118
F. M. HUENNEKENS AND H. R. WHITELEY
From 1934, when the earliest calorimetric measurements were made
(31, 32), until 1956, the AH value for ATP was taken as —12 kcal./
mole, and the AF° value was assumed to be approximately the same.
Recent measurements, however, have resulted in a decrease in
both the AF° and AH values of ATP. Podolsky and Sturtevant (36)
measured calorimetrically the AH value for ATP hydrolysis catalyzed
by purified myosin ATPase. The measured value of AH, AH ohe , at pH
8.0 was —15.9 kcal./mole. This value is equal to the sum of AH for the
hydrolysis of the substrate and AH b , the heat of neutralization of the
H
+ (produced in the primary reaction) by the buffering medium. Thus,
AH ohB = AH + AH b
(16)
For the primary hydrolytic reaction we may write:
ATP
4 " + H 2 0 -> ADP
3 - + P»
2
" + H+
(17)
From the known dissociation constants (37) for ATP, ADP, and Pi, it
can be shown that at pH 8.0, 0.97 mole of H
+ is liberated per mole of
ATP hydrolyzed. AH h is —10.6 kcal./mole for the neutralization of that
amount of H
+ , and, therefore, AH for the primary reaction is only —5.3
kcal./mole. Later, Podolsky and Morales (38) extended these calorimetric measurements and obtained a value of AH equal to —4.7
kcal./mole at pH 8.O. Values for AH near —5 kcal./mole are more
consistent with those for inorganic pyrophosphate (39) and trimetaphosphate (40).
The AF° value for ATP hydrolysis has also been revised downward.
Levintow and Meister (41) measured the equilibrium of the reaction:
ATP + NH 3 + Glutamate ^± Glutamine + ADP + P<
(18)
and obtained an equilibrium constant, K, equal to 1.2 χ 10
3 for this reaction at pH 7.0 and 37°. From this value of K, it can be calculated that
the AF° of hydrolysis for ATP is larger by 4.3 kcal./mole than the AF°
for glutamine hydrolysis catalyzed by glutaminase (Eq. 18a). The
absolute value of AF° for ATP can be obtained
Glutamine + H 2 0 ^± Glutamate + NH 3
(18a)
if the AF° for Reaction 18a is known. The above authors assumed that
Borsook's value of —3.7 kcal./mole (41a) for the hydrolysis of an
ordinary peptide bond was applicable to the amide hydrolysis in Reaction 18a. AF° for ATP was thus calculated to be —8.0 kcal./mole.
Later, Morales et al. (42) corrected these calculations for the ionization
of the various reacting species in Eqs. 17, 18, and 18a and obtained a
figure of —7.0 kcal./mole for ATP hydrolysis.
A further complication of this problem relates to the binding of Mg
2+
F. M. HUENNEKENS AND H. R. WHITELEY
From 1934, when the earliest calorimetric measurements were made
(31, 32), until 1956, the AH value for ATP was taken as —12 kcal./
mole, and the AF° value was assumed to be approximately the same.
Recent measurements, however, have resulted in a decrease in
both the AF° and AH values of ATP. Podolsky and Sturtevant (36)
measured calorimetrically the AH value for ATP hydrolysis catalyzed
by purified myosin ATPase. The measured value of AH, AH ohe , at pH
8.0 was —15.9 kcal./mole. This value is equal to the sum of AH for the
hydrolysis of the substrate and AH b , the heat of neutralization of the
H
+ (produced in the primary reaction) by the buffering medium. Thus,
AH ohB = AH + AH b
(16)
For the primary hydrolytic reaction we may write:
ATP
4 " + H 2 0 -> ADP
3 - + P»
2
" + H+
(17)
From the known dissociation constants (37) for ATP, ADP, and Pi, it
can be shown that at pH 8.0, 0.97 mole of H
+ is liberated per mole of
ATP hydrolyzed. AH h is —10.6 kcal./mole for the neutralization of that
amount of H
+ , and, therefore, AH for the primary reaction is only —5.3
kcal./mole. Later, Podolsky and Morales (38) extended these calorimetric measurements and obtained a value of AH equal to —4.7
kcal./mole at pH 8.O. Values for AH near —5 kcal./mole are more
consistent with those for inorganic pyrophosphate (39) and trimetaphosphate (40).
The AF° value for ATP hydrolysis has also been revised downward.
Levintow and Meister (41) measured the equilibrium of the reaction:
ATP + NH 3 + Glutamate ^± Glutamine + ADP + P<
(18)
and obtained an equilibrium constant, K, equal to 1.2 χ 10
3 for this reaction at pH 7.0 and 37°. From this value of K, it can be calculated that
the AF° of hydrolysis for ATP is larger by 4.3 kcal./mole than the AF°
for glutamine hydrolysis catalyzed by glutaminase (Eq. 18a). The
absolute value of AF° for ATP can be obtained
Glutamine + H 2 0 ^± Glutamate + NH 3
(18a)
if the AF° for Reaction 18a is known. The above authors assumed that
Borsook's value of —3.7 kcal./mole (41a) for the hydrolysis of an
ordinary peptide bond was applicable to the amide hydrolysis in Reaction 18a. AF° for ATP was thus calculated to be —8.0 kcal./mole.
Later, Morales et al. (42) corrected these calculations for the ionization
of the various reacting species in Eqs. 17, 18, and 18a and obtained a
figure of —7.0 kcal./mole for ATP hydrolysis.
A further complication of this problem relates to the binding of Mg
2+