Molecular Biochemistry II
Lipid Digestion & Transport
Copyright © 1999-2005 by Joyce J. Diwan.
All rights reserved.
Lipid Digestion & Transport
Digestion & transport of lipids poses unique problems
relating to the insolubility of lipids in water.
Enzymes that act on lipids are soluble proteins or
membrane proteins at the aqueous interface. Lipids,
and products of their digestion, must be transported
through aqueous compartments within the cell as well
as in the blood & tissue spaces.
O
Bile Acids
C
R1
O
R2
CH3
CH3
HO
OH
R1 = OH or H
H
R2 = H or NHCH2COOH or NHCH2CH2SO3
Bile acids (bile salts) are polar derivatives of
cholesterol, formed in liver and secreted into the gall
bladder. They pass via the bile duct into the intestine,
where they aid digestion of fats & fat-soluble vitamins.
Bile acids are amphipathic, with detergent properties.
O
Bile Acids
C
R1
O
R2
CH3
CH3
HO
OH
R1 = OH or H
H
R2 = H or NHCH2COOH or NHCH2CH2SO3
Bile acids emulsify fat globules into smaller micelles,
increasing the surface area accessible to lipid-hydrolyzing
enzymes. They also help to solubilize lipid breakdown
products (e.g., mono- & diacylglycerols from hydrolysis
of triacylglycerols).
Secretion of bile salts & cholesterol into the bile by liver
is the only mechanism by which cholesterol is excreted.
Most cholesterol & bile acids are reabsorbed in the
small intestine, returned to the liver via the portal vein,
and may be re-secreted. This is the enterohepatic cycle.
Agents that interrupt the enterohepatic cycle are used
to treat high blood cholesterol. Examples:
 Synthetic resins, as well as soluble fiber (e.g., oat bran
fiber and fruit pectin), that bind bile acids &/or
cholesterol, prevent absorption/reabsorption.
 A recently introduced drug ezetimibe acts on cells
lining the lumen of the small intestine to inhibit
absorption of cholesterol.
O
H2C
O
C
O
O
R1
HC
O
C
O
R2
H2C
O
C
R3
triacylglycerol
H2O
H2C
O
HC
O
H2C
C
O
R1
C
R2
O
 O
C
R3
OH
1,2-diacylglycerol
fatty acid
Pancreatic lipase (secreted into the intestine), catalyzes
hydrolysis of triacylglycerols at their 1 & 3 positions,
forming 1,2-diacylglycerols, & then 2-monoacylglycerols
(monoglycerides). A protein colipase is required to aid
binding of the enzyme at the lipid-water interface.
Monoacylglycerols and fatty acids are absorbed by
intestinal epithelial cells. Within intestinal epithelial cells
triacylglycerols are resynthesized.
O
Phospholipase A2
is secreted by the
pancreas into the
intestine. It
hydrolyzes the ester
linkage between the
fatty acid & the
hydroxyl on C2 of
phospholipids.
Lysophospholipids,
the products of
Phospholipase A2
reactions, are
powerful detergents.
O
R2
C
H2C
O
C
C
H
O
O
CH2O
R1
P
O
X
O
phospholipid
H2O
O
HO
H2C
O
C
C
H
O
CH2O
P
R1
O
+
O
R2
C
O
X
O
lysophospholipid
fatty acid
Lysophospholipids, produced from phospholipids
via Phospholipase A2, aid digestion of other lipids
by breaking up fat globules into small micelles.
Some phospholipid (lecithin) is secreted by the liver
in the bile, presumably to provide substrate for
Phospholipase A2 within the intestine and thus aid in
fat digestion.
Cobra & bee venoms contain Phospholipase A2.
These venoms, injected into the blood, produce
lysophospholipids that disrupt cell membranes and
lyse blood cells.
Cholesteryl Ester
O
R C O
Within intestinal cells (and other body cells) some of
the absorbed cholesterol is esterified to fatty acids,
forming cholesteryl esters.
(R = fatty acid hydrocarbon in diagram above)
The enzyme that catalyzes cholesterol esterification is
ACAT (Acyl CoA: Cholesterol Acyl Transferase).
PDB 1ICM
Within intestinal cells, fatty
acids (which are poorly
soluble & have detergent
properties) are sequestered
from the cytosol by being
bound with intestinal fatty
acid binding protein
(I-FABP).
I-FABP
Fatty acid-binding proteins, which are in several cell
types, have a ''b-clam" structure.
A fatty acid is carried in a cavity between 2 approx.
orthogonal b-sheets, each consisting of 5 antiparallel
b-strands.
Apoprotein B-100
Free fatty acids
are transported in
the blood bound
to albumin, a
serum protein
secreted by liver.
monolayer of
phospholipid &
cholesterol
core: cholesteryl
esters & some
triacylglycerols
LDL
Most other lipids are transported in the blood as part of
lipoproteins, complex particles whose structure includes:
 a core consisting of a droplet of triacylglycerols
and/or cholesteryl esters
 a surface monolayer of phospholipid, cholesterol, &
specific proteins (apolipoproteins), e.g., B-100.
Lipoproteins differ in their contents of proteins and
lipids. They are classified based on density.
 Chylomicron (largest; lowest in density due to high
lipid/protein ratio; highest % weight triacylglycerols)
 VLDL (very low density lipoprotein; 2nd highest in
triacylglycerols as % of weight)
 IDL (intermediate density lipoprotein)
 LDL (low density lipoprotein, highest in cholesteryl
esters as % of weight)
 HDL (high density lipoprotein; highest in density due
to high protein/lipid ratio)
Apolipoproteins:
Partial structures of some apolipoproteins are available.
A common motif is amphipathic a-helices (polar along
one surface & hydrophobic along the other side).
These helices may float on the phospholipid surface of
the lipoprotein.
Other domains of apolipoproteins have roles in
interaction of lipoproteins with cell surface receptors.
PDB 1AV1
The lipid-binding
domain of HDL
apoprotein-A-I
is an a-helix
regularly
interrupted by
prolines to give a
horseshoe shape.
Lipid-binding domain of HDL Apolipoprotein A-I
Hydrophobic
residues run along one edge of the amphipathic a-helix.
Antiparallel dimers form by association of hydrophobic
residues, with chains offset to form an elliptical ring.
Above right: hydrophobic = magenta; polar = cyan.
Apoprotein-A-I dimers may wrap around the spherical HDL.
Intestinal epithelial cells synthesize triacylglycerols,
cholesteryl esters, phospholipids, free cholesterol, and
apoproteins, and package them into chylomicrons.
Chylomicrons are secreted by intestinal epithelial cells,
and transported via the lymphatic system to the blood.
Apoprotein CII on the chylomicron surface activates
Lipoprotein Lipase, an enzyme attached to the lumenal
surface of small blood vessels.
Lipoprotein Lipase catalyzes hydrolytic cleavage of fatty
acids from triacylglycerols of chylomicrons.
Released fatty acids & monoacylglycerols are picked up
by body cells for use as energy sources.
As triacylglycerols are removed by hydrolysis,
chylomicrons shrink in size, becoming chylomicron
remnants with lipid cores having a relatively high
concentration of cholesteryl esters.
Chylomicron remnants are taken up by liver cells, via
receptor-mediated endocytosis (to be discussed later).
The process involves recognition of apoprotein E of the
chylomicron remnant by receptors on the liver cell surface.
Liver cells produce, and secrete into the blood, very low
density lipoprotein (VLDL).
The VLDL core has a relatively high triacylglycerol
content.
One of the apoproteins of VLDL is B-100.
MTP (microsomal triglyceride transfer protein), in the
lumen of the endoplasmic reticulum in liver, has an
essential role in VLDL assembly.
MTP facilitates transfer of lipids to apoprotein B-100
while B-100 is being translocated into the ER lumen
during translation.
Control of VLDL production:
 VLDL assembly is dependent on availability of lipids.
Transcription of genes for enzymes that catalyze lipid
synthesis is controlled by SREBP.
 Availability of apoprotein B-100 for VLDL assembly
depends at least in part on regulated transfer of B-100
out of the ER for degradation via the proteasome.
As VLDL particles are transported in the bloodstream,
Lipoprotein Lipase catalyzes triacylglycerol removal
by hydrolysis.
With removal of triacylglycerols and some proteins, the
% weight that is cholesteryl esters increases.
VLDL are converted to IDL, and eventually to LDL.
VLDL  IDL  LDL
The lipid core of LDL is predominantly cholesteryl esters.
Whereas VLDL contains 5 apoprotein types (B-100, C-I,
C-II, C-III, & E), only one protein, apoprotein B-100, is
associated with the surface monolayer of LDL.
LDL
extracellular space
cytosol
Cells take up LDL by
receptor-mediated
endocytosis.
LDL
receptor
receptor-mediated
endocytosis
The cholesterol in LDL is then used by cells, e.g., for
synthesis of cellular membranes.
The LDL receptor was identified by M. Brown & J.
Goldstein, who were awarded the Nobel prize.
The LDL receptor is a single-pass transmembrane
glycoprotein with a modular design.
LDL
extracellular space
The cytosolic domain
of the LDL receptor
binds adapter proteins
that mediate formation
of a clathrin coat.
cytosol
This allows the receptor
to be selected into
budding vesicles.
receptor-mediated
endocytosis
LDL
receptor
The LDL-binding domain on the exterior side of the
plasma membrane recognizes & binds apoprotein B-100.
Once the receptor with bound LDL is taken into a cell by
endocytosis, the LDL-binding domain faces the lumen of
the vesicle.
The vesicle then fuses with an endosomal compartment.
N- R1 R2 R3 R4 R5 R6 R7 EGF-A EGF-B b-propeller EGF-C GD TM Cyt -C
Order of domains in primary structure of the LDL Receptor
 The N-terminal LDL-binding (apoprotein B-100binding) domain of the receptor consists of a series of
cysteine-rich repeats (R1-R7), each of which is
stabilized by 3 disulfide linkages and has a bound Ca++.
 Between the cysteine-rich repeats & the transmembrane
(TM) segment are 3 epidermal growth factor-like
domains (EGF-A, B, C) & a b-propeller.
 A domain subject to O-linked glysosylation (GD),
between the innermost EGF domain & the transmembrane a-helix, may act as a spacer to extend the
LDL-binding region out from the cell surface.
PDB 1N7D
b-propeller
The long, flexible,
modular structure
allows association of
N-terminal domains of
the receptor with ligand
on the surface of a
lipoprotein that may
vary in size.
LDL receptor: LDL-binding domain
Under acidic conditions of the endosome the b-propeller
forms a complex with two of the cysteine-rich repeats.
This causes the receptor to release LDL, which is then
carried via a vesicle to a lysosome to be degraded.
Regulation:
Synthesis of LDL Receptor is suppressed by high
intracellular cholesterol.
This process involves decreased release of SREBP.
Members of the SREBP family of transcription factors
activate transcription of genes for the LDL receptor,
as well as for enzymes essential to cholesterol synthesis
such as HMG-CoA Reductase.
The decreased synthesis of LDL receptor prevents
excessive cholesterol uptake by cells.
It has the deleterious consequence that excess dietary
cholesterol remains in the blood as LDL.
The lowered intracellular cholesterol that results from
treatment with statin drugs, leads to activation of SREBP,
increasing transcription of the gene for LDL receptor.
Thus statins lower plasma cholesterol both by inhibiting
HMG-CoA Reductase (decreasing cholesterol synthesis)
and by promoting removal of LDL from the blood.
Mutations affecting the LDL receptor are associated
with the most common form of the disease familial
hypercholesterolemia (high blood cholesterol).
Cells lacking functional LDL receptors cannot take up
LDL.
As a result, the amount of circulating LDL increases,
leading to enhanced risk of developing atherosclerosis.
Other hereditary hypercholesterolemias relate to genetic
defects in structure of apolipoproteins.
E.g., familial defective apoprotein B100 leads to
impaired binding of LDL to cell surface receptors, with
elevated levels of circulating LDL.
HDL (high density lipoprotein) is secreted as a small
protein-rich particle by liver (and intestine).
One HDL apoprotein, A-1, activates LCAT (LecithinCholesterol Acyl Transferase), which catalyzes
synthesis of cholesteryl esters using fatty acids cleaved
from the membrane lipid lecithin.
The cholesterol is scavenged from cell surfaces & from
other lipoproteins.
HDL may transfer cholesteryl esters to other lipoproteins.
Some remain associated with HDL, which may be taken up
by liver & degraded.
HDL thus transports cholesterol from tissues & other
lipoproteins to the liver, which can excrete excess
cholesterol as bile acids.
High blood levels of HDL ("good" cholesterol) correlate
with low incidence of atherosclerosis.
Bacterial & viral infections, & some inflammatory
disease states decrease HDL & increase VLDL production
by the liver.
These & other changes associated with inflammation can
lead to increased risk of atherosclerosis if prolonged.
blood vessel lumen
elastic
lamina
endothelial cells
smooth muscle cells
Cell layers adjacent to the lumen of arterial blood vessel.
Development of an atherosclerotic plaque:
Various conditions can initiate formation of a lesion in the
endothelium lining the arterial lumen.
 Inflammatory response, including cytokine production
that may be activated by oxidized lipids present in LDL.
 Risk factors include elevated circulating LDL, high
blood pressure, exposure to nicotine, etc.
blood vessel lumen
 
 LDL  

endothelial
cells
 foam cell

 
smooth muscle cells
Lipoproteins (e.g., LDL) leak across the endothelium and
accumulate in the subendothelial space. Lipoproteins
accumulate in part through binding to proteoglycans.
Macrophages accumulate at the lesion and enter the
subendothelial space. They ingest lipoproteins and appear
as “foam cells” due to cytoplasmic lipid droplets.
blood vessel lumen
 
 LDL  

endothelial
cells
 foam cell

 
smooth muscle cells
Smooth muscle cells may also migrate into the
subendothelial space & become foam cells.
As foam cells eventually die, they may release harmful
cellular contents that can contribute to rupturing of the
plaque and development of blood clots.