4. ENERGY-RICH COMPOUNDS
113
ooo
oII
I
II
I
R—C—O—P—O- + H 2 0 -► R—C—OH + HO—P—0~
(4)
h
i
The ionization of the carboxyl group, as shown in Eq. 5, would conO
O
II
II
R—C—OH i± R—C—O- + H+
(5)
tribute a further term of —3.2 kcal./mole* to the total AF of hydrolysis
if the dissociation constant, K u of the carboxyl group is assumed to be
2 X 10"
5 and the hydrolysis is carried out at pH 7.
The theoretical approach was further expanded by Hill and Morales
(13). These writers utilized the concepts developed previously by
Kalckar and Oesper, but added refinements in the form of a semiquantitative interpretation of the contribution of various factors to the "energy-rich" nature of these compounds. A typical hydrolysis reaction,
accompanied by the principal ionization effects, is depicted in Eq. 6:
X—P0 3
2 " + H 2 0 i=± HX + HPO 4
2
~
+
H+
Κι][
Η++Χ][Kt
X—PO3HH 2 P0 4 -
AF° is the free energy change for the primary reaction as written across
the top line; AF
0
l5 AF° 2 , and AF° 3 are the free energy changes for the
corresponding ionization reactions indicated by K u K 2 , and K 3 . (AF°) a
is the free energy change for the actual equilibrium mixture of the
various ionized forms of reactants and products. The contribution of
the dissociation reactions was treated by Hill and Morales as corrections to the primary reaction:
(AF°) a = AF° + AF° 2 + Δ^° 3 - AF°i
(7)
The hydrolysis of the phosphate bond in AMP was taken as the
prototype for an "energy-poor" bond; the experimental value for this
reaction is ca. —2 kcal./mole (7, 14). For typical "energy-rich" compounds, such as acetyl phosphate, 1,3-diphosphoglycerate, phosphoenolpyruvate, phosphocreatine,f phosphoarginine, adenosine diphosphate,
* Oesper used the expression AF = RT In [ (H
+ ) /K] for this calculation; later
Hill and Morales (12) suggested the more rigorous function
+
Kl
H
+
(6)
AF =
-RT\n
b
+
w>}
f N-Phosphates can be treated similarly to the O-phosphate compounds, since
the larger stability (by approx. 28 kcal./mole) of the O-phosphate bond, as compared to the N-phosphate bond, is nearly compensated by the greater stability of
the O—H bond in the product (approx. 26.5 kcal./mole).
113
ooo
oII
I
II
I
R—C—O—P—O- + H 2 0 -► R—C—OH + HO—P—0~
(4)
h
i
The ionization of the carboxyl group, as shown in Eq. 5, would conO
O
II
II
R—C—OH i± R—C—O- + H+
(5)
tribute a further term of —3.2 kcal./mole* to the total AF of hydrolysis
if the dissociation constant, K u of the carboxyl group is assumed to be
2 X 10"
5 and the hydrolysis is carried out at pH 7.
The theoretical approach was further expanded by Hill and Morales
(13). These writers utilized the concepts developed previously by
Kalckar and Oesper, but added refinements in the form of a semiquantitative interpretation of the contribution of various factors to the "energy-rich" nature of these compounds. A typical hydrolysis reaction,
accompanied by the principal ionization effects, is depicted in Eq. 6:
X—P0 3
2 " + H 2 0 i=± HX + HPO 4
2
~
+
H+
Κι][
Η++Χ][Kt
X—PO3HH 2 P0 4 -
AF° is the free energy change for the primary reaction as written across
the top line; AF
0
l5 AF° 2 , and AF° 3 are the free energy changes for the
corresponding ionization reactions indicated by K u K 2 , and K 3 . (AF°) a
is the free energy change for the actual equilibrium mixture of the
various ionized forms of reactants and products. The contribution of
the dissociation reactions was treated by Hill and Morales as corrections to the primary reaction:
(AF°) a = AF° + AF° 2 + Δ^° 3 - AF°i
(7)
The hydrolysis of the phosphate bond in AMP was taken as the
prototype for an "energy-poor" bond; the experimental value for this
reaction is ca. —2 kcal./mole (7, 14). For typical "energy-rich" compounds, such as acetyl phosphate, 1,3-diphosphoglycerate, phosphoenolpyruvate, phosphocreatine,f phosphoarginine, adenosine diphosphate,
* Oesper used the expression AF = RT In [ (H
+ ) /K] for this calculation; later
Hill and Morales (12) suggested the more rigorous function
+
Kl
H
+
(6)
AF =
-RT\n
b
+
w>}
f N-Phosphates can be treated similarly to the O-phosphate compounds, since
the larger stability (by approx. 28 kcal./mole) of the O-phosphate bond, as compared to the N-phosphate bond, is nearly compensated by the greater stability of
the O—H bond in the product (approx. 26.5 kcal./mole).