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

CATALYSIS AND THE USE OF ENERGY BY CELLS
71
(A) POLYSACCHARIDES
(B) NUCLEIC ACIDS
glucose
glycogen
CH 2 OH
O
OH
HO OH
CH 2 OH
OH
HO
O
O
CH 2 OH
OH
O
O
O
CH 2
O
A
O
CH 2
O
A
OH
OH
OH
H 2 O
energy from nucleoside
triphosphate hydrolysis
O
O
P
OH
O _
O
O
P
OH
O _
CH 2 OH
O
CH 2 OH
O
CH 2 OH
O
RNA
O
CH 2
O
C
O
CH 2
O
C
OH
HO
OH
O
OH
OH
O
OH
OH
O
OH
OH
H 2 O
O
OH
glycogen
(C) PROTEINS
protein
H O
C C
R
N
H
R
C
H
C
O
OH
H
H
amino acid
N
H
C
R
C
O
OH
O
nucleotide
OH
P O _
O
CH 2
O
energy from nucleoside
triphosphate hydrolysis
G
O
RNA
P O _
O
CH 2
O
OH
OH
G
H 2 O
energy from nucleoside
triphosphate hydrolysis
OH
OH
H O
C C
R
protein
N
H
R
C
H
O
C
N
H
H
C
R
C
O
OH
Figure 2–41 The synthesis of polysaccharides, proteins, and nucleic acids. Synthesis
of each kind of biological polymer involves the loss of water in a condensation reaction.
Not shown is the consumption of high-energy nucleoside triphosphates that is required to
activate each monomer before its addition. In contrast, the reverse reaction—the breakdown
of all three types of polymers—occurs by the simple addition of water (hydrolysis).
be removed in the condensation reaction is first activated by becoming involved
in a high-energy linkage to a second molecule. However, the actual mechanisms
used to link ATP hydrolysis to the synthesis of proteins and polysaccharides are
more complex than that used for glutamine synthesis, since a series of high-energy
intermediates is required to generate the final high-energy bond that is broken
during the condensation step (discussed in Chapter 6 for protein synthesis).
Each activated carrier has limits in its ability to drive a biosynthetic reaction.
MBoC6 m2.65/2.41
The ∆G for the hydrolysis of ATP to ADP and inorganic phosphate (P i ) depends
on the concentrations of all of the reactants, but under the usual conditions in a
cell it is between –46 and –54 kJ/mole. In principle, this hydrolysis reaction could
drive an unfavorable reaction with a ∆G of, perhaps, +40 kJ/mole, provided that a
suitable reaction path is available. For some biosynthetic reactions, however, even
–50 kJ/mole does not provide enough of a driving force. In these cases, the path
of ATP hydrolysis can be altered so that it initially produces AMP and pyrophosphate
(PP i ), which is itself then hydrolyzed in a subsequent step (Figure 2–42).
The whole process makes available a total free-energy change of about –100 kJ/
mole. An important type of biosynthetic reaction that is driven in this way is the