Fatty acid synthesis is regulated, both in plants and
animals. Excess carbohydrate and protein in the diet are converted into fat.
Only a relatively small amount of energy is stored in animals as glycogen or
other carbohydrates, and the level of glycogen is closely regulated.
Protein storage doesn’t take place in animals. Except for the
small amount that circulates in the cells, amino acids exist in the body only
in muscle or other protein-containing tissues. If the animal or human needs
specific amino acids, they must either be synthesized or obtained from the
breakdown of muscle protein. Adipose tissue serves as the major storage area
for fats in animals. A normal human weighing 70 kg contains about 160 kcal of
usable energy. Less than 1 kcal exists as glycogen, about 24 kcal exist as amino acids in muscle, and the
balance-more than 80 percent of the total-exists as fat. Plants make oils for
energy storage in seeds. Because plants must synthesize all their cellular
components from simple inorganic compounds, plants-but usually not animals-can
use fatty acids from these oils to make carbohydrates and amino acids for later
growth after germination.
Fatty Acid Biosynthesis
The biosynthetic reaction pathway to a compound is usually
not a simple opposite of its breakdown. Chapter 12 of Volume 1 discusses this
concept in regard to carbohydrate metabolism and gluconeogenesis. In fatty acid
synthesis, acetyl-CoA is the direct precursor only of the methyl end of the growing fatty acid
chain. All the other carbons come from the acetyl group of acetyl-CoA but only
after it is modified to provide the actual substrate for fatty acid synthase,
Malonyl-CoA contains a 3-carbon dicarboxylic acid, malonate, bound to Coenzyme A. Malonate is
formed from acetyl-CoA by the addition of CO2 using the biotin cofactor of the
enzyme acetyl-CoA carboxylase.
– Acetyl-CoA + HCO3
– + ATP Malonyl-CoA + ADP + Pi
Formation of malonyl-CoA is the commitment step for fatty
acid synthesis, because malonyl-CoA has no metabolic role other than serving as
a precursor to fatty acids.
Fatty acid synthase (FAS) carries out the chain elongation
steps of fatty acid biosynthesis. FAS is a large multienzyme complex. In
mammals, FAS contains two subunits, each containing multiple enzyme activities. In bacteria and
plants, individual proteins, which associate into a large complex, catalyze the
individual steps of the synthesis scheme.
Fatty acid synthesis starts with acetyl-CoA, and the chain
grows from the “tail end” so that carbon 1 and the alpha-carbon of the complete
fatty acid are added last. The first reaction is the transfer of the acetyl
group to a pantothenate group of acyl carrier protein (ACP), a region of the
large mammalian FAS protein. (The acyl carrier protein is a small, independent
peptide in bacterial FAS, hence the name).
The pantothenate group of ACP is the same as is found on
Coenzyme A, so the transfer requires no energy input: Acetyl~S-CoA + HS-ACP®
HS-CoA + Acetyl~S-ACP.
In the preceding reaction, the S and SH refer to the thio
group on the end of Coenzyme A or the pantothenate groups. The ~ is a reminder
that the bond between the carbonyl carbon of the acetyl group and the thio group is a “high
energy” bond (that is, the activated acetyl group is easily donated to an
acceptor). The second reaction is another transfer, this time, from the
pantothenate of the ACP to cysteine sulfhydral (–SH) group on FAS.
Acetyl~ACP + HS-FAS ® HS-ACP + Acetyl~S-FAS
Note that at this point, the FAS has two activated substrates,
the acetyl group bound on the cysteine –SH and the malonyl group bound on the
pantothenate –SH. Transfer of the 2-carbon acetyl unit from
Acetyl~S-cysteine to malonyl-CoA has two features:
Release of the CO2 group of malonyic acid that was originally
put on by acetyl-CoA carboxylase
Generation of a 4-carbon b-keto acid derivative, bound to the
pantothenate of the ACP protein
The ketoacid is now reduced to the methylene (CH2) state in a
three-step reaction sequence.
The elongated 4-carbon chain is now ready to accept a new
2-carbon unit from malonyl-CoA. The 2-carbon unit, which is added to the
growing fatty acid chain, becomes carbons 1 and 2 of hexanoic acid (6-carbons).
The cycle of transfer, elongation, reduction, dehydration,
and reduction continues until palmitoyl-ACP is made. Then the thioesterase
activity of the FAS complex releases the 16-carbon fatty acid palmitate from
Note that fatty acid synthesis provides an extreme example of
the phenomenon of metabolic channeling: neither free fatty acids with more than
four carbons nor their CoA derivatives can directly participate in the
synthesis of palmitate. Instead they must be broken down to acetyl-CoA and reincorporated into
the fatty acid.
Fatty acids are generated cytoplasmically while acetyl-CoA is
made in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttle
system must exist to get the acetyl-CoA or its equivalent out of the
mitochondrion. The shuttle system operates in the following:
way: Acetyl-CoA is first converted to citrate by citrate
synthase in the TCA-cycle reaction. Then citrate is transferred out of the
mitochondrion by either of two carriers, driven by the electroosmotic
gradient: either a citrate/phosphate antiport or a citrate/malate
antiport as shown in Figure 2-2.
Fatty acid biosynthesis (and most biosynthetic reactions)
requires NADPH to supply the reducing equivalents. Oxaloacetate is used to
generate NADPH for biosynthesis in a two-step sequence.
The first step is the malate dehydrogenase reaction found in
the TCA cycle. This reaction results in the formation of NAD from NADH (the
NADH primarily comes from glycolysis). The malate formed is a substrate for the
malic enzyme reaction, which makes pyruvate, CO2, and NADPH. Pyruvate is
transported into the mitochondria where pyruvate carboxylase uses ATP energy to
Palmitate is the starting point for other fatty acids that
use a set of related reactions to generate the modified chains and head groups
of the lipid classes. Microsomal enzymes primarily catalyze these chain modifications. Desaturation
uses O2 as the ultimate electron acceptor to introduce double bonds at the
nine, six, and five positions of an acyl-CoA.
Elongation is similar to synthesis of palmitate because it
uses malonyl-CoA as an intermediate. See Figure 2-3.
Glycerol accepts fatty acids from acyl-CoAs to synthesize
glycerol lipids. Glycerol phosphate comes from glycolysis-specifically from the
reduction of dihydroxyacetone phosphate using NADH as a cofactor. Then the
glycerol phosphate accepts two fatty acids from fatty acyl-CoA. The fatty acyl-CoA is
formed by the expenditure of two high-energy phosphate bonds from ATP.
Cholesterol Biosynthesis and its Control
Despite a lot of bad press, cholesterol remains an essential
and important biomolecule in animals. As much as half of the membrane lipid in
a cellular membrane is cholesterol, where it helps maintain constant fluidity
and electrical properties. Cholesterol is especially prominent in membranes of the nervous
Cholesterol also serves as a precursor to other important
molecules. Bile acids aid in lipid absorption during digestion. Steroid
hormones all derive from cholesterol, including the adrenal hormones that
maintain fluid balance; Vitamin D, which is an important regulator of calcium status; and the male and
female sex hormones.
Although humans wouldn’t survive in one sense or another
without cholesterol metabolites, cholesterol brings with it some well-known
side effects. Doctors find cholesterol derivatives, being essentially insoluble
in water, in the deposits (plaque) that characterize diseased arteries.
HMG CoA Reductase
HMG-CoA reductase is the committed and therefore the
regulatory step in cholesterol biosynthesis. If HMG-CoA is reduced to
mevalonate, cholesterol is the only product that can result. The reduction is a
two-step reaction, which releases the Coenzyme A cofactor and converts the
thiol-bound carboxylic group of HMG-CoA to a free alcohol. Two NADPH molecules
supply the reducing equivalents because the thioester must first be reduced to
the level of an aldehyde and then to an alcohol.
Mevalonate molecules are condensed to a 30-carbon compound,
squalene. The alcohol groups of mevalonate are first phosphorylated. Then they
multiply phosphorylated mevalonate decarboxylates to make the two compounds
isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP).
mevalonate ® phosphomevalonate ® pyrophosphomevalonate
First, the other hydroxyl group of mevalonate accepts a
phosphate from ATP. The resulting compound rearranges in an enzyme-catalyzed
reaction, eliminating both CO2 and phosphate. The 5-carbon compound that results, IPP,
is rapidly isomerized with DMAPP.
In plants and fungi, IPP and DMAPP are the precursors to many
so-called isoprenoid compounds, including natural rubber. In animals, they are
mainly precursors to sterols, such as cholesterol. The first step is
condensation of one of each to geranyl pyrophosphate, which then condenses with another
molecule of IPP to make farnesyl pyrophosphate. Some important membrane-bound
proteins have a farnesyl group added on to them; however, the primary fate of
farnesyl pyrophosphate is to accept a pair of electrons from NADPH and condense
with another molecule of itself to release both pyrophosphate groups.
The resulting 30-carbon compound is squalene; it folds into a
structure that closely resembles the structure of the steroid rings, although
the rings are not closed yet.
Squalene ® Lanosterol
The first recognizable steroid ring system is lanosterol; it
is formed first by the epoxidation of the double bond of squalene that was
originally derived from a DMAPP through farnesyl pyrophosphate, and then by the
cyclization of squalene epoxide. The enzyme that forms the epoxide uses NADPH
to reduce molecular oxygen to make the epoxide.
Lanosterol ® Cholesterol
This sequence of reactions is incompletely understood but
involves numerous oxidations of carbon groups, for example, the conversion of
methyl groups to carboxylic acids, followed bydecarboxylation. The end product,
cholesterol, is the precursor to cholesterol esters in the liver and is
transported to the peripheral tissues where it is a precursor to membranes (all
cells), bile salts (liver), steroid hormones (adrenals and reproductive
tissues), and vitamin D (skin, then liver, and finally kidney).
Cholesterol Transport, Uptake, and ControlCholesterol is
expor ted to the peripheral tissues in LDL and VLDL (see Chapter 1). About 70
percent of the cholesterol molecules in LDL are esterified with a fatty acid
(for example, palmitate) on the OH group (at Carbon 3; see Figure 2-5). Cells
take up cholesterol from the LDL by means of LDL receptors in the outer cell