Equilibrium Constants of the Reactions of Acetyl Coenzyme A Synthetase and the Hydrolysis of Adenosine Triphosphate to Adenosine Monophosphate and Inorganic Pyrophosphate

• Published in: The Journal of biological chemistry 1974-05, Vol.249 (10), p.3248-3254
• Main Authors: Guynn, Robert W., Webster, Leslie T., Veech, Richard L.
• Format: Article
• Language: English
• Subjects: Acetyl Coenzyme A - metabolism; Adenosine Monophosphate - metabolism; Adenosine Triphosphatases - metabolism; Adenosine Triphosphate - metabolism; Animals; Cations, Monovalent; Cattle; Coenzyme A Ligases - metabolism; Computers; Hydrogen-Ion Concentration; Hydrolysis; Kinetics; Magnesium - metabolism; Mathematics; Mitochondria, Muscle - enzymology; Myocardium - enzymology; Osmolar Concentration; Phosphates - metabolism; Spectrophotometry; Spectrophotometry, Atomic; Temperature; Thermodynamics
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)42664-1

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+] = 10-4m, and 9.88 (38°) and 11.4 (25°) at free [Mg2+] = 10-3m. 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+] = 10-4m; and 3.6 Cal per mole (15.3 kJ per mole) at free [Mg2+] = 10-3m. 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+] = 10-4m; and -9.96 Cal per mole (-41.67 kJ per mole) at free [Mg2+] = 10-3m. The corresponding equilibrium constants for the hydrolysis of ATP to AMP and inorganic pyrophosphate under these conditions are 10.8 x 106m ([Mg2+] = 0), 9.26 x 106m (free [Mg2+] = 10-4m), and 9.98 x 106m (free [Mg2+] = 10-3m).