Essential Cell Biology 5th edition

110 CHAPTER 3 Energy, Catalysis, and Biosynthesis
Figure 3−38 Biotin transfers a carboxyl
group to a substrate. Biotin is a vitamin
that is used by a number of enzymes to
transfer a carboxyl group to a substrate.
Shown here is the reaction in which biotin,
held by the enzyme pyruvate carboxylase,
accepts a carboxyl group from bicarbonate
and transfers it to pyruvate, producing
oxaloacetate, a molecule required in the
citric acid cycle (discussed in Chapter
13). Other enzymes use biotin to transfer
carboxyl groups to other molecules.
Note that the synthesis of carboxylated
biotin requires energy derived from ATP
hydrolysis—a general feature that applies to
many activated carriers.
carboxylated
biotin
O O –
C
high-energy
N bond
S
O
N
ADP
H
O
TRANSFER OF CH 3
P
ENZYME
CARBOXYL GROUP
pyruvate
C O
carboxylase
C
O O –
CARBOXYLATION
pyruvate
OF BIOTIN
ATP
biotin
H
N
O O –
O O – S
O
C
C
N
CH
H
2
OH
O
C O
bicarbonate
ENZYME
C
pyruvate
O O –
carboxylase
oxaloacetate
The Synthesis of Biological Polymers Requires
an Energy Input
The macromolecules of the cell constitute the vast majority of its dry
mass—that is, the mass not due to water. These molecules are made
from subunits (or monomers) that are linked together by bonds formed
during an enzyme-catalyzed condensation reaction. The reverse reaction—the
breakdown of ECB5 polymers—occurs e3.37-3.38 through enzyme-catalyzed
hydrolysis reactions. These hydrolysis reactions are energetically favorable,
whereas the corresponding biosynthetic reactions require an energy
input and are more complex (Figure 3−39).
The nucleic acids (DNA and RNA), proteins, and polysaccharides are all
polymers that are produced by the repeated addition of a subunit onto
one end of a growing chain. The mode of synthesis of each of these
macromolecules is outlined in Figure 3−40. As indicated, the condensation
step in each case depends on energy provided by the hydrolysis of a
nucleoside triphosphate. And yet, except for the nucleic acids, there are
no phosphate groups left in the final product molecules. How, then, is the
energy of ATP hydrolysis coupled to polymer synthesis?
Each type of macromolecule is generated by an enzyme-catalyzed pathway
that resembles the one discussed previously for the synthesis of
the amino acid glutamine (see Figure 3−32). The principle is exactly the
same, in that the –OH group that will be removed in the condensation
reaction is first activated by forming a high-energy linkage to a second
molecule. The mechanisms used to link ATP hydrolysis to the synthesis
of proteins and polysaccharides, however, are more complex than
that used for glutamine synthesis. In the biosynthetic pathways leading
Figure 3−39 In cells, macromolecules are
synthesized by condensation reactions
and broken down by hydrolysis reactions.
Condensation reactions are all energetically
unfavorable, whereas hydrolysis reactions
are all energetically favorable.
H 2 O
H 2 O
A OH + H B A B
A OH + H B
CONDENSATION
HYDROLYSIS
energetically
unfavorable
energetically
favorable