Biochemistry, 4th Edition P77 ppsx

24.1 How Are Fatty Acids Synthesized? 723
(Levels of free fatty acids are very low in the typical cell. The palmitate made in this
process is rapidly converted to CoA esters in preparation for the formation of tri-
acylglycerols and phospholipids.)
Cells Must Provide Cytosolic Acetyl-CoA and Reducing Power
for Fatty Acid Synthesis
Eukaryotic cells face a dilemma in providing suitable amounts of substrate for
fatty acid synthesis. Sufficient quantities of acetyl-CoA, malonyl-CoA, and
NADPH must be generated in the cytosol for fatty acid synthesis. Malonyl-CoA is
made by carboxylation of acetyl-CoA, so the problem reduces to generating suf-
ficient acetyl-CoA and NADPH.
There are three principal sources of acetyl-CoA (Figure 24.1):
1. Amino acid degradation produces cytosolic acetyl-CoA.
2. Fatty acid oxidation produces mitochondrial acetyl-CoA.
3. Glycolysis yields cytosolic pyruvate, which (after transport into the mitochondria)
is converted to acetyl-CoA by pyruvate dehydrogenase.
Fatty acyl-
carnitine
Citrate
Fatty acyl-
carnitine
Citrate
Amino acid catabolism
Malate
Malate
Oxaloacetate
Oxaloacetate
Fatty acid
oxidation
Mitochondrial matrix Cytosol
Fatty

acids
Fatty
acids
Amino
acids
+
+
Inner
mitochondrial
membrane
Pyruvate Pyruvate
Glycolysis
Glucose
TCA
cycle
ATP
CO
2
CO
2
CO
2
NAD
+
NAD
+
NAD
+
ADP P
i

NADH
NADH
NADH
NAD
+
NADH
NADP
+
NADPH
Acetyl-CoA
Acetyl-CoA
Fatty acyl-CoA
Malic
enzyme
Malate
dehydrogenase
Malate
dehydrogenase
Pyruvate
carboxylase
Pyruvate
dehydrogenase
ATP-citrate
lyase
Citrate
synthase
FIGURE 24.1 The citrate–malate–pyruvate shuttle provides cytosolic acetate units and some reducing equiva-
lents (electrons) for fatty acid synthesis.The shuttle collects carbon substrates, primarily from glycolysis but
also from fatty acid oxidation and amino acid catabolism. Pathways that provide carbon for fatty acid synthe-
sis are shown in blue; pathways that supply electrons for fatty acid synthesis are shown in red.

724 Chapter 24 Lipid Biosynthesis
The acetyl-CoA derived from amino acid degradation is normally insufficient for
fatty acid biosynthesis, and the acetyl-CoA produced by pyruvate dehydrogenase
and by fatty acid oxidation cannot cross the mitochondrial membrane to participate
directly in fatty acid synthesis. Instead, acetyl-CoA is linked with oxaloacetate to
form citrate, which is transported from the mitochondrial matrix to the cytosol (Fig-
ure 24.1). Here it can be converted back into acetyl-CoA and oxaloacetate by
ATP–citrate lyase. In this manner, mitochondrial acetyl-CoA becomes the substrate
for cytosolic fatty acid synthesis. (Oxaloacetate returns to the mitochondria in the
form of either pyruvate or malate, which is then reconverted to acetyl-CoA and
oxaloacetate, respectively.)
NADPH can be produced in the pentose phosphate pathway as well as by malic
enzyme (Figure 24.1). Reducing equivalents (electrons) derived from glycolysis in
the form of NADH can be transformed into NADPH by the combined action of
malate dehydrogenase and malic enzyme:
Oxaloacetate ϩ NADH ϩ H
ϩ
⎯⎯→malate ϩ NAD
ϩ
Malate ϩ NADP
ϩ
⎯⎯→pyruvate ϩ CO
2
ϩ NADPH ϩ H
ϩ
How many of the 14 NADPH needed to form one palmitate (see equation on
page 722) can be made in this way? The answer depends on the status of malate. Every
citrate entering the cytosol produces one acetyl-CoA and one malate (Figure 24.1).
Every malate oxidized by malic enzyme produces one NADPH, at the expense of a de-
carboxylation to pyruvate. Thus, when malate is oxidized, one NADPH is pro-

duced for every acetyl-CoA. Conversion of 8 acetyl-CoA units to one palmitate would
then be accompanied by production of 8 NADPH. (The other 6 NADPH required, as
shown in the equation on page 722, would be provided by the pentose phosphate
pathway.) On the other hand, for every malate returned to the mitochondria, one
NADPH fewer is produced.
Acetate Units Are Committed to Fatty Acid Synthesis by Formation
of Malonyl-CoA
Rittenberg and Bloch showed in the late 1940s that acetate units are the building
blocks of fatty acids. Their work, together with the discovery by Salih Wakil that bi-
carbonate is required for fatty acid biosynthesis, eventually made clear that this
pathway involves synthesis of malonyl-CoA. The carboxylation of acetyl-CoA to form
malonyl-CoA is essentially irreversible and is the committed step in the synthesis of
fatty acids (Figure 24.2). The reaction is catalyzed by acetyl-CoA carboxylase, which
contains a biotin prosthetic group. This carboxylase is the only enzyme of fatty acid
synthesis in animals that is not part of the multienzyme complex called fatty acid
synthase.
Acetyl-CoA Carboxylase Is Biotin-Dependent and Displays
Ping-Pong Kinetics
The biotin prosthetic group of acetyl-CoA carboxylase is covalently linked to the
⑀-amino group of an active-site lysine in a manner similar to pyruvate carboxylase
(see Figure 22.2). The reaction mechanism is also analogous to that of pyruvate car-
boxylase (see Figure 22.3): ATP-driven carboxylation of biotin is followed by trans-
fer of the activated CO
2
to acetyl-CoA to form malonyl-CoA. The enzyme from
Escherichia coli has three subunits: (1) a biotin carboxyl carrier protein (a dimer of
22.5-kD subunits); (2) biotin carboxylase (a dimer of 51-kD subunits), which adds
CO
2
to the prosthetic group; and (3) carboxyltransferase (an ␣

2
␤
2
tetramer with
30- and 35-kD subunits), which transfers the activated CO
2
unit to acetyl-CoA.
The long, flexible biotin–lysine chain (biocytin) enables the activated carboxyl
group to be carried between the biotin carboxylase and the carboxyltransferase
(Figure 24.3).
Biotin carboxylase
domain of human acetyl-CoA
carboxylase 2 (
p
db id = 2HJW)
The biotin carboxylase domain from human acetyl-CoA
carboxylase 2, with the A-subdomain in blue, the
B-subdomain in red, the A–B linker in green, and the
C-subdomain in yellow.
24.1 How Are Fatty Acids Synthesized? 725
Acetyl-CoA Carboxylase in Animals Is a Multifunctional Protein
In animals, acetyl-CoA carboxylase (ACC) is a filamentous polymer (4 to 8 ϫ 10
6
D)
composed of 265-kD protomers. Each of these subunits contains the biotin carboxyl
carrier moiety, biotin carboxylase, and carboxyltransferase activities, as well as al-
losteric regulatory sites. Animal ACC is thus a multifunctional protein. The polymeric
form is active, but the 265-kD protomers are inactive. The activity of ACC is thus de-
pendent upon the position of the equilibrium between these two forms:
Inactive protomers 34 active polymer

Because this enzyme catalyzes the committed step in fatty acid biosynthesis, it is
carefully regulated. Palmitoyl-CoA, the final product of fatty acid biosynthesis, shifts
the equilibrium toward the inactive protomers, whereas citrate, an important allosteric
activator of this enzyme, shifts the equilibrium toward the active polymeric form of
the enzyme. Acetyl-CoA carboxylase shows the kinetic behavior of a Monod–Wyman–
Changeux V-system allosteric enzyme in which allosteric effectors shift the T/R equi-
librium between active R conformers and inactive T conformers.
Carboxyltransferase domain of
yeast acetyl-CoA carboxylase
(
p
db id = 1OD2)
The carboxyltransferase domain dimer of acetyl-CoA
carboxylase-1 from Saccharomyces cerevisiae. The N-
and C-subdomains of one monomer are cyan and yel-
low, whereas those of the other monomer are purple
and green. CoA is shown as a ball-and-stick model in
one subunit.
+
–
HCO
3
– –
O C O
O
P
O
O
–
O

–
O
NNH
S
O
HN NH
S
–
O C
O
O
NNH
S
–
O
C
O
C SCoA
O
H
2
C
H
2
C
C SCoA
O
COO
–
+ HN NH

S
Lys
Lys
Lys
Lys
O
CH
2
–
O
–
C
C SCH
3
CoA
O
++HCO
3
O
C S CoA
O
+
+
+
H
+
P
i
P
i

ATP
ATP
ADP
ADP
Step 1
The carboxylation of biotin
Step 2
The transcarboxylation reaction
Biotin
(a)
(b)
ACTIVE FIGURE 24.2 (a) The acetyl-
CoA carboxylase reaction produces malonyl-CoA for
fatty acid synthesis. (b) A mechanism for the acetyl-CoA
carboxylase reaction. Bicarbonate is activated for car-
boxylation reactions by formation of N-carboxybiotin.
ATP drives the reaction forward, with transient forma-
tion of a carbonylphosphate intermediate (Step 1). In a
typical biotin-dependent reaction, nucleophilic attack
by the acetyl-CoA carbanion on the carboxyl carbon of
N-carboxybiotin—a transcarboxylation—yields the
carboxylated product (Step 2). Test yourself on
the concepts in this figure at www.cengage.com/
login.
726 Chapter 24 Lipid Biosynthesis
Phosphorylation of ACC Modulates Activation by Citrate
and Inhibition by Palmitoyl-CoA
The regulatory effects of citrate and palmitoyl-CoA are dependent on the phosphorylation state of
acetyl-CoA carboxylase. The animal enzyme is phosphorylated at eight to ten sites on
each enzyme subunit (Figure 24.4). Some of these sites are regulatory, whereas oth-

ers are “silent” and have no effect on enzyme activity. Unphosphorylated acetyl-CoA
carboxylase binds citrate with high affinity and thus is active at very low citrate con-
O
HN NH
S
O
C
N
H
O
C
SCoA
CH
3
C
O
–
O
O
PO
3
2
–
O
N
NH
S
O
C
N

H
O
C
SCoA
CH
2
C
–
O
O
N
NH
S
C
N
H
C
O
–
O
O
N
NH
S
C
N
C
O
–
O

O
N
NH
S
C
N
H
C
O
–
O
–
P
O
O
O
Biotin carboxyl
carrier protein
Biotin carboxylase Carboxyltransferase
FIGURE 24.3 In the acetyl-CoA carboxylase reaction, the
biotin ring, on its flexible tether, acquires carboxyl
groups from carbonylphosphate on the biotin
carboxylase subunit and transfers them to acyl-CoA
molecules on the carboxyltransferase subunits. Colors
of the domains correspond to those in Figure 24.4.
P
P
P
P
P

P
P
1
83
621
661
821
1200
1574
2346
cAMP-dependent protein kinase (PKA),
AMP-dependent protein kinase (AMPK)
95 Protein kinase C (PKC)
Carboxyl-
transferase
BCCP
Biotin
carboxylase
77
AMP-dependent protein kinase (AMPK)
76
cAMP-dependent protein kinase (PKA),
protein kinase C (PKC)
29
25
23
Casein kinase II
Calmodulin-dependent protein kinase
Residue
number

FIGURE 24.4 Schematic of the acetyl-CoA carboxylase
polypeptide, with domains and phosphorylation sites
indicated, along with the protein kinases responsible.
Phosphorylation at Ser
1200
is primarily responsible for
decreasing the affinity for citrate.
24.1 How Are Fatty Acids Synthesized? 727
centrations (Figure 24.5). Phosphorylation of the regulatory sites decreases the affin-
ity of the enzyme for citrate, and in this case, high levels of citrate are required to
activate the carboxylase. The inhibition by fatty acyl-CoAs operates in a similar but op-
posite manner. Thus, low levels of fatty acyl-CoA inhibit the phosphorylated carboxy-
lase, but the dephosphoenzyme is inhibited only by high levels of fatty acyl-CoA. Spe-
cific phosphatases act to dephosphorylate ACC, thereby increasing the sensitivity to
citrate.
Acyl Carrier Proteins Carry the Intermediates in Fatty Acid Synthesis
The basic building blocks of fatty acid synthesis are acetyl and malonyl groups, but
they are not transferred directly from CoA to the growing fatty acid chain. Rather,
they are first passed to ACP. This protein consists (in E. coli) of a single polypeptide
chain of 77 residues to which is attached (on a serine residue) a phosphopante-
theine group, the same group that forms the “business end” of coenzyme A. Thus,
ACP is a somewhat larger version of coenzyme A, specialized for use in fatty acid
biosynthesis (Figure 24.6).
In Some Organisms, Fatty Acid Synthesis Takes Place
in Multienzyme Complexes
The enzymes that catalyze formation of acetyl-ACP and malonyl-ACP and the subse-
quent reactions of fatty acid synthesis are organized quite differently in different or-
ganisms. Fatty acid synthesis in mammals occurs on homodimeric fatty acyl synthase
I (FAS I), each 270-kD polypeptide of which contains all reaction centers required to
produce a fatty acid. In lower eukaryotes, such as yeast and fungi, the enzymatic

P
P
P
P
P
P
P
P
P
ATP
Dephospho-acetyl-CoA carboxylase
(Low [citrate] activates, high [fatty acyl-CoA]
inhibits)
H
2
O
P
i
PhosphatasesKinases
Phospho-acetyl-CoA carboxylase
(High [citrate] activates, low [fatty acyl-CoA]
inhibits)
ADP
FIGURE 24.5 The activity of acetyl-CoA carboxylase is
modulated by phosphorylation and dephosphorylation.
The dephospho form of the enzyme is activated by low
[citrate] and inhibited only by high levels of fatty acyl-
CoA. In contrast, the phosphorylated form of the enzyme
is activated only by high levels of citrate but is very sensi-
tive to inhibition by fatty acyl-CoA.

CH
2
H
CH
3
O
O
P
O
–
HS CH
2
N
H
C
O
CH
2
CH
2
N
H
C
O
C
HO
C
CH
3
CH

2
O
O
O
P
O
–
CH
2
HH
HH
O
Adenine
OH
2–
O
3
PO
CH
2
H
CH
3
O
O
P
O
–
HS CH
2

N
H
C
O
CH
2
CH
2
N
H
C
O
C
HO
C
CH
3
CH
2
O CH
2
Ser Acyl carrier protein
Phosphopantetheine group of coenzyme A
Phosphopantetheine prosthetic group of ACP
FIGURE 24.6 Fatty acids are conjugated both to coenzyme A and to acyl carrier protein through the sulfhydryl
of phosphopantetheine prosthetic groups.
A DEEPER LOOK
Choosing the Best Organism for the Experiment
The selection of a suitable and relevant organism is an important
part of any biochemical investigation. The studies that revealed

the secrets of fatty acid synthesis are a good case in point.
The paradigm for fatty acid synthesis in plants has been the avo-
cado, which has one of the highest fatty acid contents in the plant
kingdom. Early animal studies centered primarily on pigeons, which
are easily bred and handled and which possess high levels of fats in
their tissues. Other animals, richer in fatty tissues, might be even
more attractive but more challenging to maintain. Grizzly bears, for
example, carry very large fat reserves but are difficult to work with in
the lab!
728 Chapter 24 Lipid Biosynthesis
activities of FAS are distributed on two multifunctional peptide chains, which form
2.6-megadalton ␣
6
␤
6
complexes. In plants, most bacteria, and parasites, the enzymes
of fatty acid synthesis are separated and independent, and this collection of enzymes
is referred to as fatty acyl synthase II (FAS II).
The individual steps in the elongation of the fatty acid chain are quite similar
across all organisms. The mammalian pathway (Figure 24.7) is a cycle of elongation
that involves six enzyme activities. The elongation cycle is initiated by transfer of the
␤-Ketoacyl
synthase
MAT
MAT
Thioesterase
␤-Ketoacyl-ACP
reductase
Palmitate
␤-Hydroxyacyl-

ACP dehydratase
␤-Enoyl-ACP
reductase
CoASH
KR
KR
DH
DH
ER
ER
TE
KS
KS
KS
56
234
7
COO
–
1
Acetyl-CoA
O
CH
3
C
S-CoA
O
CH
3
CO

2
CO
2
C
S-KSase
Malonyl-CoA
COO
–
ACP-SH
O
CH
2
C
S-CoA
COO
–
O
CH
2
C
S-ACP
O
CH
3
C
S-ACP
S ACP
Acetoacetyl-ACP
O
CH

3
C
O
CH
2
C
CH
3
C
O
CH
2
C
OH
H
S ACP
D-␤-Hydroxybutyryl-ACP
CH
3
C
H
C
H
O
C S ACP
Crotonyl-ACP
CH
3
O
CH

2
CH
2
C S ACP
Butyryl-ACP
NADP
+
NADPH
+ H
+
H
2
O
H
2
O
NADP
+
NADPH
+ H
+
␤-Hydroxyacyl-ACP
␤-Ketoacyl-ACP
␤-Enoyl-ACP
Acyl (C
n+2
)-ACP
NADP
+
NADPH

+ H
+
NADP
+
NADPH + H
+
FIGURE 24.7 The pathway of palmitate synthesis from acetyl-CoA and
malonyl-CoA. Acetyl and malonyl building blocks are introduced as
ACP conjugates. Decarboxylation drives the ␤-ketoacyl-ACP synthase
and results in the addition of two-carbon units to the growing chain.
The first turn of the cycle begins at ➊ and goes to butryrl-ACP; subse-
quent turns of the cycle are indicated as ➋ through ➏.
24.1 How Are Fatty Acids Synthesized? 729
acyl moiety of acetyl-CoA to the acyl carrier protein by the malonyl-CoA–acetyl-CoA-
ACP transacylase (MAT), which also transfers the malonyl group of malonyl-CoA
to ACP.
Decarboxylation Drives the Condensation of Acetyl-CoA
and Malonyl-CoA
The ␤-ketoacyl-ACP synthase (KS) catalyzes the decarboxylative condensation of the
acyl group with malonyl-ACP to produce a ␤-ketoacyl-ACP intermediate (acetoacetyl-
ACP in the first cycle). The mechanism (Figure 24.8) begins with acetyl group trans-
fer to MAT, followed with attack by the ACP thiol sulfur to form an acetyl-ACP. The
acetyl group is transferred to a cysteine sulfur on KS, freeing the ACP thiol to acquire
the malonyl group. In the condensation reaction that follows, decarboxylation of the
malonyl group creates a transient, highly nucleophilic carbanion that can attack the
acetate group.
The net reaction for each turn of this cycle (see Figure 24.7) is addition of a two-
carbon unit to the acyl group. Why is the three-carbon malonyl group used here as a two-
carbon donor? The answer is that this is yet another example of a decarboxylation
driving a desired but otherwise thermodynamically unfavorable reaction. The de-

carboxylation that accompanies the reaction with malonyl-ACP drives the synthesis
of acetoacetyl-ACP. Note that hydrolysis of ATP drove the carboxylation of acetyl-
CoA to form malonyl-ACP, so, indirectly, ATP is responsible for the condensation re-
action to form acetoacetyl-ACP. Malonyl-CoA can be viewed as a form of stored en-
ergy for driving fatty acid synthesis.
It is also worth noting that the carbon of the carboxyl group that was added to
drive this reaction is the one removed by the condensing enzyme. Thus, all the car-
bons of acetoacetyl-ACP (and of the fatty acids to be made) are derived from acetate
units of acetyl-CoA.
Reduction of the ␤-Carbonyl Group Follows a Now-Familiar Route
The next three steps—reduction of the ␤-carbonyl group by ␤-ketoacyl-ACP re-
ductase (KR) to form a ␤-alcohol, then dehydration by ␤-hydroxyacyl-ACP dehy-
dratase (DH) and reduction by 2,3-trans-enoyl-ACP reductase (ER) to saturate the
chain (see Figure 24.7)—look very similar to the fatty acid degradation pathway in
reverse. However, there are two crucial differences between fatty acid biosynthesis
and fatty acid oxidation (besides the fact that different enzymes are involved): First,
the alcohol formed in biosynthesis has the
D-configuration rather than the L-form
MAT
H
3
CC
C
CH
3
SH
ACP
O
MAT
O

S
ACP
SCoA
H
3
CO
HS KS
C
OO
–
MAT
OH
–
OOC CH
2
C SCoA
O
C
CH
2
O
MAT
O
COO
–
C
CH
3
SH
ACP

O
KS
S
C
CH
2
O
C
S
ACP
C
CH
2
CH
3
O
C
KS
SH
S
ACP
O
C
CH
3
O
KS
S
OH
O

CO
2
FIGURE 24.8 A mechanism for mammalian ketoacyl synthase. An acetyl group
is transferred from CoA to MAT, then to the acyl carrier protein, and then to
ketoacyl synthase. Next, a malonyl group is transferred to MAT and then to the
acyl carrier protein. Decarboxylation of the malonyl group creates a transient
carbanion on the acyl group of ACP, which attacks the KS acetyl group to form
a ketoacyl-ACP. A cycle (see Figure 24.7) of keto group reduction (by KR), water
removal (by DH), and double bond reduction (by ER; see next section) will
finally produce an acyl group increased in length by two carbons.
730 Chapter 24 Lipid Biosynthesis
seen in catabolism; second, the reducing coenzyme is NADPH, whereas NAD
ϩ
and
FAD are the oxidants in the catabolic pathway.
The net result of the first turn of the biosynthetic cycle is the synthesis of a four-
carbon unit, a butyryl group, from two smaller building blocks. In the next cycle of
the process, this butyryl-ACP condenses with another malonyl-ACP to make a six-
carbon ␤-ketoacyl-ACP and CO
2
. Subsequent reduction to a ␤-alcohol, dehydration,
and another reduction yield a six-carbon saturated acyl-ACP. This cycle continues
with the net addition of a two-carbon unit in each turn until the chain is 16 carbons
long (see Figure 24.7). The KS cannot accommodate larger substrates, so the reac-
tion cycle ends with a 16-carbon chain. Hydrolysis of the C
16
-acyl-ACP yields a
palmitic acid and the free ACP.
In the end, seven malonyl-CoA molecules and one acetyl-CoA yield a palmitate
(shown here as palmitoyl-CoA):

Acetyl-CoA ϩ 7 malonyl-CoA
Ϫ
ϩ 14 NADPH ϩ 14 H
ϩ
⎯⎯→
palmitoyl-CoA ϩ 7 HCO
3
Ϫ
ϩ 14 NADP
ϩ
ϩ 7 CoASH
The formation of seven malonyl-CoA molecules requires
7 Acetyl-CoA ϩ 7 HCO
3
Ϫ
ϩ 7 ATP
4Ϫ
⎯⎯→
7 malonyl-CoA
Ϫ
ϩ 7 ADP
3Ϫ
ϩ 7 P
i
2Ϫ
ϩ 7 H
ϩ
Thus, the overall reaction of acetyl-CoA to yield palmitic acid is
8 Acetyl-CoA ϩ 7 ATP
4Ϫ

ϩ 14 NADPH ϩ 7 H
ϩ
⎯⎯→
palmitoyl-CoA ϩ 14 NADP
ϩ
ϩ 7 CoASH ϩ 7 ADP
3Ϫ
ϩ 7 P
i
2Ϫ
Note: These equations are stoichiometric and are charge balanced. See problem 1
at the end of the chapter for practice in balancing these equations.
Eukaryotes Build Fatty Acids on Megasynthase Complexes
The multiple enzyme domains of eukaryotic fatty acyl synthases are arrayed on large
protein structures termed megasynthases. The individual enzyme domains of these
KS
KS
2
␣
MAT
MAT
MAT
ER
A
C
P
TEKR
KR
KR
DH

DH
DH
DH
DH
␣
PPTKR
KR
2
KR
2
␤
AT
AT
AT
MPT
MPT
MPT
MPT
MPT
ER
ER
DH
DH
DH
KS
KS
2
Reaction chamber
Reaction chamber
ER

ER
2
Fungal fatty acid synthase Mammalian fatty acid synthase
Reaction
chamber
Reaction
chamber
ϭ Active sites
A
T: Acetyl transferase
MPT: Malonyl/palmitoyl transferase
MAT: Malonyl-CoA–acetyl-CoA-ACP
transacylase
TE: Thioesterase
A
CP: Acyl carrier protein
PPT: Phosphopantetheinyl
transferase
KR: ␤-Ketoacyl reductase
KS: ␤-Ketoacyl synthase
ER: ␤-Enoyl reductase
DH: Dehydratase
A
C
P
FIGURE 24.9 Organization of enzyme functions on two eukaryotic fatty acid synthases. (left) Fungal FAS is a
closed barrel 260 Å high and 230 Å wide. (right) Mammalian (pig) FAS is an asymmetric X-shape 210 Å high,
180 Å wide, and 90 Å deep.The arrangement of functional domains along the FAS polypeptides is shown at
the bottom of the figure. KS domains form dimers in both structures. KR domains form dimers in the fungal
enzyme, whereas ER and DH domains form dimers in the mammalian complex.

24.1 How Are Fatty Acids Synthesized? 731
structures in all eukaryotes are homologous to the corresponding small, discrete en-
zymes of bacterial FAS pathways. Remarkably, however, lower eukaryotes such as
fungi and higher eukaryotes such as mammals have evolved entirely different
megasynthase architectures for fatty acid synthesis. Mammalian homodimeric FAS
has a flattened X-shape, whereas the fungal dodecameric FAS is a large, closed bar-
rel, with two reaction chambers separated by equatorial stabilizing struts (Figure
24.9). In the fungal structure, the six ␣-subunits form a central ring that is a “trimer
of dimers” (Figure 24.10a,b). Each ␣-subunit contributes an extended ␣-helical seg-
ment to the center of the structure. Pairs of these helices form three coiled-coil
struts anchored by a six-helix bundle in the center of the barrel. Each ␣-subunit
(b) Central wheel(a)
(c)
Tilted side view
Lower
reaction
chamber
KS dimer
Upper
reaction
chamber
Interchamber
opening
L1
L2
ACP
PPT
Central
anchor
Peripheral

anchor
KR dimer
90°
FIGURE 24.10 (a) FAS from S. cerevisiae possesses two trimeric reaction chambers
separated by equatorial stabilizing struts. ACP domains are located at the equa-
torial base of each reaction chamber, close to the catalytic site of KS. (b) The
equatorial base of this structure is an ␣
6
trimer of dimers, with alternating KS and
KR domains.The central stabilizing struts are ␣-helical extensions of the function-
al domains arranged around the outside of the ring. A central six-helix bundle
stabilizes the structure. (c) The ACP domains (red) are tethered on flexible linkers
(yellow) so that they can move from one active site to the next in the catalytic
cycle. Comparison of the ACP domains of S.cerevisiae and E.coli reveals that the
ACP domains probably extend the acyl-phosphopantetheine group to active
sites but then retract the acyl group into a hydrophobic cleft while moving from
one site to the next (pdb ϭ 2UV8; images courtesy of Marc Leibundgut and
Nenad Ban, ETH [Zurich]).
732 Chapter 24 Lipid Biosynthesis
contains KR and KS domains. Three KR and three KS active sites are oriented to-
ward the upper reaction chamber, and three of each face the lower chamber. The
␤-subunit trimers form rounded caps over the upper and lower reaction chambers.
Each chamber contains three pores that allow substrates (acetyl-CoA and malonyl-
CoA) to diffuse in and palmitoyl-CoA to exit. On each end of the structure, the ac-
tive sites of the four ␤-subunit enzyme domains (see Figure 24.9) are oriented to-
ward the interior of the reaction chamber. Three ACP domains in each chamber
shuttle growing acyl chains from site to site during the catalytic cycle. Each ACP is
tethered by two flexible linker peptides, which facilitate its site-to-site movement
(Figure 24.10c). The phosphopantetheine arm on each ACP can extend outward to
reach into active sites or may retract to insert its acyl chain in a protective hy-

drophobic cavity during intersite transport.
The homodimeric mammalian FAS contains all six functional enzyme domains
on each subunit (Figures 24.9 and 24.11). In the X-shaped dimer, three of the
domains—including KS, ER, and DH—form dimeric structures, whereas the KR
and MAT domains are separated and lie near the ends of the extended “arms.” The
arms form reaction chambers on either side of the structure. The flexible ACP do-
mains do not appear in this structure (probably because they are not fixed in any
one position in the crystals used for the structural studies). However, since it follows
the KR domain in the polypeptide sequence, the ACP domain probably lies at the
end of each KR arm, where it can rotate to interact with the adjacent active sites.
In both the fungal and the mammalian FAS structures, the close association of
enzymic domains within one large complex permits efficient transfer of intermedi-
ates from one active site to the next. In addition, the presence of all these enzyme
domains on one or two polypeptides allows the cell to coordinate synthesis of all en-
zymes needed for fatty acid synthesis.
KR
KR
DH
DH
KS
2
MAT
MAT
ER
2
FIGURE 24.11 Structural studies reveal that mammalian FAS homodimer is X-shaped. The ACP domains are
probably located adjacent to the KR domains at the ends of the arms (pdb id ϭ 2VZ8; image courtesy of Marc
Leibundgut and Nenad Ban, ETH [Zurich]).