Essential Cell Biology 5th edition

Activated Carriers and Biosynthesis
105
ATP
phosphoanhydride bonds
_ O O CH 2
O _ O _ O _
P O P O P
ADENINE
O O O
RIBOSE
energy from
sunlight or from ΔGº > 0
ΔGº < 0
the breakdown
of food
O _ +
O _
ADENINE
P O _ _ O P O P O CH 2
O
O O
RIBOSE
phosphate ( P )
ADP
energy available
to drive energetically
unfavorable
reactions
Figure 3−30 The interconversion of
ATP and ADP occurs in a cycle. The two
outermost phosphate groups in ATP are
held to the rest of the molecule by “highenergy”
phosphoanhydride bonds and
are readily transferred to other organic
molecules. Water can be added to ATP to
form ADP and inorganic phosphate (P i) .
Inside a cell, this hydrolysis of the terminal
phosphate of ATP yields between 46 and
54 kJ/mole of usable energy. (Although the
ΔGº of this reaction is –30.5 kJ/mole, its ΔG
inside cells is much more negative, because
the ratio of ATP to the products ADP and P i
is kept so high.)
The formation of ATP from ADP and P i
reverses the hydrolysis reaction; because
this condensation reaction is energetically
unfavorable, it must be coupled to a highly
energetically favorable reaction to occur.
The large negative ΔGº of the ATP hydrolysis reaction arises from a
number of factors. Release of the terminal phosphate group removes an
unfavorable repulsion between adjacent negative charges; in addition,
the inorganic phosphate ion (P i ) released is stabilized by favorable hydrogen-bond
formation with water.
The energetically favorable reaction of ATP hydrolysis is coupled to
many otherwise unfavorable reactions through which other molecules
are synthesized. We will encounter several of these reactions in this
chapter, where we will see ECB5 exactly e3.31/3.30 how this coupling is carried out. ATP
hydrolysis is often accompanied by a transfer of the terminal phosphate
in ATP to another molecule, as illustrated in Figure 3−31. Any reaction
that involves the transfer of a phosphate group to a molecule is termed
a phosphorylation reaction. Phosphorylation reactions are examples of
condensation reactions (see Figure 2−19), and they occur in many important
cell processes: they activate substrates for a subsequent reaction,
mediate movement, and serve as key constituents of intracellular signaling
pathways (discussed in Chapter 16).
ATP is the most abundant activated carrier in cells. It is used to supply
energy for many of the pumps that actively transport substances into
_ O
phosphoester
bond
O _ P O
O
phosphorylated
molecule
hydroxyl
group on
another
molecule
HO
C
C
_ O O CH 2 O _ O _ O _
P O P O P
ADENINE
O O O
ATP
phosphoanhydride
RIBOSE
bond
ΔGº < 0 PHOSPHATE TRANSFER
O _ O _
ADENINE
C C +
O O
ADP
RIBOSE
QUESTION 3–8
The phosphoanhydride bond that
links two phosphate groups in ATP
in a high-energy linkage has a ΔG°
of –30.5 kJ/mole. Hydrolysis of this
bond in a cell liberates from 46 to
54 kJ/mole of usable energy. How
can this be? Why do you think a
range of energies is given, rather
than a precise number as for ΔG°?
Figure 3−31 The terminal phosphate
of ATP can be readily transferred to
other molecules. Because an energyrich
phosphoanhydride bond in ATP
is converted to a less energy-rich
phosphoester bond in the phosphateaccepting
molecule, this reaction is
energetically favorable, having a large
negative ΔGº (see Panel 3–1, pp. 94–95).
Phosphorylation reactions of this type are
involved in the synthesis of phospholipids
and in the initial steps of the breakdown of
sugars, as well as in many other metabolic
and intracellular signaling pathways.