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F. M. HUENNEKENS AND H. R. WHITELEY
At equilibrium AF = 0 and AF° = —RT In K, as before. ΔΡ° values
have the desirable feature of uniformity and can be added or subtracted directly when equations involving the compounds are combined.
AF values, on the other hand, differ with various concentrations and
are, therefore, of limited value in making comparisons.
C. METHODS FOR DETERMINING AF AND AH
1. Calorimetric Measurement of AH of Hydrolysis
By employing the standard calorimetric apparatus and technique
(29, 30), it has been possible to determine AH directly for a number of
enzymatic reactions including the hydrolysis of some phosphorylated
compounds (31, 32). Measurements of AH were initially of considerable value in differentiating "energy-rich" and "energy-poor" phosphate
compounds.
2. Calculation of AF° from Thermal Data
AF° for a reaction may be calculated from known or estimated data
of free energy of formation for the reactants and products. For example,
Lipmann's estimation (7) of the AF° value for the hydrolysis of phosphoenolpyruvate (Reaction 11) made use of this method.
Phosphoenolpyruvate + H 2 0 —> Pyruvate + P»
(11)
This reaction may be considered as the sum of three other reactions:
Phosphoenolpyruvate + H 2 0 ^ Phosphoglycerate
(12)
Phosphoglycerate + H2O —> Glycerate + P»
(13)
Glycerate —> Pyruvate + H 2 0
(14)
Reactions such as Eq. 12, involving the hydration of a double bond,
usually have a AF° value of less than —1 kcal./mole and may be
neglected. The AF° for Reaction 13 may be estimated at —3 kcal./
mole, which is an average value for "energy-poor" phosphate compounds (7, 24, 27). The dehydration Reaction 14 is undoubtedly more
complicated in a mechanistic sense than Reaction 12* and it might
well be expected, a priori, to have a considerably larger AF° value.
Using the heats of combustion for the components of Reaction 14, Lipmann (7) calculated that AF° = approx. —13 kcal./mole; Kaplan (23)
arrived at a value of —14 kcal./mole from the free energies of formation. The average of these two values, —13.5 kcal./mole, when added to
* Reaction 12 is similar to the reaction:
Fumarate + H 2 0 ;=± Malate
which has a AF° value of —0.88 kcal./mole (27).
F. M. HUENNEKENS AND H. R. WHITELEY
At equilibrium AF = 0 and AF° = —RT In K, as before. ΔΡ° values
have the desirable feature of uniformity and can be added or subtracted directly when equations involving the compounds are combined.
AF values, on the other hand, differ with various concentrations and
are, therefore, of limited value in making comparisons.
C. METHODS FOR DETERMINING AF AND AH
1. Calorimetric Measurement of AH of Hydrolysis
By employing the standard calorimetric apparatus and technique
(29, 30), it has been possible to determine AH directly for a number of
enzymatic reactions including the hydrolysis of some phosphorylated
compounds (31, 32). Measurements of AH were initially of considerable value in differentiating "energy-rich" and "energy-poor" phosphate
compounds.
2. Calculation of AF° from Thermal Data
AF° for a reaction may be calculated from known or estimated data
of free energy of formation for the reactants and products. For example,
Lipmann's estimation (7) of the AF° value for the hydrolysis of phosphoenolpyruvate (Reaction 11) made use of this method.
Phosphoenolpyruvate + H 2 0 —> Pyruvate + P»
(11)
This reaction may be considered as the sum of three other reactions:
Phosphoenolpyruvate + H 2 0 ^ Phosphoglycerate
(12)
Phosphoglycerate + H2O —> Glycerate + P»
(13)
Glycerate —> Pyruvate + H 2 0
(14)
Reactions such as Eq. 12, involving the hydration of a double bond,
usually have a AF° value of less than —1 kcal./mole and may be
neglected. The AF° for Reaction 13 may be estimated at —3 kcal./
mole, which is an average value for "energy-poor" phosphate compounds (7, 24, 27). The dehydration Reaction 14 is undoubtedly more
complicated in a mechanistic sense than Reaction 12* and it might
well be expected, a priori, to have a considerably larger AF° value.
Using the heats of combustion for the components of Reaction 14, Lipmann (7) calculated that AF° = approx. —13 kcal./mole; Kaplan (23)
arrived at a value of —14 kcal./mole from the free energies of formation. The average of these two values, —13.5 kcal./mole, when added to
* Reaction 12 is similar to the reaction:
Fumarate + H 2 0 ;=± Malate
which has a AF° value of —0.88 kcal./mole (27).