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

72 Chapter 2: Cell Chemistry and Bioenergetics
(A)
_ O _ _ _ O P O CH2
O O O
ADENINE
P O P O P O CH 2
O _ O O
RIBOSE
adenosine triphosphate (ATP)
H 2 O
O O
O
ADENINE
_ O P O P +
O _ O _
O _
(B)
H 2 O
ATP
+
AMP
Figure 2–42 An alternative pathway of
ATP hydrolysis, in which pyrophosphate
is first formed and then hydrolyzed. This
route releases about twice as much free
energy (approximately –100 kJ/mole) as
the reaction shown earlier in Figure 2–33,
and it forms AMP instead of ADP. (A) In
the two successive hydrolysis reactions,
oxygen atoms from the participating water
molecules are retained in the products, as
indicated, whereas the hydrogen atoms
dissociate to form free hydrogen ions
(H + , not shown). (B) Summary of overall
reaction.
pyrophosphate
RIBOSE
H 2 O
adenosine monophosphate (AMP)
H 2 O
_ O
O
P OH
O _
+
_ O
O _ O
P OH
O _
P i
P P i
P i
+
phosphate
phosphate
synthesis of nucleic acids (polynucleotides) from nucleoside triphosphates, as
illustrated on the right side of Figure 2–43.
Note that the repetitive condensation reactions that produce macromolecules
can be oriented in one of two ways, giving rise to either the head polymerization
or the tail polymerization of monomers. In so-called head polymerization, the
reactive bond required for the condensation reaction is carried on the end of the
2 ATP
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base
3
P P P O
sugar
OH
high-energy intermediate
P
P i
P
O
sugar
P
O
base
1
sugar
base
2
P
O
sugar
base
3
OH
nucleoside
monophosphate
2 ADP
2 P i
products of
ATP hydrolysis
H 2 O
O
sugar
O
polynucleotide chain
containing three nucleotides
P
P
base
1
sugar
P
OH
polynucleotide
chain containing
two nucleotides
O
base
2
sugar
base
3
Figure 2–43 Synthesis of a polynucleotide, RNA or DNA, is a multistep process driven by ATP
hydrolysis. In the first step, a nucleoside monophosphate is activated by the sequential transfer of
the terminal phosphate groups from two ATP molecules. The high-energy intermediate formed—a
nucleoside triphosphate—exists free in solution until it reacts with the growing end of an RNA or a
DNA chain with release of pyrophosphate. Hydrolysis of the latter to inorganic phosphate is highly
favorable and helps to drive the overall reaction in the direction of polynucleotide synthesis. For
details, see Chapter 5.
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OH