Essential Cell Biology 5th edition

106 CHAPTER 3 Energy, Catalysis, and Biosynthesis
or out of the cell (discussed in Chapter 12) and to power the molecular
motors that enable muscle cells to contract and nerve cells to transport
materials along their lengthy axons (discussed in Chapter 17), to name
just two important examples. Why evolution selected this particular
nucleoside triphosphate over the others as the major carrier of energy,
however, remains a mystery. GTP, although chemically similar to ATP, is
involved in a different set of functions in the cell, as we discuss in later
chapters.
Figure 3–32 An energetically unfavorable
biosynthetic reaction can be driven by
ATP hydrolysis. (A) Schematic illustration of
the condensation reaction described in the
text. In this set of reactions, a phosphate
group is first donated by ATP to form a
high-energy intermediate, A−O−PO 3 ,
which then reacts with the other substrate,
B−H, to form the product A−B. (B) Reaction
showing the biosynthesis of the amino acid
glutamine from glutamic acid. Glutamic
acid, which corresponds to the A−OH
shown in (A), is first converted to a highenergy
phosphorylated intermediate, which
corresponds to A–O–PO 3. This intermediate
then reacts with ammonia (which
corresponds to B–H) to form glutamine.
In this example, both steps occur on the
surface of the same enzyme, glutamine
synthetase (not shown). ATP hydrolysis can
drive this energetically unfavorable reaction
because it produces a favorable free-energy
change (ΔG° of –30.5 kJ/mole) that is larger
in magnitude than the energy required for
the synthesis of glutamine from glutamic
acid plus NH 3 (ΔG° of +14.2 kJ/mole). For
clarity, the glutamic acid side chain is shown
in its uncharged form.
Energy Stored in ATP Is Often Harnessed to Join Two
Molecules Together
A common type of reaction that is needed for biosynthesis is one in which
two molecules, A and B, are joined together by a covalent bond to produce
A–B in an energetically unfavorable condensation reaction:
A–OH + B–H → A–B + H 2 O
ATP hydrolysis can be coupled indirectly to this reaction to make it go
forward. In this case, energy from ATP hydrolysis is first used to convert
A–OH to a higher-energy intermediate compound, which then reacts
directly with B–H to give A–B. The simplest mechanism involves the
transfer of a phosphate from ATP to A–OH to make A–O–PO 3 , in which
case the reaction pathway contains only two steps (Figure 3−32A). The
condensation reaction, which by itself is energetically unfavorable, has
been forced to occur by being coupled to ATP hydrolysis in an enzymecatalyzed
reaction pathway.
A biosynthetic reaction of exactly this type is employed to synthesize the
amino acid glutamine, as illustrated in Figure 3−32B. We will see later
in the chapter that very similar (but more complex) mechanisms are also
used to produce nearly all of the large molecules of the cell.
NADH and NADPH Are Both Activated Carriers of
Electrons
Other important activated carriers participate in oxidation–reduction
reactions and are also commonly part of coupled reactions in cells. These
P
O
O
ATP
ADP
C
A
STEP 1
A
STEP 2
OH
O
in the ACTIVATION step, ATP transfers
a phosphate, P , to A–OH to produce a
high-energy intermediate
P
B
H
P
in the CONDENSATION step, the activated
intermediate reacts with B–H to form the
product A–B, a reaction accompanied by
the release of inorganic phosphate
A
O
A
P
B
ACTIVATION
STEP
O
C
CH 2
OH
ATP
CH 2
CH 2
H 3 N + CH COO –
high-energy intermediate
(A–O–P)
ADP
P
products of
ATP hydrolysis
H 2 N H
ammonia (B–H)
O
CONDENSATION
STEP
C
CH 2
NH 2
NET RESULT
A
OH + B H + ATP A B + ADP + P
CH 2
H 3 N + CH COO –
CH 2
H 3 N + CH COO –
(A) (B) glutamic acid (A–OH)
glutamine (A–B)