Digestion and absorption of lipids
and Fatty acid synthesis
脂类消化、吸收和脂肪酸合成
Deqiao Sheng PhD
Biochemistry Department
Most lipids are ingested in the form of
triacylglycerols.
 Dietary lipids are digested by pancreatic
lipases .
 Triacylglycerols in the intestinal lumen are
incorporated into micelles formed with the
aid of bile salts, amphipathic molecules
synthesized from cholesterol in the liver and
secreted from the gallbladder.

Dietary Lipids Are Digested by
Pancreatic Lipases
Most lipids are ingested in the form of
triacylglycerols but must be degraded to fatty
acids for absorption across the intestinal
epithelium. Recall that lipids are not easily
solubilized, yet they must be in order to be
degraded.
 Triacylglycerols in the intestinal lumen are
incorporated into micelles(微胶粒) formed
with the aid of bile salts.

Absorption


Most lipids in the diets of mammals are
triacylglycerols with smaller amounts of
phospholipids and cholesterol.
The digestion of dietary lipids occurs mainly
in the small intestine, where suspended fat
particles are coated with bile salts.
(Emulsification)



Bile salts are amphipathic cholesterol
derivatives synthesized in the liver, collected
in the gallbladder, and secreted into the
lumen of the intestine.
Bile salts play an important role in the
absorption of dietary lipids. (Emulsification)
Pancreatic lipase acts to degrade
triacylglycerols in the fat particles. This
lipase catalyzes hydrolysis at the C-1 and C3 positions of a triacylglycerol, producing
free fatty acids and a 2-monoacylglycerol.
pancreatic lipases
Action of pancreatic lipase.
Removal of the C-1 and C-3 acyl chains
produces free fatty acids and a 2monoacylglycerol.


A small protein called colipase(辅脂酶) helps bind
the water-soluble lipase to the lipid substrates.
Colipase also activates lipase by holding it in a
conformation with an open active site.
The initial products of fat hydrolysis are free fatty
acids and monoacylglycerols. These molecules are
transported to the intestinal wall in bile-salt
micelles where they are absorbed by the cells
lining the intestinal wall and the bile salts are
released.


Fatty acids are converted to fatty acyl CoA
molecules within the intestinal cells.
Three of these molecules can combine with
glycerol, or two with a monoacylglycerol, to
form a triacylglycerol. These waterinsoluble triglycerides combine with
cholesterol and specific proteins to form
chylomicrons for transport to other tissues.
Chylomicron Formation.
Free fatty acids and monoacylglycerols are absorbed by
intestinal epithelial cells. Triacylglycerols are resynthesized
and packaged with other lipids and apoprotein B-48 to form
chylomicrons, which are then released into the lymph
system.
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
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Triglycerides combine with cholesterol and
specific proteins to form chylomicrons(CM, 乳糜
微粒) for transport to other tissues.
The fate of dietary phospholipids is similar to that
of triacylglycerols. Pancreatic phospholipases
secreted into the intestine catalyze the hydrolysis
of phospholipids, which aggregate in micelles.
The major phospholipase in the pancreatic
secretion is phospholipase A2, which catalyzes
hydrolysis of the ester bond at C-2 of a
glycerophospholipid to form a
lysophosphoglyceride and a fatty acid.
Action of four phospholipases.
Phospholipases A1, A2, C, and D can be used to dissect
glycerophospholipid structure. Phospholipases catalyze the
selective removal of fatty acids from C-1 or C-2 or convert
glycerophospholipids to diacylglycerols or phosphatidates.
Dietary Lipids Are Transported in
Chylomicrons
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In the intestinal mucosal cells, the triacylglycerols
are resynthesized from fatty acids and monoacylglycerols and then packaged into lipoprotein
(脂蛋白) transport particles called chylomicrons.
These particles are composed mainly of
triacylglycerols, with apoprotein B-48 as the main
protein component. Protein constituents of
lipoprotein particles are called apolipoproteins (载
脂蛋白).
Chylomicrons also function in the transport of fatsoluble vitamins and cholesterol.
Lipoproteins

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Triacylglycerols, cholesterol, and cholesteryl esters
cannot be transported in blood or lymph as free
molecules because they are insoluble in water.
Instead, these lipids assemble with phospholipids
and amphipathic lipid binding proteins to form
spherical macromolecular particles known as
lipoproteins.
A lipoprotein has a hydrophobic core containing
triacylglycerols and cholesteryl esters and a
hydrophilic surface consisting of a layer of
amphipathic molecules such as cholesterol,
phospholipids, and proteins.
Structure of a lipoprotein

The largest lipoproteins are chylomicrons,
which deliver triacylglycerols and
cholesterol from the intestine via the lymph
and blood to tissues such as muscle (for
oxidation) and adipose tissue (for storage) .
Summary of lipoprotein metabolism
Fatty Acid Synthesis


Fatty acids are synthesized by the repetitive
addition of two-carbon units to the growing
end of a hydrocarbon chain.
Fatty acid biosynthesis and breakdown
occur by different pathways, are catalyzed
by different sets of enzymes, and take place
in different parts of the cell.
ACP--acyl
carrier protein
An overview of
fatty acid synthesis
The first steps in the fatty acid
synthesis pathway are the
synthesis of acetyl
ACP and malonyl ACP from
acetyl CoA.
ACP--acyl
carrier protein
ACP
Addition of two carbons to a growing
fatty acyl chain: a four-step sequence.
A. Synthesis of Malonyl ACP and Acetyl
ACP

Malonyl ACP is the main substrate for fatty acid
biosynthesis. It is made in two steps, the first of
which is the carboxylation of acetyl CoA in the
cytosol to form malonyl CoA . The carboxylation
reaction is catalyzed by the biotin- dependent
enzyme acetyl-CoA carboxylase using a
mechanism similar to the reaction catalyzed by
pyruvate carboxylase . The ATP-dependent
activation of HCO3- forms carboxybiotin. This
reaction is followed by the transfer of activated
CO2 to acetyl CoA, forming malonyl CoA.
the first step
The second step

The second step in the synthesis of malonyl
ACP is the transfer of the malonyl moiety
from coenzyme A to ACP. This reaction is
catalyzed by malonyl CoA:ACP
transacylase . A similar enzyme called acetyl
CoA:ACP transacylase converts acetyl CoA
to acetyl ACP.
B. The Initiation Reaction of Fatty Acid
Synthesis

The synthesis of long-chain fatty acids
begins with the formation of a four-carbon
unit attached to ACP. This molecule, called
acetoacetyl ACP, is formed by condensation
of a two-carbon substrate (acetyl CoA or
acetyl ACP) and a three-carbon substrate
(malonyl ACP) with the loss of CO2. The
reaction is catalyzed by 3-ketoacyl ACP
synthase (KAS).
C. The Elongation Reactions of Fatty
Acid Synthesis
Acetoacetyl ACP contains the smallest 3ketoacyl moiety.
 In order to prepare for subsequent
condensation reactions, this oxidized 3ketoacyl moiety has to be reduced to an acyl
form by the transfer of electrons (and
protons) to the 3-carbon position. Three
separate reactions are required.
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In the first reduction the ketone is converted to an
alcohol. The second step is the removal of water by
a dehydratase producing a C=C double bond.
Finally, a second reduction adds hydrogens to
create the fully reduced acyl group.
The final product of the reduction, dehydration,
and reduction steps is an acyl ACP that is two
carbons longer. This acyl ACP becomes the
substrate for the elongation forms of 3-ketoacyl
ACP synthase (KAS I and KAS II).
The end product of saturated fatty
acid synthesis is 16- and 18-carbon
fatty acids. Larger chain lengths
cannot be accommodated in the
binding site of the condensing
enzyme.
The elongation stage
of fatty acid synthesis.
D. Activation of Fatty Acids

The thioesterase reaction results in release
of free fatty acids but subsequent
modifications of these fatty acids, and their
incorporation into membrane lipids, require
an activation step where they are converted
to thioesters of coenzyme A in an ATPdependent reaction catalyzed by acyl-CoA
synthetase .
Activation of fatty acids.
Fatty Acid Extension and
Desaturation
The fatty acid synthase pathway cannot
make fatty acids that are longer than 16 or
18 carbons (C16 or C18).
 Longer fatty acids are made by extending
palmitoyl CoA or stearoyl CoA. The
enzymes that catalyze such extensions are
known as elongases and they use malonyl
CoA (not malonyl ACP) as the source of the
2-carbon extension unit.

The Fatty Acid Synthase Complex Is a
Polypeptide Containing Seven Enzyme Activities

The complex is a dimer of two identical
polypeptide monomers, 1 and 2, each
consisting of seven enzyme activities and the
acyl carrier protein (ACP). (Cys-SH,
cysteinethiol.) The -SH of the 4′phosphopantetheine of one monomer is in
close proximity to the -SH of the cysteine
residue of the ketoacyl synthase of the other
monomer, suggesting a “head-to-tail”
arrangement of the two monomers.

Though each monomer contains all the
partial activities of the reaction sequence,
the actual functional unit consists of onehalf of one monomer interacting with the
complementary half of the other. Thus, two
acyl chains are produced simultaneously.
The sequence of the enzymes in each
monomer is based on Wakil.
acetyl
malonyl
Fatty acid synthase multienzyme complex
By Salih Jawad Wakil was born in 1927 in Kerballa, Iraq
The Main Source of NADPH for Lipogenesis
Is the Pentose Phosphate Pathway
• NADPH is involved as donor of reducing
equivalents in both the reduction of the 3ketoacyl and of the 2,3- unsaturated acyl
derivatives. The oxidative reactions of the
pentose phosphate pathway are the chief
source of the hydrogen required for the
reductive synthesis of fatty acids.
Long-Chain Saturated 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 .
 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 in mitochondria.

Routes of synthesis of other fatty
acids.
Palmitate is the precursor of stearate
and longer-chain saturated fatty acids,
as well as the monounsaturated acids
palmitoleate and oleate.
Mammals cannot convert oleate to
linoleate or -linolenate (shaded pink),
which are therefore required in the diet
as essential fatty acids. Conversion
of linoleate to other polyunsaturated
fatty acids and eicosanoids is outlined.
Unsaturated fatty acids are symbolized
by indicating the number of carbons
and the number and position of the
double bonds.
Desaturation of Fatty Acids Requires a
Mixed-Function Oxidase

Palmitate and stearate serve as precursors
of the two most common monounsaturated
fatty acids of animal tissues: palmitoleate,
16:1(Δ9), and oleate, 18:1(Δ 9); both of these
fatty acids have a single cis double bond
between C-9 and C-10.

The double bond is introduced into the fatty
acid chain by an oxidative reaction
catalyzed by fatty acyl–CoA desaturase , a
mixed-function oxidase .
– Oxygenases, and Cytochrome P-450

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Mammalian hepatocytes can readily introduce
double bonds at the Δ9 position of fatty acids but
cannot introduce additional double bonds between
C-10 and the methyl-terminal end. Thus mammals
cannot synthesize linoleate, 18:2(Δ 9,12), or linolenate, 18:3(Δ 9,12,15).
Because they are necessary precursors for the
synthesis of other products, linoleate (亚油酸) and
linolenate (亚麻酸) are essential fatty acids for
mammals.

Synthesis of unsaturated FA
unsaturated FA ：软油酸、Oleate、linoleate、
linolenate、arachidonic acid （ Essential FA ）
Essential FA：required for the growth of
mammals and they must be obtained from food.
Including linoleate、linolenate, arachidonic
acid
Overview
ACP
Eicosanoid Hormones Are Derived from
Polyunsaturated Fatty Acids

Eicosanoids(廿碳) are a family of very
potent biological signaling molecules that
act as short-range messengers, affecting
tissues near the cells that produce them. In
response to hormonal or other stimuli,
phospholipase A2, present in most types of
mammalian cells, attacks membrane
phospholipids, releasing arachidonate(花生
四烯酸 ) from the middle carbon of glycerol.

Arachidonate, a 20:4 fatty acid derived
from linoleate, is the major precursor of
several classes of signal molecules:
prostaglandins (PGs), prostacyclins,
thromboxanes (TXs), and leukotrienes (LTs)

“Eicosanoid”(eicosa-, Greek for "twenty")
is the collective term for oxygenated
derivatives of three different 20-carbon
essential fatty acids:
Eicosapentaenoic acid (二十碳五烯酸 ，EPA), an
ω-3 fatty acid with 5 double bonds;
Arachidonic acid (AA), an ω-6 fatty acid, with 4
double bonds;
Dihomo-gamma-linolenic acid (DGLA), an ω-6,
with 3 double bonds.
花生四烯酸
Structures of Several Eicosanoids.
LTs
TXs
PGs
Arachidonate Is the Major Precursor of Eicosanoid
Hormones. Prostaglandin synthase catalyzes the first step
in a pathway leading to prostaglandins, prostacyclins,
and thromboxanes. Lipoxygenase catalyzes the initial
step in a pathway leading to leukotrienes.
Regulation of FA synthesis
 Dietary
factors: carbohydrate promotes
synthesis
 Hormone factors
 Insulin，“store
hormone”，increase FA
synthesis
 Glucagon ，“release hormon”，inhibit FA
synthesis
Citrate is an
allosteric activator
Regulation of fatty acid synthesis.
In the cells of vertebrates, both allosteric
regulation and hormone-dependent covalent
modification influence the flow of precursors
into malonyl-CoA.
Summary
1. Long-chain saturated fatty acids are synthesized
from acetyl-CoA by a cytosolic complex of six
enzyme activities plus acyl carrier protein (ACP).
The fatty acid synthase complex, which in some
organisms consists of multifunctional
polypeptides, contains two types of -SH groups
(one furnished by the phosphopantetheine of
ACP, the other by a Cys residue of -ketoacylACP synthase) that function as carriers of the
fatty acyl intermediates.
2. Malonyl-ACP, formed from acetyl-CoA (shuttled
out of mitochondria) and CO2, condenses with
an acetyl bound to the Cys-SH to yield
acetoacetyl-ACP, with release of CO2. This is
followed by reduction to the D-b-hydroxy
derivative, dehydration to the trans-Δ2unsaturated acyl-ACP, and reduction to butyrylACP. NADPH is the electron donor for both
reductions. Fatty acid synthesis is regulated at
the level of malonyl-CoA formation.
3.
Six more molecules of malonyl-ACP react
successively at the carboxyl end of the
growing fatty acid chain to form
palmitoyl-ACP—the end product of the
fatty acid synthase reaction. Free
palmitate is released by hydrolysis.
4.
Palmitate may be elongated to the 18carbon stearate. Palmitate and stearate
can be desaturated to yield palmitoleate
and oleate, respectively, by the action of
mixed-function oxidases.
5.
Mammals cannot make linoleate and
must obtain it from plant sources; they
convert exogenous linoleate to
arachidonate, the parent compound of
eicosanoids (prostaglandins,
thromboxanes, and leukotrienes), a
family of very potent signaling molecules.