Essential Cell Biology 5th edition

Activated Carriers and Biosynthesis
109
TABLE 3–2 SOME ACTIVATED CARRIERS 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
In acetyl CoA and the other activated carriers in Table 3−2, the transferable
group makes up only a small part of the molecule. 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. This
curious fact may be a relic from an early stage of cell evolution. It is
thought that the main catalysts for early life-forms on Earth were RNA
molecules (or their close relatives) and that proteins were a later evolutionary
addition. It is therefore tempting to speculate that many of the
activated carriers that we find today originated in an earlier RNA world,
where their nucleotide portions would have been useful for binding these
carriers to RNA-based catalysts, or ribozymes (discussed in Chapter 7).
Activated carriers are usually generated in reactions coupled to ATP
hydrolysis, as shown for biotin in Figure 3–38. Therefore, the energy that
enables their groups to be used for biosynthesis ultimately comes from
the catabolic reactions that generate ATP. The same principle applies to
the synthesis of large macromolecules—nucleic acids, proteins, and polysaccharides—as
we discuss next.
acetyl
group
nucleotide
ADENINE
H 3 C
C
O
acetyl group
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 O
N C C C C O P O
H OH CH 3 H O –
coenzyme A (CoA)
O
P
O –
O CH 2
RIBOSE
O
– O P O
O –
Figure 3–37 Acetyl coenzyme A (CoA) is
another important activated carrier.
A ball-and-stick model is shown above the
structure of acetyl CoA. The sulfur atom
(orange) forms a thioester bond to acetate.
Because the thioester bond is a high-energy
linkage, it releases a large amount of free
energy when it is hydrolyzed. Thus the
acetyl group carried by CoA can be readily
transferred to other molecules.