4. ENERGY-RICH COMPOUNDS
117
AF° for Reaction 13 of —3 kcal./mole and AF° = 0 for Reaction 12
give an over-all figure of AF° = —16.5 kcal./mole for Reaction 11.
Other methods have yielded a lower value for the AF° of hydrolysis of
phosphoenolpyruvate (cf. Table I).
3. Calculation of AF° from Equilibrium Data
Many phosphate compounds can be brought into equilibrium with
one another by means of transphosphorylating enzymes. This type of
linked reaction can be used to determine an unknown AF° of hydrolysis
from a known value as, for example, in Biicher's (33) study of the reaction:
1,3-Diphosphoglycerate + ADP ^ 3-Phosphoglycerate + ATP
(15)
The equilibrium constant for this reaction was found to be 3 X 10
3 . At
25°, AF° for the reaction would be —4.3 kcal./mole according to Eq. 9.
Since the reaction ATP —» ADP has a AF° value of —7.0 kcal./mole
(cf. Section II,D), AF° of hydrolysis for glyceryl phosphate is —11.3
kcal./mole.
4. Calculation of AF° from Hydrolysis Data
From Eq. 9 the AF° for hydrolysis can be ascertained directly if
the reaction is measurable under equilibrium conditions. Obviously, for
"energy-rich" phosphate esters the hydrolytic reaction proceeds too far
to permit an equilibrium to be measured: for example, if AF = —10
kcal./mole, K is 2 X 10
7 . For "energy-poor" compounds, however, K is
usually not larger than 10
2 . Lipmann (7) has used Kay's data (34) for
the hydrolysis of glycerophosphate by intestinal phosphatase to calculate a value of AF° = —2.28 kcal./mole. For this reaction Colowick
(35) made the interesting observation that if all the reactants, including
water, were in their true standard state, i.e., at 1 M, AF° would be
+0.2 kcal./mole. In the aqueous medium where the hydrolysis is
carried out, the concentration of water is 55.5 M. The change in concentration of water from 1 M to 55.5 M contributes —2.5 kcal./mole,
thereby giving the commonly used AF° value of —2.3 kcal./mole.
D. THE PRIMARY STANDARD: AF
0
FOR THE HYDROLYSIS OF ATP
From the preceding discussion it is evident that there are relatively
few independent measurements for establishing the AF° values of the
"energy-rich" compounds. Most of the methods require an accurate
value for a given AF° in order to determine the value in question. It is
evident that the AF° value for ATP is the most widely applicable
primary standard.
117
AF° for Reaction 13 of —3 kcal./mole and AF° = 0 for Reaction 12
give an over-all figure of AF° = —16.5 kcal./mole for Reaction 11.
Other methods have yielded a lower value for the AF° of hydrolysis of
phosphoenolpyruvate (cf. Table I).
3. Calculation of AF° from Equilibrium Data
Many phosphate compounds can be brought into equilibrium with
one another by means of transphosphorylating enzymes. This type of
linked reaction can be used to determine an unknown AF° of hydrolysis
from a known value as, for example, in Biicher's (33) study of the reaction:
1,3-Diphosphoglycerate + ADP ^ 3-Phosphoglycerate + ATP
(15)
The equilibrium constant for this reaction was found to be 3 X 10
3 . At
25°, AF° for the reaction would be —4.3 kcal./mole according to Eq. 9.
Since the reaction ATP —» ADP has a AF° value of —7.0 kcal./mole
(cf. Section II,D), AF° of hydrolysis for glyceryl phosphate is —11.3
kcal./mole.
4. Calculation of AF° from Hydrolysis Data
From Eq. 9 the AF° for hydrolysis can be ascertained directly if
the reaction is measurable under equilibrium conditions. Obviously, for
"energy-rich" phosphate esters the hydrolytic reaction proceeds too far
to permit an equilibrium to be measured: for example, if AF = —10
kcal./mole, K is 2 X 10
7 . For "energy-poor" compounds, however, K is
usually not larger than 10
2 . Lipmann (7) has used Kay's data (34) for
the hydrolysis of glycerophosphate by intestinal phosphatase to calculate a value of AF° = —2.28 kcal./mole. For this reaction Colowick
(35) made the interesting observation that if all the reactants, including
water, were in their true standard state, i.e., at 1 M, AF° would be
+0.2 kcal./mole. In the aqueous medium where the hydrolysis is
carried out, the concentration of water is 55.5 M. The change in concentration of water from 1 M to 55.5 M contributes —2.5 kcal./mole,
thereby giving the commonly used AF° value of —2.3 kcal./mole.
D. THE PRIMARY STANDARD: AF
0
FOR THE HYDROLYSIS OF ATP
From the preceding discussion it is evident that there are relatively
few independent measurements for establishing the AF° values of the
"energy-rich" compounds. Most of the methods require an accurate
value for a given AF° in order to determine the value in question. It is
evident that the AF° value for ATP is the most widely applicable
primary standard.