Lipid Metabolism

Fatty acids have four major physiologic roles in the cell:
Building blocks of phospholipids and glycolipids
Added onto proteins to create lipoproteins, which targets them to membrane
locations
Fuel molecules - source of ATP
Fatty acid derivatives serve as hormones and intracellular messengers
Absorption and Mobilization of Fatty Acids
Most lipids are triacylglycerols, some are phospholipids and cholesterol.
Digestion occurs primarily in the small intestine.
Fat particles are coated with bile salts (amphipathic) from gall bladder.
Degraded by pancreatic lipase (hydrolyzes C-1 and C-3 ---> 2 fatty acids and 2-
monoacylglycerol).
Can then be absorbed by intestinal epithelial cells; bile salts are recirculated after
being absorbed by the intestinal epithelial cells.
In the cells, fatty acids are converted by fatty acyl CoA molecules.
Phospholipids are hydrolyzed by pancreatic phospholipases, primarily phospholipase
A2.
Cholesterol esters are hydrolyzed by esterases to form free cholesterol, which is
solubilized by bile salts and absorbed by the cells.
Lipids are transported throughout the body as lipoproteins.
Lipoproteins consist of a lipid (tryacylglycerol, cholesterol, cholesterol ester) core
with amphipathic molecules forming layer on outside.
Lipoproteins
Both transported in form of lipoprotein particles, which solubilize hydrophobic lipids
and contain cell-targeting signals.
Lipoproteins classified according to their densities:
o chylomicrons - contain dietary triacylglycerols
o chylomicron remnants - contain dietary cholesterol esters
o very low density lipoproteins (VLDLs) - transport endogenous
triacylglycerols, which are hydrolyzed by lipoprotein lipase at capillary
surface
o intermediate-density lipoproteins (IDL) - contain endogenous cholesterol
esters, which are taken up by liver cells via receptor-mediated endocytosis
and converted to LDLs
o low-density lipoproteins (LDL) - contain endogenous cholesterol esters,
which are taken up by liver cells via receptor-mediated endocytosis; major
carrier of cholesterol in blood; regulates de novo cholesterol synthesis at
level of target cell
o high-density lipoproteins - contain endogenous cholesterol esters released
from dying cells and membranes undergoing turnover
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Storage of Fatty Acids
Triacylglycerols are transported as chylomicrons and VLDLs to adipose tissue;
there, they are hydrolyzed to fatty acids, which enter adipocytes and are esterified
for storage.
Mobilization is controlled by hormones, particularly epinephrine, which binds to -
adrenergic receptors on adipocyte membrane --> protein kinase A activated -->
phosphorylates hormone-sensitive lipase --> converts triacylglycerols to free fatty
acids and monoacylglycerols.
Insulin inhibits lipid mobilization (example of reciprocal regulation).
Monoacylglycerols formed are phosphorylated and oxidized to DHAP (intermediate
of glycolysis and gluconeogenesis).
ATP ADP NAD
+
NADH + H
+
glycerol glycerol 3-phosphate dihydroxyacetone
phosphate
glycerol kinase glycerol phosphate
dehydrogenase
Can be converted to glucose (gluconeogenesis) or pyruvate (glycolysis) in the liver.
Fatty Acid Oxidation (-oxidation)
Fatty acids are degraded by oxidation of the carbon by -oxidation.
Pathway that removes 2-C units at a time --> acetyl CoA --> citric acid cycle --> ATP
There are three stages in -oxidation:
o Activation of fatty acids in cytosol catalyzed by acyl CoA synthetase; two
high energy bonds are broken to produce AMP
o 2) Transport of fatty acyl CoA into mitochondria via carnitine shuttle
o 3) -oxidation - cyclic pathway in which many of the same enzymes are used
repeatedly (see pathway sheet)
-oxidation of odd chain and unsaturated fatty acids
Odd chain fatty acids undergo -oxidation until propionyl CoA is formed.
Propionyl CoA is then converted to succinyl CoA, which then enters the Krebs cycle.
See pathway sheet for details
Unsaturated fatty acids need two additional enzymes besides those of -oxidation.
o enoyl-CoA isomerase
o 2,4-dienoyl-CoA reductase
How the pathway looks depends upon the location of the double bond, but there are
two possibilities.
See pathway sheets for details.
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ATP generation from Fatty Acid Oxidation:
Can be estimated from the amount of acetyl CoA, QH2, and NADH produced.
See pathway sheet.
Regulation of Fatty Acid Oxidation
Already talked about fatty acid mobilization via epinephrine.
Net result is high concentrations of acetyl CoA and NADH via -oxidation.
Both molecules allosterically inhibit pyruvate dehydrogenase complex.
Most of acetyl CoA produced goes to Krebs cycle; during periods of fasting, excess
acetyl CoA is produced, too much for Krebs cycle.
Also in diabetes, oxaloacetate is used to form glucose by gluconeogenesis -->
concentration of oxaloacetate is lowered.
Result is the diversion of acetyl CoA to form acetoacetate and 3-hydroxybutyrate;
these two molecules plus acetone are known as ketone bodies.
Acetoacetate is formed via the following reactions:
acetyl
CoA CoA acetyl CoA
2 acetyl CoA 3-hydroxy- acetoacetate
HMG-CoA lyase
3-methylglutaryl CoA
NADH + H
+ -hydroxy H
+
NAD
+ butyrate CO2
Dehydrogenase
3-hydroxybutyrate acetone
The major site of ketone body synthesis is the liver, within the mitochondrial matrix
---> transported to the bloodstream.
Acetoacetate and 3-hydroxybutyrate are used in respiration and are important
sources of energy.
Cardiac muscle and the renal cortex perferentially use acetoacetate over glucose.
Glucose is used by brain and RBCs; in brain, ketone bodies substitute for glucose as
fuel because the brain cannot undergo gluconeogenesis.
Acetoacetate can be converted to acetyl CoA and oxidized in citric acid cycle only in
nonhepatic tissues.
Diabetes (insulin-dependent diabetes mellitus; IDDM)
Decreased insulin secretion by beta cells of pancreas; could be caused by viruses (?)
Juvenile onset
Patients are thin, hyperglycemic, dehydrated, polyuric (pee a lot), hungry, thirsty
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In these patients, glycogen mobilization, gluconeogenesis, fatty acid oxidation occurs ---
> massive ketone body production; also, some of the glucose is in urine (tends to pull
water out of body) ----> diabetic ketoacidosis
FATTY ACID SYNTHESIS
Important features of this pathway:
1) Synthesis takes place in cytosol; -oxidation takes place in mitochondrial matrix.
2) Intermediates are bound to sulfhydral groups of acyl carrier protein (ACP);
intermediates of -oxidation are bonded to CoA
3) Growing fatty acid chain is elongated by sequential addition of two-carbon units
derived from acetyl CoA
4) Reducing power comes from NADPH; oxidants in -oxidation are NAD
+ and FAD
5) Elongation of fatty acid stops when palmitate (C16) is formed; further elongation and
insertion of double bonds carried out later by other enzymes
Fatty acid synthesis takes place in three stages:
1) Mitochondrial acetyl CoA is transported into cytosol via citrate transport system
Acetyl CoA is condensed with oxaloacetate to form citrate ---> antiported out with
inward movement of anion
Citrate cleaved by cytosolic citrate lyase --> oxaloacetate + acetyl CoA
2) Formation of malonyl CoA
Acetyl CoA carboxylase is key regulatory enzyme
Influenced by glucagon --> inactivates enzyme in liver
Epinephrine inactivates enzyme in adipocytes
Citrate allosterically activates enzyme
Fatty acyl CoA allosterically inhibits enzyme
3) Assembly of fatty acid chain via fatty acid synthase
Consists of five separate stages:
1) Loading - acetyl CoA and malonyl CoA are attached to acyl carrier protein
2) Condensation - both are condensed by fatty acid synthase to from
acetoacetyl-ACP
3) Reduction - NADPH is oxidized to form hydroxybutyryl ACP
4) Dehydration - formation of double bond
5) Reduction - NADPH is source of e
-
and H
+
to form butyryl-ACP
Last four steps are repeated, each time with malonyl-ACP to elongate chain, until palmitate
is produced.
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Overall reaction:
acetyl CoA + 7 malonyl CoA + 14 NADPH + 20 H
+
---> palmitate + 7CO2 + 14 NADP
+
+ 8 HS-CoA + 6
H2O
Regulation of Fatty Acid Synthesis
Metabolism of fatty acids is under hormonal regulation by glucagons, epinephrine,
and insulin.
Fatty acid synthesis is maximal when carbohydrate and energy are plentiful.
Important points of control are release of fatty acids from adipocytes and
regulation of carnitine acyltransferase I in the liver.
High insulin levels also stimulate formation of malonyl CoA, which allosterically
inhibits carnitine acyltransferase I _ fatty acids remain in cytosol and are not
transported to mitochondria for oxidation.
Key regulatory enzyme is acetyl-CoA carboxylase (catalyzes first committed step
in fatty acid synthesis).
Insulin stimulates fatty acid synthesis and inhibits hydrolysis of stored
triacylglycerols.
Glucagon and epinephrine inhibit fatty acid synthesis (enzyme is phosphorylated by
protein kinase A; removal of phosphate group catalyzed by protein phosphatase 2A).
Citrate is an allosteric activator, but its biological relevance has not been
established.
Fatty acyl CoA acts as an inhibitor.
Palmitoyl CoA and AMP are allosteric inhibitors.
Synthesis of Eicosanoids
Precursors for eicosanoids are 20-carbon polyunsaturated fatty acids such as
arachidonate.
Part of inner leaflet of cell membrane.
There are two classes of eicosanoids:
1) prostaglandins and thromboxanes
Synthesized by enzyme cyclooxygenase
Localized molecules such as thromboxane A2, prostaglandins, prostacyclin ae
produced.
Thromboxane A2 leads to platelet aggregation and blood clots _ reduced
blood flow in tissues.
Aspirin binds irreversibly to COX enzymes and prevents prostaglandin
synthesis.
2) leukotrienes
Produced by lipoxygenases.
Products were once called “slow-acting substances of anaphylaxis”,
responsible for fatal effects of some immunizations.
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Synthesis of Triacylglycerols and Glycerophospholipids
Most fatty acids are esterified as triacylglycerols or glycerophospholipids.
Intermediate molecule in synthesis of these two molecules is phosphatidic acid or
phosphatidate.
There are two pathways:
1) de novo – “from scratch”
2) salvage pathway - uses “old” pieces and parts to make new molecules
Synthesis of phosphatidate:
Common intermediate in synthesis of phosphoglycerides and triacylglycerols
Formed from glycerol 3-phosphate and 2 acetyl CoA molecules
Enzyme is glycerol phosphate acyltransferase
Synthesis of triacylglycerols and neutral phospholipids:
Uses phosphatidate, which is dephosphorylated to produce 1,2-diacylglycerol
If acetylated ---> triacylglyerol
If reacted with nucleotide derivative --> phosphatidylcholine or
phosphatidylethanolamine
Synthesis of acidic phospholipids:
Uses phosphatidate and reacts it with CTP ---> CDP-diacylglycerol
Addition of serine --> phosphatidylserine
Addition of inositol ---> phosphatidylinositol
In mammals, phosphatidylserine and phosphatidylethanolamine can be interconverted
- base-exchange occurs in ER.
Decarboxylation occurs in mitochondria and procaryotes
Synthesis of Sphingolipids
All have C18 unsaturated alcohol (sphingosine) as structural backbone, rather than
glycerol
Palmitoyl CoA and serine condense ---> dehydrosphinganine ---> sphingosine
Acetylation of amino group of sphingosine ---> ceramide
Substitution of terminal hydroxyl group gives:
sphingomyelin -- addition of phosphatidylcholine
cerebroside -- substitute UDP-glucose or UDP-galactose
gangliosides -- substitute oligosaccharide
Tay-Sachs disease = inherited disorder of ganglioside breakdown.
Deficient or missing enzyme is -N-acetylhexosaminidase, which removes the
terminal N-acetylgalactosamine residue from its ganglioside.
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One in 30 Jewish Americans of eastern European descent are carriers of a
defective allele.
Can be diagnosed during fetal development by assaying amniotic fluid for enzyme
activity.
Causes weakness, retarded psychomotor development, blindness by age two, and
death around age three.
Synthesis of Cholesterol
Precursor of steroid hormones and bile salts.
Most cholesterol is synthesized in liver cells, although most animal cells can synthesize
it.
Starts with 3 molecules of acetyl CoA to form 3-hydroxy-3-methyl-glutaryl CoA, which
is reduced to mevalonate (C6) by HMG-CoA reductase (first committed step of
cholesterol synthesis)
Amount of cholesterol formation by liver and intestine is highly responsive to cellular
levels of cholesterol.
Enzyme HMG-CoA reductase is controlled in multiple ways:
1) Rate of enzyme synthesis is controlled by sterol regulatory element (SRE); SRE
inhibits mRNA production
2) Translation of reductase mRNA is inhibited by nonsterol metabolites derived
from mevalonate
3) Degradation of the enzyme occurs at high enzyme levels
4) Phosphorylation of enzyme
If enzyme is phosphorylated via glucagon pathway --> decreased activity-->
cholesterol synthesis ceases when ATP levels are low
If enzyme is dephosphorylated via insulin pathway --> increased activity
Cells outside liver and intestine obtain cholesterol from blood instead of synthesizing it
de novo.
Steps in the uptake of cholesterol by LDL pathway:
1) apolipoprotein on surface of LDL particle binds to receptor on membrane of
nonhepatic cells
2) LDL-receptor complex internalized by endocytosis
3) vesicles formed fuse with lysosomes, which breaks apart protein part of lipoprotein
to amino acids and hydrolyzes cholesterol esters
4) released unesterified cholesterol can be used for membrane biosynthesis or be
reesterified for storage
Defects in LDL receptor lead to familial hypercholesterolemia (FH), in which
cholesterol and LDL levels are markedly elevated.
Result is deposition of cholesterol in tissues because of high levels of LDL-cholesterol in
blood
Heterozygotes suffer from atherosclerosis and increased risk of stroke
Homozygotes usually die in childhood from coronary artery disease
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Disease is the result of an absence (homozygotes) or reduction (heterozygotes) in
number of LDL receptors.
LDL entry into liver and other cells is impaired.
Drug therapy can help heterozygotes
1) can inhibit intestinal absorption of bile salts (which promote absorption of dietary
cholesterol)
2) lovastatin - competitive inhibitor of HMG-CoA reductase ---> blocks cholesterol
synthesis