Transfer by ATP
We have seen that endergonic reactions
are those that will not proceed
spontaneously by themselves because
the products require an input of free
energy. However, an endergonic reaction
may be driven by coupling the
energy-requiring reaction with an
energy-yielding reaction. ATP is the
most common intermediate in coupled
, and because it can
drive such energetically unfavorable
reactions, it is of central importance in
The ATP molecule consists of
adenosine (the purine adenine and the
5-carbon sugar ribose) and a triphosphate
group (Figures 4-6 and 4-7).
Most of the free energy in ATP resides
in the triphosphate group, especially
in two phosphoanhydride bonds
the three phosphate groups.
These two bonds are called “highenergy
” because a great deal
of free energy in the bonds is liberated
when ATP is hydrolyzed to adenosine
diphosphate (ADP) and inorganic
|Figure 4-6 A, Structure of ATP. B, ATP formation from ADP.
|Figure 4-7 Space-filling model of ATP. In this model, carbon is shown in black;
nitrogen in blue; oxygen in red; and phosphorus in yellow.
represents inorganic phosphate
(i = inorganic). The high-energy
groups in ATP are designated by the
“tilde” symbol ~. A high-energy phosphate
bond is shown as ~P and a lowenergy
bond (such as the bond linking
the triphosphate group to adenosine)
as —P. ATP may be symbolized as
A—P~P~P and ADP as A—P~P.
|Figure 4-8 A coupled reaction. The endergonic
conversion of substrate A to product A
occur spontaneously but requires an input of
energy from another
reaction involving a large
release of energy. ATP is the intermediate through
which the energy is shuttled.
The way that ATP can act to drive a
coupled reaction is shown in Figure 4-
8. A coupled reaction is really a system
involving two reactions linked by an
energy shuttle (ATP). The conversion
of substrate A to product A is endergonic
because the product contains
more free energy than the substrate.
Therefore energy must be supplied by
coupling the reaction to one that is
exergonic, the conversion of substrate
B to product B. Substrate B in this reaction
is commonly called a fuel
example, glucose or a lipid). Bond
energy that is released in reaction B is
transferred to ADP, which in turn is
converted to ATP. ATP now contributes
its phosphate-bond energy to reaction
A, and ADP is produced again.
The high-energy bonds of ATP are
actually rather weak, unstable bonds.
Because they are unstable, the energy
of ATP is readily released when ATP is
hydrolyzed in cellular reactions. Note
that ATP is an energy-coupling agent
and not a fuel. It is not a storehouse of
energy set aside for some future need.
Rather it is produced by one set of
reactions and is almost immediately
consumed by another. ATP is formed
as it is needed, primarily by oxidative
processes in the mitochondria. Oxygen
is not consumed unless ADP and phosphate
molecules are available, and
these do not become available until
ATP is hydrolyzed by some energyconsuming
process. Metabolism is
therefore mostly self-regulating.