114
F. M. HUENNEKENS AND H. R. WHITELEY
and adenosine triphosphate, the free energy of hydrolysis was taken as
ca. —10 kcal./mole, although this value is now known to be high. For
each of the above compounds, Hill and Morales assessed the various
factors that contribute to the enhanced free energy of hydrolysis over
AMP. The largest contribution in all cases is due to the relative resonance stabilization of the products, but no quantitative assessment can
be made of this factor. The pK values for the dissociation reaction in
Eq. 6 were converted into values for AF° 1? AF° 2 , and AF° 3 . In addition
to resonance stabilization and ionization, these authors suggested an
additional contributing factor, electrostatic repulsion, which is certainly
applicable to ATP and ADP, and possibly to phosphoenolpyruvate. This
repulsion is caused by the positive centers $ on adjacent phosphate
atoms, viz.:
oI
I
R—O—P
+ —O—P
+ —OA- Aand is estimated from a theoretical model to be 5-6 kcal./mole for ATP
and 3-4 kcal./mole for ADP and phosphoenolpyruvate.
Finally, the conclusions drawn by Hill and Morales may be recapitulated briefly: the AF of hydrolysis is not a reflection of a particular, localized bond energy, but is, rather, the difference in free energies between the products and reactants; and important contributions
to the net difference in free energy may be provided by resonance
stabilization, ionization effects, and electrostatic repulsions. Thus, the
term§ "energy-rich," as customarily applied to a certain class of phosphate compounds, is erroneous. On the other hand, the term is admittedly convenient and vivid, and it will continue to be used in this
chapter with the foregoing reservations implicitly understood. Accordingly, we shall define an "energy-rich" compound as one that undergoes
hydrolysis at a particular bond with a AF° value greater than —5 kcal./
mole.
| The P=0 double bond is more correctly written (15) as a "semi-polar"
double bond:
O
ΐ
—P—
§ We have preferred to use the expression "energy-rich" rather than the equally
widespread term "high energy" as applied to phosphate compounds; the latter
term is often denoted by the symbol ~P. Gillespie et al. (15a) have also cautioned
against the improper use, in a thermodynamic sense, of the term "high energy"
bond.
F. M. HUENNEKENS AND H. R. WHITELEY
and adenosine triphosphate, the free energy of hydrolysis was taken as
ca. —10 kcal./mole, although this value is now known to be high. For
each of the above compounds, Hill and Morales assessed the various
factors that contribute to the enhanced free energy of hydrolysis over
AMP. The largest contribution in all cases is due to the relative resonance stabilization of the products, but no quantitative assessment can
be made of this factor. The pK values for the dissociation reaction in
Eq. 6 were converted into values for AF° 1? AF° 2 , and AF° 3 . In addition
to resonance stabilization and ionization, these authors suggested an
additional contributing factor, electrostatic repulsion, which is certainly
applicable to ATP and ADP, and possibly to phosphoenolpyruvate. This
repulsion is caused by the positive centers $ on adjacent phosphate
atoms, viz.:
oI
I
R—O—P
+ —O—P
+ —OA- Aand is estimated from a theoretical model to be 5-6 kcal./mole for ATP
and 3-4 kcal./mole for ADP and phosphoenolpyruvate.
Finally, the conclusions drawn by Hill and Morales may be recapitulated briefly: the AF of hydrolysis is not a reflection of a particular, localized bond energy, but is, rather, the difference in free energies between the products and reactants; and important contributions
to the net difference in free energy may be provided by resonance
stabilization, ionization effects, and electrostatic repulsions. Thus, the
term§ "energy-rich," as customarily applied to a certain class of phosphate compounds, is erroneous. On the other hand, the term is admittedly convenient and vivid, and it will continue to be used in this
chapter with the foregoing reservations implicitly understood. Accordingly, we shall define an "energy-rich" compound as one that undergoes
hydrolysis at a particular bond with a AF° value greater than —5 kcal./
mole.
| The P=0 double bond is more correctly written (15) as a "semi-polar"
double bond:
O
ΐ
—P—
§ We have preferred to use the expression "energy-rich" rather than the equally
widespread term "high energy" as applied to phosphate compounds; the latter
term is often denoted by the symbol ~P. Gillespie et al. (15a) have also cautioned
against the improper use, in a thermodynamic sense, of the term "high energy"
bond.