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

66 Chapter 2: Cell Chemistry and Bioenergetics
hydroxyl
group on
another
molecule
HO
O _
P O
O
C
phosphoanhydride
bond
C
O _ O _
ADENINE
P O P O CH 2
O O
RIBOSE
ATP
Figure 2–34 An example of a phosphate
transfer reaction. Because an energyrich
phosphoanhydride bond in ATP
is converted to a phosphoester bond,
this reaction is energetically favorable,
having a large negative ΔG. Reactions of
this type are involved in the synthesis of
phospholipids and in the initial steps of
reactions that catabolize sugars.
_ O
O _ P O
C
C
+
O _
_ O
_ O P O _
O P O CH 2 ADENINE
ΔG < 0
PHOSPHATE TRANSFER
O
phosphoester
bond
O
O
RIBOSE
ADP
A typical biosynthetic reaction is one in which two molecules, A and B, are
joined together to produce A–B in the energetically unfavorable condensation
reaction
A–H + B–OH → A–B + H 2 O
There is an indirect pathway MBoC6 that m2.58/2.34 allows A–H and B–OH to form A–B, in which
a coupling to ATP hydrolysis makes the reaction go. Here, energy from ATP hydrolysis
is first used to convert B–OH to a higher-energy intermediate compound,
which then reacts directly with A–H to give A–B. The simplest possible mechanism
involves the transfer of a phosphate from ATP to B–OH to make B–O–PO 3 , in
which case the reaction pathway contains only two steps:
1. B–OH + ATP → B–O–PO 3 + ADP
2. A–H + B–O–PO 3 → A–B + P i
Net result: B–OH + ATP + A–H → A–B + ADP + P i
The condensation reaction, which by itself is energetically unfavorable, is forced
to occur by being directly coupled to ATP hydrolysis in an enzyme-catalyzed reaction
pathway (Figure 2–35A).
A biosynthetic reaction of exactly this type synthesizes the amino acid glutamine
(Figure 2–35B). We will see shortly that similar (but more complex) mechanisms
are also used to produce nearly all of the large molecules of the cell.
Figure 2–35 An example of an
energetically unfavorable biosynthetic
reaction driven by ATP hydrolysis. (A)
Schematic illustration of the formation of A–B
in the condensation reaction described in
the text. (B) The biosynthesis of the common
amino acid glutamine from glutamic acid and
ammonia. Glutamic acid is first converted to
a high-energy phosphorylated intermediate
(corresponding to the compound B–O–PO 3
described in the text), which then reacts
with ammonia (corresponding to A–H) to
form glutamine. In this example, both steps
occur on the surface of the same enzyme,
glutamine synthetase. The high-energy
bonds are shaded red; here, as elsewhere
throughout the book, the symbol P i =
HPO 4 2– , and a yellow “circled P” = PO 3 2– .
(B)
O
C
CH 2
P
O
CH 2
H 3 N + CH COO –
(A)
B
ATP
ACTIVATION
STEP
OH
O
B
high-energy intermediate
ADP
P
P i
products of
ATP hydrolysis
A
H
A
CONDENSATION
STEP
B
O
ATP
NH 3
ammonia
ADP P i
OH
products of
C
ATP hydrolysis
O NH 2
CH 2
C
CH 2
ACTIVATION
STEP
H 3 N + CH COO –
glutamic acid
high-energy intermediate
CONDENSATION
STEP
CH 2
CH 2
H CH COO –
3 N +
glutamine