -8.54 Cal per mole (-35.75 kJ per mole) under the same conditions of temperature, pH, and ionic strength. Therefore, at 38°, pH 7.0, ionic strength 0.25, and taking the standard state of liquid water to have unit activity ([H2O] = 1), the ΔG0obs for the reaction [see PDF for equation] can be calculated at 38° and ionic strength of 0.25 to be -10.0 Cal per mole (-41.84 kJ per mole) at free [Mg2+] = 0; -9.91 Cal per mole (-41.46 kJ per mole) at free [Mg2+] = 10-4 m; and -9.96 Cal per mole (-41.67 kJ per mole) at free [Mg2+] = 10-3 m. The corresponding equilibrium constants for the hydrolysis of ATP to AMP and inorganic pyrophosphate under these conditions are 10.8 x 106 m ([Mg2+] = 0), 9.26 x 106 m (free [Mg2+] = 10-4 m), and 9.98 x 106 m (free [Mg2+] = 10-3 m).
Highlights
The AGzbsfor the hydrolysis of acetyl-CoA is virtually unaffected by the free [Mg2f] and has been previously determined to be -8.54 Cal per mole (-35.75 kJ per mole) under the same conditions of temperature, pH, and ionic strength
The current study reports the determination of the AGzba for another fundamental reaction, the hydrolysis of the ATP to AMP and PPi
Using x and square brackets to indicate total concentration, for the hydrolysis of the inner pyrophosphate bond of ATP, &bs = [x AMP1 Lx PPil/[E ATP] [Hz01 *. Essential to these calculations is an accurate estimate of the equilibrium constant of the acetyl-CoA synthetase reaction
Summary
The observed standard free energy change (AGtbs) for the ATP-pyrophosphorylase reaction (EC 3.6.1.8) has been calculated for near physjological conditions of temperature, ionic strength, and free magnesium concentration. The observed equilibrium constant (Kobs)for the acetyl-CoA synthetase reaction (EC 6.2.1.1) has been determined at both 25 and 38”, pH 7.0, ionic strength 0.25, and varying free [Mg’+]. The Kobs of this reaction reflects the difference between the AGtbS for the hydrolysis of acetyl-CoA and the AGO,,I,*for the hydrolysis of ATP to AMP and inorganic PPi. Using c and square brackets to indicate total concentrations of all the ionic speciespresent:. In previous studies the observed standard free energy change (AGfb,)l for the hydrolysis of acetyl-CoA [1] and the hydrolysis of ATP to ADP and Pi [2] have been determined under physiological conditions. Using x and square brackets to indicate total concentration, for the hydrolysis of the inner pyrophosphate bond of ATP, &bs = [x AMP1 Lx PPil/[E ATP] [Hz01 * Essential to these calculations is an accurate estimate of the equilibrium constant of the acetyl-CoA synthetase reaction. The value of the equilibrium constant for the acetyl-CoA synthetase reaction determined in the current study differs significantly from either of the previous determinations [5, 6]
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