section 5.3
Standard Free Energy of Hydrolysis of ATP
73
The choice of a negative sign for the standard emf to sig-
nify “easier to remove electrons” is arbitrary
.2
Since free electrons combine rapidly with whatever is
at hand, half-reactions never occur by themselves. Some-
thing must accept electrons as fast as they are released. The
substance releasing electrons (or H- ) is the
reductant,
or
reducing agent
(because it is oxidized), and the substance
accepting electrons is the
oxidant,
or
oxidizing agent
(be-
cause it is reduced). Two half-reactions when combined
give a redox reaction. When balanced, such reactions never
show free (uncombined) electrons. For example,
H+ + NADH + ^ 0
2
A
E°'
= -0.32 - (+0.816)= -1.136 V.
A
E°'
for this reaction is calculated according to
A
E°'
= A
E°'
(reductant) —
A
E°'
(oxidant).
Under standard conditions (and pH = 7.0), the reaction
will occur as written if A
E°'
< 0. Otherwise the reac-
tion will proceed from right to left (the reverse of the way
it is written).
When A
E°'
in volts is converted to calories per mole,
the result is A
G°'
for the redox reaction being considered.
This conversion is accomplished by means of the equation
n F ■
A
F°'
AG°' = ------------- = (23.061)
n ■
A
E°'
4.184
where, as before,
F
is Faraday’s constant = 96,487 J/V;
n
is the number of electrons transferred per mole of ma-
terial oxidized or reduced; A
E°'
is
E°'
(reductant) —
E°'
(oxidant) in volts; and A
G°'
is the standard free energy
change in calories per mole of material oxidized or reduced
(since there are 4.184 J/cal).
In the case of the reaction just considered
H+ + NADH + ^ 0
2
A
E°'=
-1.136 V,
n
equals 4 gram-equivalents of electrons per mole of 0
2
or
2 gram-equivalents of electrons per mole of H
2
A
G°'
= 23.061« x (-1.136) = -26.197«
= -26.197 x 2 = -52.394
calories/mol of H20 or NAD+ formed
= -26.197 x 4
= —104.788 calories/mol of 0
2
transformed
2Although the International Union of Pure and Applied Chemistry rec-
ommends the opposite convention, the older convention is retained here
because it is commonly encountered in medically and biologically related
publications.
As in all reactions, the actual value of A
G°'
depends on
the reactant or product for which it is calculated.
The significance of free-energy changes and redox re-
actions lies in the fact that life on earth depends on the
redox reaction in which C 0
2
is reduced by H20 to yield
(CH
2
0), using sunlight as the energy source:
Reduction
(photosynthesis)
C 0
2
+ H20 + energy <
----4 0
2
+ (CH
2
0)
Oxidation
(respiration)
where (CH
2
0) represents carbohydrates. The A
E°'
for
the forward reaction is about +1.24 V, or A
G°'
114.3 kcal/(CH
2
0). Thus, for glucose formation, A
G°'
is +685.8 kcal.
In plants, energy from the sun is absorbed by the chloro-
plasts of green plants, causing water to be oxidized and car-
bon dioxide reduced, producing oxygen and carbohydrate
[represented by (CH
2
0)m]. In oxidation reactions, carbo-
hydrate (or lipids) and oxygen are consumed, and energy,
water, and carbon dioxide are released. The energy is cap-
tured primarily as adenosine triphosphate (ATP), which is
used as an immediate source of energy in most cellular
endergonic processes.
5.3
Standard Free Energy of Hydrolysis of ATP
In the living organism, ATP functions as the most impor-
tant energy intermediate, linking exergonic with ender-
gonic processes. Exergonic processes (e.g., oxidation of
glucose, glycogen, and lipids) are coupled to the forma-
tion of ATP, and endergonic processes are coupled to the
expenditure of ATP (e.g., biosynthesis, muscle contrac-
tion, active transport across membranes). ATP is called
a “high-energy compound” because of its large negative
free energy of hydrolysis
ATP4- + H20 -> ADP3” + HPO2“ (or Pj) + H+
AG0/ = —7.3 kcal/mol (—30.5 kJ/mol).
The reason for the large release of free energy associ-
ated with hydrolysis of ATP is that the products of the
reaction, ADP and Pi; are much more stable than ATP.
Several factors contribute to their increased stability: re-
lief of electrostatic repulsion, resonance stabilization, and
ionization.
At pH 7.0, ATP has four closely spaced negative charges
with a strong repulsion between these charges (Figure 5-4).
Upon hydrolysis, the strain in the molecule due to elec-
trostatic repulsion is relieved by formation of less nega-
tively charged products (ADP3~ and P
2
). Furthermore,
since these two products are also negatively charged, their