120
F. M. HUENNEKENS AND H. R. WHITELEY
approximately equal, and that the concentration of HP0 4 is approx.
10
-2 M, the second term on the right side of Eq. 21 will be negative.
In fact, at any concentration of HP0 4 below 1 M, AF will be more
negative than AF°.
The reduction in thermodynamic values for ATP hydrolysis has
evoked the following apt comment by Morales et al. (see reference
42, p. 476):
"Some time ago we suggested that substances like ATP, whose hydrolyses were thought to be highly exergonic, not be described as compounds containing a Tiigh-energy phosphate bond,' for this description
violated established concepts of both 'bond' and 'energy'; possibly it
also violates the established concept of 'high.'"
E. SUMMARY OF THERMODYNAMIC VALUES FOR THE HYDROLYSIS
OF "ENERGY-RICH" COMPOUNDS
It is apparent from the above discussion that the inherent value of
the AF° for hydrolysis of ATP is affected by the following factors: (a)
pH; (b) temperature; and (c) concentration of divalent metal ion
(usually Mg
2+ or Mn
2+ ). To this should be added the possible effect of
the enzyme (if present at an appreciable concentration) in shifting the
equilibrium of the reaction (44b, 44c). Thus, the accuracy of any reported value of AF° for ATP is dependent upon the extent to which the
above parameters were controlled in the original measurements. For
the purpose of this review, we have chosen to use the most recently
determined value (43c) of —7.0 kcal./mole for the ΔΡ° of ATP at pH
7.0, 25°, excess Mg
2+ , and catalytic quantities of enzyme.
All of these conditions apply, as well, in coupled reactions where
the AF° of another "energy-rich" compound is to be determined from
the measured equilibrium constant of the reaction between the compound and ATP. Unfortunately, most of the existing equilibrium data
for the coupled reactions have been obtained under diverse conditions
of pH and temperature, while the effects of metal ion and enzyme
concentration have usually not been considered. Approximate values
(probably accurate to only ±1-2 kcal./mole) for the AF° of hydrolysis
for other "energy-rich" compounds (summarized in Table I) were obtained by attempting to correct the data to a uniform set of conditions,
i.e., pH 7.0 and 25°, and making the assumption that the original determinations were made in the presence of Mg
2+ .
The above AF° values are used most frequently in linked reactions, e.g.,
ATP -j- Creatine ^ ADP + Phosphocreatine
(22)
F. M. HUENNEKENS AND H. R. WHITELEY
approximately equal, and that the concentration of HP0 4 is approx.
10
-2 M, the second term on the right side of Eq. 21 will be negative.
In fact, at any concentration of HP0 4 below 1 M, AF will be more
negative than AF°.
The reduction in thermodynamic values for ATP hydrolysis has
evoked the following apt comment by Morales et al. (see reference
42, p. 476):
"Some time ago we suggested that substances like ATP, whose hydrolyses were thought to be highly exergonic, not be described as compounds containing a Tiigh-energy phosphate bond,' for this description
violated established concepts of both 'bond' and 'energy'; possibly it
also violates the established concept of 'high.'"
E. SUMMARY OF THERMODYNAMIC VALUES FOR THE HYDROLYSIS
OF "ENERGY-RICH" COMPOUNDS
It is apparent from the above discussion that the inherent value of
the AF° for hydrolysis of ATP is affected by the following factors: (a)
pH; (b) temperature; and (c) concentration of divalent metal ion
(usually Mg
2+ or Mn
2+ ). To this should be added the possible effect of
the enzyme (if present at an appreciable concentration) in shifting the
equilibrium of the reaction (44b, 44c). Thus, the accuracy of any reported value of AF° for ATP is dependent upon the extent to which the
above parameters were controlled in the original measurements. For
the purpose of this review, we have chosen to use the most recently
determined value (43c) of —7.0 kcal./mole for the ΔΡ° of ATP at pH
7.0, 25°, excess Mg
2+ , and catalytic quantities of enzyme.
All of these conditions apply, as well, in coupled reactions where
the AF° of another "energy-rich" compound is to be determined from
the measured equilibrium constant of the reaction between the compound and ATP. Unfortunately, most of the existing equilibrium data
for the coupled reactions have been obtained under diverse conditions
of pH and temperature, while the effects of metal ion and enzyme
concentration have usually not been considered. Approximate values
(probably accurate to only ±1-2 kcal./mole) for the AF° of hydrolysis
for other "energy-rich" compounds (summarized in Table I) were obtained by attempting to correct the data to a uniform set of conditions,
i.e., pH 7.0 and 25°, and making the assumption that the original determinations were made in the presence of Mg
2+ .
The above AF° values are used most frequently in linked reactions, e.g.,
ATP -j- Creatine ^ ADP + Phosphocreatine
(22)