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
69
Figure 2–38 The structure of the
important activated carrier molecule
acetyl CoA. A ball-and-stick model is
shown above the structure. The sulfur
atom (yellow) forms a thioester bond to
acetate. Because this is a high-energy
linkage, releasing a large amount of free
energy when it is hydrolyzed, the acetate
molecule can be readily transferred to other
molecules.
acetyl
group
nucleotide
ADENINE
H 3 C
C
O
S
high-energy
bond
H
C
H
H
C
H
O H
N C C
H H
H
C
H
O H CH 3 H
N C C C C
H OH CH 3 H
O
O
P
O –
O
O
P
O –
O CH 2
RIBOSE
O
– O P O
O –
acetyl group
coenzyme A (CoA)
acetyl group in a thioester linkage, and in this activated form is known as acetyl
CoA (acetyl coenzyme A). Acetyl CoA (Figure 2–38) is used to add two carbon
units in the biosynthesis of larger molecules.
In acetyl CoA, as in other carrier molecules, the transferable group makes up
only a small part of the molecule.
MBoC6 e3.36/2.39
The rest consists of a large organic portion that
serves as a convenient “handle,” facilitating the recognition of the carrier molecule
by specific enzymes. As with acetyl CoA, this handle portion very often contains
a nucleotide (usually adenosine), a curious fact that may be a relic from an
early stage of evolution. It is currently thought that the main catalysts for early
life-forms—before DNA or proteins—were RNA molecules (or their close relatives),
as described in Chapter 6. It is tempting to speculate that many of the carrier
molecules that we find today originated in this earlier RNA world, where their
nucleotide portions could have been useful for binding them to RNA enzymes
(ribozymes).
Thus, ATP transfers phosphate, NADPH transfers electrons and hydrogen, and
acetyl CoA transfers two-carbon acetyl groups. FADH 2 (reduced flavin adenine
dinucleotide) is used like NADH in electron and proton transfers (Figure 2–39).
The reactions of other activated carrier molecules involve the transfer of a methyl,
carboxyl, or glucose group for biosyntheses (Table 2–3). These activated carriers
(A)
CH 3
C
C
CH 3
FADH 2
H
C
C
H
C
C
H
H
H
H
N
N
CH 2
C
C
C
C
C
OH
OH
OH
O
C
N
H
NH
C
O
H 2 C O P P O CH 2 ADENINE
Table 2–3 Some Activated Carrier Molecules Widely Used in Metabolism
Activated carrier
Group carried in high-energy linkage
ATP
phosphate
NADH, NADPH, FADH 2
Electrons and hydrogens
Acetyl CoA
acetyl group
Carboxylated biotin
Carboxyl group
S-Adenosylmethionine
Methyl group
Uridine diphosphate glucose
Glucose
(B)
2H + 2e –
RIBOSE
FAD FADH 2
Figure 2–39 FADH 2 is a carrier of
hydrogens and high-energy electrons,
like NADH and NADPH. (A) Structure of
FADH 2 , with its hydrogen-carrying atoms
highlighted in yellow. (B) The formation of
FADH 2 from FAD.