Essential Cell Biology 5th edition

112 CHAPTER 3 Energy, Catalysis, and Biosynthesis
Figure 3–41 In an alternative route for
the hydrolysis of ATP, pyrophosphate
is first formed and then hydrolyzed in
solution. This route releases about twice
as much free energy as the reaction shown
earlier in Figure 3–30. (A) In each of the two
successive hydrolysis reactions, an oxygen
atom from the participating water molecule
is retained in the products, whereas the
hydrogen atoms from water form free
hydrogen ions, H + . (B) The overall reaction
shown in summary form.
(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 _
pyrophosphate
RIBOSE
(B)
ATP
H 2 O
+ P P +
AMP
H 2 O
adenosine monophosphate (AMP)
_ O
O
P OH
O _
+
O _ O
H 2 O
_ O
P OH
O _
P
+
P
phosphate
phosphate
ATP will make many appearances throughout the book as a molecule
that powers reactions in the cell. And in Chapters 13 and 14, we discuss
how the cell uses the energy from food to generate ATP. In the next chapter,
we learn more about the proteins that make such reactions possible.
ECB5 e3.40/3.41
2 ATP
P P P
O
sugar
base
3
OH
high-energy intermediate
P
P
P
O
sugar
P
O
base
1
sugar
base
2
P
O
sugar
base
3
OH
nucleoside
monophosphate
2 ADP
2 P
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 3–42 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 ECB5 e3.41/3.42 with the growing end of an RNA or a DNA
chain, with release of pyrophosphate. Hydrolysis of the pyrophosphate to inorganic
phosphate is highly favorable and helps to drive the overall reaction in the direction
of polynucleotide synthesis.
OH