Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

70 Chapter 2: Cell Chemistry and Bioenergetics
are generated in reactions that are coupled to ATP hydrolysis, as in the example
in Figure 2–40. Therefore, the energy that enables their groups to be used for biosynthesis
ultimately comes from the catabolic reactions that generate ATP. Similar
processes occur in the synthesis of the very large molecules of the cell—the
nucleic acids, proteins, and polysaccharides—that we discuss next.
The Synthesis of Biological Polymers Is Driven by ATP Hydrolysis
As discussed previously, the macromolecules of the cell constitute most of its dry
mass (see Figure 2–7). These molecules are made from subunits (or monomers)
that are linked together in a condensation reaction, in which the constituents of a
water molecule (OH plus H) are removed from the two reactants. Consequently,
the reverse reaction—the breakdown of all three types of polymers—occurs by the
enzyme-catalyzed addition of water (hydrolysis). This hydrolysis reaction is energetically
favorable, whereas the biosynthetic reactions require an energy input
(see Figure 2–9).
The nucleic acids (DNA and RNA), proteins, and polysaccharides are all polymers
that are produced by the repeated addition of a monomer onto one end of
a growing chain. The synthesis reactions for these three types of macromolecules
are outlined in Figure 2–41. As indicated, the condensation step in each case
depends on energy from nucleoside triphosphate hydrolysis. And yet, except for
the nucleic acids, there are no phosphate groups left in the final product molecules.
How are the reactions that release the energy of ATP hydrolysis coupled to
polymer synthesis?
For each type of macromolecule, an enzyme-catalyzed pathway exists which
resembles that discussed previously for the synthesis of the amino acid glutamine
(see Figure 2–35). The principle is exactly the same, in that the –OH group that will
CARBOXYL GROUP ACTIVATION
ADP
P P O
ATP
P P P
CH 2
RIBOSE
O CH 2
ADENINE
ADENINE
carboxylated
biotin
P i
O
S
C
N
N
H
ENZYME
O O – C
high-energy
bond
O
CH 3
C O
C
O O –
pyruvate
RIBOSE
O O –
C
OH
bicarbonate
O
biotin
S
H
N
N
H
ENZYME
O
pyruvate carboxylase
O
O –
CH 2
C O
C
O O –
oxaloacetate
CARBOXYL GROUP TRANSFER
Figure 2–40 A carboxyl group-transfer reaction using an activated carrier molecule. Carboxylated biotin is used by the enzyme pyruvate
carboxylase to transfer a carboxyl group in the production of oxaloacetate, a molecule needed for the citric acid cycle. The acceptor molecule for
this group-transfer reaction is pyruvate. Other enzymes use biotin, a B-complex vitamin, to transfer carboxyl groups to other acceptor molecules.
Note that synthesis of carboxylated biotin requires energy that is derived from ATP—a general feature of many activated carriers.
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