The production of the four-carbon, saturated fatty acyl-ACP completes one
pass through the fatty acid synthase complex. The butyryl group is now
transferred from the phosphopantetheine -SH group of ACP to the Cys -SH group of
β-ketoacyl-ACP synthase, which initially bore the acetyl group (Fig. 20-5). To
start the next cycle of four reactions that lengthens the chain by two more
carbons, another malonyl group is linked to the now unoccupied
phosphopantetheine -SH group of ACP (Fig. 20-6). Condensation occurs as the
butyryl group, acting exactly as did the acetyl group in the first cycle, is
linked to two carbons of the malonyl-ACP group with concurrent loss of
CO
2. The product of this condensation is a six-carbon
acyl group, covalently bound to the phosphopantetheine -SH group. Its β-keto
group is reduced in the next three steps of the synthase cycle to yield the
six-carbon saturated acyl group, exactly as in the first round of reactions.
Seven cycles of condensation and reduction produce the 16-carbon saturated
palmitoyl group, still bound to ACP. For reasons not well understood, chain
elongation generally stops at this point, and free palmitate is released from
the ACP molecule by the action of a hydrolytic activity in the synthase complex.
Small amounts of longer fatty acids such as stearate (18:0) are also formed. In
certain plants (coconut and palm, for example) chain termination occurs earlier;
up to 90% of the fatty acids in the oils of these plants are between 8 and 14
carbons long.
The overall reaction for the synthesis of palmitate from acetyl-CoA can be
broken down into two parts. First, the formation of seven malonyl-CoA
molecules:
7 Acetyl-CoA + 7CO
2 + 7ATP
![]()
-------->7 malonyl-CoA + 7ADP +
7P
i (20-1)
then seven cycles of condensation and reduction:
Acetyl-CoA + 7 malonyl-CoA + l4NADPH +
14H
+ ![]()
-------------->
palmitate + 7C0
2 + 8CoA +
l4NADP
+ + 6H
2O (20-2) The
overall process (the sum of Eqns 20-1 and 20-2) is
8 Acetyl-CoA + 7ATP + l4NADPH + 14H
+
![]()
------------> palmitate + 8CoA +
6H
2O + 7ADP + 7P
i +
l4NADP
+ (20-3)
The biosynthesis of fatty acids such as palmitate thus requires acetyl-CoA
and the input of chemical energy in two forms: the group transfer potential of
ATP and the reducing power of NADPH. The ATP is required to attach
CO
2 to acetyl-CoA to make malonyl-CoA; the NADPH is
required to reduce the double bonds. We shall return to the sources of
acetyl-CoA and NADPH soon, but let us first consider the structure of the
remarkable enzyme complex that catalyzes the synthesis of fatty acids.
The Fatty Acid Synthase of Some Organisms Is Composed of
Multifunctional Proteins
We noted earlier that the seven active sites for fatty acid synthesis (six
enzymes and ACP) reside in seven separate polypeptides in the fatty acid
synthase of E. coli; the same is true of the enzyme complex from higher plants
(Fig. 20-7). In these complexes each enzyme is positioned with its active site
near that of the preceding and succeeding enzymes of the sequence. The flexible
pantetheine arm of ACP can reach all of the active sites, and it carries the
growing fatty acyl chain from one site to the next; the intermediates are not
released from the enzyxne complex until the finished product is obtained. As in
the cases we have encountered in earlier chapters, this channeling of
intermediates from one active site to the next increases the efficiency of the
overall process.
The fatty acid synthases of yeast and of vertebrates are also
multienzyme complexes, but their integration is even more complete than in E.
coli and plants. In yeast, the seven distinct active sites reside in only two
large, multifunctional polypeptides, and in vertebrates, a single large
polypeptide (Mr 240,000) contains all seven enzymatic
activities as well as a hydrolytic activity that cleaves the fatty acid from the
ACP-like part of the enzyme complex. The active form of this multifunctional
protein is a dimer (Mr 480,000).
Fatty Acid Synthesis Occurs in the Cytosol of Many Organisms but in
the Chloroplasts of Plants
In mammals, the fatty acid synthase complex is found exclusively in the
cytosol (Fig. 20-8), as are the biosynthetic enzymes for nucleotides, amino
acids, and glucose. This location segregates synthetic processes from
degradative reactions, many of which take place in the mitochondrial matrix.
There is a corresponding segregation of electron-carrying cofactors for
anabolism (generally a reductive process) and those for catabolism (generally
oxidative). Usually, NADPH is the electron carrier for anabolic reactions, and
NAD+ serves in catabolic reactions. In hepatocytes,
the ratio [NADPH]/[NADP+] is very high (about 75) in
the cytosol, furnishing a strongly reducing environment for the reductive
synthesis of fatty acids and other biomolecules. Because the cytosolic
[NADH]/[NAD+] ratio is much smaller (only about 8
×10-4), the NAD+-dependent oxidative catabolism of
glucose can occur in the same compartment, at the same time, as fatty acid
synthesis. The [NADH]/ [NAD+] ratio within the
mitochondrion is much higher than in the cytosol because of the flow of
electrons into NAD+ from the oxidation of fatty acids,
amino acids, pyruvate, and acetyl-CoA. This high [NADH]/
[NAD+] ratio favors the reduction of oxygen via the
respiratory chain.
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In adipocytes cytosolic NADPH is largely generated by malic enzyme
(Fig. 20-9). (We encountered an NAD-linked malic enzyme in the carbon fixation
pathway of C4 plants (see Fig. 19-37); this enzyme is unrelated in function.)
The pyruvate produced in this reaction reenters the mitochondrion. In
hepatocytes and in the mammary gland of lactating animals, the NADPH required
for fatty acid biosynthesis is supplied primarily by the reactions of the
pentose phosphate pathway
In the photosynthetic cells of plants, fatty acid synthesis occurs not in the
cytosol, but in the chloroplast stroma (Fig. 20-8). This location makes sense
when we recall that NADPH is produced in chloroplasts by the light reactions of
photosynthesis (Fig. 20-10). Again, the resulting high [NADPH I/[NADP+] ratio
provides the reducing environment that favors reductive anabolic processes such
as fatty acid synthesis.
light H2O +
NADP+ > z0~ + NADPH + H~
Figure 20-10 Production of NADPH by
photosynthesis.
Acetate Is Shuttled out of Mitochondria as Citrate
In nonphotosynthetic eukaryotes, nearly all the acetyl-CoA used in fatty acid
synthesis is formed in mitochondria from pyruvate oxidation and from the
catabolism of the carbon skeletons of amino acids. AcetylCoA arising from the
oxidation of fatty acids does not represent a significant source of acetyl-CoA
for fatty acid biosynthesis in animals because the two pathways are regulated
reciprocally, as described below. Because the mitochondrial inner membrane is
impermeable to acetylCoA, an indirect shuttle transfers acetyl group equivalents
across the inner membrane (Fig. 20-11). Intramitochondrial acetyl-CoA first
reacts with oxaloacetate to form citrate, in the citric acid cycle reaction
catalyzed by citrate synthase (see Fig. 15-7). Citrate then
passes into the cytosol through the mitochondrial inner membrane on the
tricarboxylate transporter. In the cytosol, citrate cleavage by
citrate lyase regenerates acetyl-CoA; this reaction is driven by the investment
of energy from ATP. Oxaloacetate cannot return to the matrix directly; there is
no transporter for it. Instead, oxaloacetate is reduced by cytosolic malate
dehydrogenase to malate, which returns to the mitochondrial matrix on the
malate-a-ketoglutarate transporter in exchange for citrate, and is reoxidized to
oxaloacetate to complete the shuttle. Alternatively, the malate produced in the
cytosol is used to generate cytosolic NADPH through the activity of malic
enzyme, as described above.
Plants have another means of acquiring acetyl-CoA for fatty acid synthesis.
They produce acetyl-CoA from pyruvate using a stromal isozyme of
pyruvate dehydrogenase (see Fig. 15-2).
When a cell or organism has more than enough metabolic fuel available to meet
its energetic needs, the excess is generally converted to fatty acids and stored
as lipids such as triacylglycerols. The reaction catalyzed by acetyl-CoA
carboxylase is the rate-limiting step in the biosynthesis of fatty acids, and
this enzyme is an important site of regulation. In vertebrates, palmitoyl-CoA,
the principal product of fatty acid synthesis, acts as a feedback inhibitor of
the enzyme, and citrate is an allosteric activator (Fig. 20-12a). When there is
an increase in the concentrations of mitochondrial acetyl-CoA and of ATP,
citrate is transported out of the mitochondria and becomes both the precursor of
cytosolic acetyl-CoA and an allosteric signal for the activation of acetyl-CoA
carboxylase.
Acetyl-CoA carboxylase is also regulated by covalent alteration.
Phosphorylation triggered by the hormones glucagon and epinephrine inactivates
it, thereby slowing fatty acid synthesis. In its active (dephosphorylated) form,
acetyl-CoA carboxylase polymerizes into long filaments (Fig. 20-12b);
phosphorylation is accompanied by dissociation into monomeric subunits and loss
of activity.
The acetyl-CoA carboxylase from plants and bacteria is not regulated by
citrate or by a phosphorylation-dephosphorylation cycle. The plant enzyme is
activated by an increase in stromal pH and Mg2+
concentration, both of which occur upon illumination of the plant (p. 630).
Bacteria do not use triacylglycerols as energy stores. The primary role of fatty
acid synthesis in E. coli is to provide precursors for membrane lipids, and the
regulation of this process is complex, involving certain guanine nucleotides
that coordinate cell growth with membrane formation.
Other enzymes in the pathway of fatty acid synthesis are also regulated. The
pyruvate dehydrogenase complex and citrate lyase, both of which supply
acetyl-CoA, are activated by insulin (Fig. 20-12a) through a cascade of protein
phosphorylation. Insulin and glucagon are released in response to blood glucose
concentrations that are too high or too low, respectively. Their broad metabolic
effects and molecular mechanisms will be discussed further in
If fatty acid synthesis and β oxidation were to occur simultaneously, the two
processes would constitute a futile cycle, wasting energy. We noted earlier (p.
496) that β oxidation is blocked by malonylCoA, which inhibits carnitine
acyltransferase I. Thus during fatty acid synthesis, the production of the first
intermediate, malonyl-CoA, shuts down β oxidation at the level of a transport
system in the mitochondrial inner membrane. This control mechanism illustrates
another advantage to a cell of segregating synthetic and degradative pathways in
different cellular compartments.
Long-Chain Fatty Acids Are Synthesized from Palmitate
Palmitate, the principal product of the fatty acid synthase system in animal
cells, is the precursor of other long-chain fatty acids (Fig. 2013). It may be
lengthened to form stearate (18:0) or even longer saturated fatty acids by
further additions of acetyl groups, through the action of fatty acid elongation
systems present in the smooth endoplasmic reticulum and the mitochondria. The
more active elongation system of the endoplasmic reticulum extends the 16-carbon
chain of palmitoyl-CoA by two carbons, forming stearoyl-CoA. Although different
enzyme systems are involved, and coenzyme A rather than ACP is the acyl carrier
directly involved in the reaction, the mechanism of elongation is otherwise
identical with that employed in palmitate synthesis: donation of two carbons by
malonyl-ACP, followed by reduction, dehydration, and reduction to the saturated
18-carbon product, stearoyl-CoA.
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