LIPID METAMOLISM

1. Unit-7
Metabolism of Lipids

2. Chapter contents
1. Digestion & Absorption
a. Bile acid & Fat Digestion
b. Fat soluble Vitamins
c. Transportation & Circulation
2. Fatty acid Oxidation & Generation of Energy
3. Deposition of Fat ( synthesis of Triacyl glycerol)
4. Mobilization of Fat ( Catabolism of triacyl glycerol)
5. Phospholipid Metabolism
6. Lipid Transport
a) Lipoproteins ( Chylomicrons,VLDL,LDL,HDL )
7. Clinical Correlates – a)Ketosis ;b) Steatorrhoea &
malabsorption syndromes ; c)Fatty Liver ; d)Disorders
of Plasma lipoprotein (hypercholestrolemia &
atheroscelerosis)

3. Digestion and Absorption of Lipids

7. A. PREPARATORY PHASE
1. Digestion in mouth and stomach: It was
believed earlier that little or no fat digestion takes place in
the mouth. Recently a lipase has been detected called
lingual lipase which is secreted by the dorsal surface of
the tongue (Ebner’s gland).
• Lingual lipase: Lingual lipase activity is continued in the stomach
also where the pH value is low. Due to retention of food bolus
for 2 to 3 hours, about 30 per cent of dietary triacyl glycerol
(TG) may be digested.
Lingual lipase is more active on TG having shorter FA chains
and is found to be more specific for ester linkage at 3-position
rather than position-1.

8. • Gastric lipase: There is evidence of presence of small
amounts of gastric lipase in gastric secretion. The overall
digestion of fats, brought about by gastric lipase is negligible
because:
• No emulsification of fats takes place in stomach
• The enzyme secreted in small quantity
• pH of gastric juice is not conducive which is highly acidic,
whereas gastric lipase activity is more effective at relatively
alkaline pH.

9. 2. Digestion in small intestine: The major site
of fat digestion is the small intestine.
This is due to the presence of a powerful lipase
(steapsin) in the pancreatic juice and presence of
bile salts, which acts as an effective emulsifying
agent for fats.
Pancreatic juice and bile enter the upper small
intestine, the duodenum, by way of the pancreatic
and bile ducts respectively. Secretion of
pancreatic juice is stimulated by the:
• Passage of an acid gastric contents (acid
chyme) into the duodenum
• By secretion of the GI hormones, secretin and
CCKPZ.

10. – Secretin: Increases the secretion of electrolytes
and fluid components of pancreatic juice.
– Pancreozymin of CCK-PZ: Stimulates the secretion
of the pancreatic enzymes.
– Cholecystokinin of CCK-PZ: Causes contraction of
the gallbladder and discharge the bile into the
duodenum. Discharge of bile is also stimulated by
secretin and bile salts themselves.
– Hepatocrinin: Released by the intestinal mucosa
stimulates more bile formation which is relatively poor
in the bile salt content.

11. • Pancreatic juice has been shown to
contain a number of lipolytic enzymes:
1. Pancreatic lipase (steapsin)
2. Phospholipase A2 (Lecithinase)
3. Cholesterol esterase.
• The pancreatic lipase is the most important
which hydrolyses TG containing short-chain
FA as well as long chain FA. Other two
enzymes are required for phospholipids and
cholesterol respectively.

12. • Pancreatic lipase (steapsin):
1. Role of bile salts in pancreatic lipase activity: Bile salts
are required for proper functioning of the enzyme.
• (a) Bile salts help in combination of ‘lipase’ with two
molecules of a small protein called as colipase in the
intestinal lumen. This combination of lipase with colipase
has two effects:
• Enhances the lipase activity of the intestinal pH.
• Also protects the enzyme against inhibitory effects of bile
salts and against surface denaturation.
• (b) Bile salts also help in emulsification of fats.

14. The complete hydrolysis of fats (TG) produces glycerol and FA. Pancreatic
lipase is virtually specific for the hydrolysis of primary ester linkage. It
cannot readily hydrolyze the ester linkage of position 2. Digestion of TG
molecule by pancreatic lipase proceeds
- First by removal of a terminal FA to produce diglyceride
- The other terminal FA is then removed to produce monoglyceride.

16. B. TRANSPORT PHASE
• Products of fat digestion, FFA and monoglycerides mainly enter the
microvilli and the apical pole of the absorptive epithelial cells by
simple diffusion.
• Short and medium chain FA and unsaturated FA are absorbed more
readily than long chain FA.
• Products of digestion is taken up by SER and resynthesized into TG
again by the enzymes in the intestinal epithelial cells.
• This maintains a sharp gradient of concentration within the mucosal
cells that favours continued rapid diffusion of FA into the cell from
intestinal lumen.
• There is a merging of SER into RER- in which probably
enzymes for TG resynthesis are formed as well as the
protein component (apo-B48) of lipoprotein complex,
chylomicron.

17. C. TRANSPORTATION PHASE

21. • Resynthesized TG cannot pass to lymphatics nor to portal
blood as it is insoluble. Hence, converted to lipoprotein
complex chylomicrons.
• Each droplet of hydrophobic and water insoluble TG gets
covered with a layer of hydrophilic PL,
cholesterol/cholesterol esters and an apoprotein apo-B48.

22. Fat soluble vitamins
• Found in the fats and oils of food.
• Absorbed into the lymph and carried in blood with
protein transporters = chylomicrons.
• Stored in liver and body fat and can become toxic if
large amounts are consumed.

23. • The fat-soluble vitamins, A, D, E, and K, are stored in
the body for long periods of time and generally pose a
greater risk for toxicity when consumed in excess than
water-soluble vitamins. Eating a normal, well-
balanced diet will not lead to toxicity in otherwise
healthy individuals

24. • Unlike water-soluble vitamins, these vitamins dissolve
in fat and are stored in body tissues. Because they are
stored, over time they can accumulate to dangerous
levels and can lead to a condition called
hypervitaminosis, meaning excess amounts of a
vitamin in the body, if more than the recommended
amount is taken.

25. • This substance, which is produced in the liver, flows
into the small intestine, where it breaks down fats.
Nutrients are then absorbed through the wall of the
small intestine. Upon absorption, the fat-soluble
vitamins enter the lymph vessels before making their
way into the bloodstream.

26. • Transporting fat through water based blood and
lymphatic system is a challenge.
• Short and medium chain fatty acids travel via
cardiovascular system to liver.
• Long chain enter lymph system.
• Fat is transported via lipoproteins
• Lipid core, shell composed of protein , phospholipid &
cholesterol that allow transportation.

27. Degradation of Fatty Acids
Fatty acids are aerobically oxidized by β-oxidation in the mitochondrial
matrix of lever, muscle, heart, renal cortex and brown adipose tissue for
energy production during starvation, carbohydrate deprivation and
sustained muscular work.
β -Oxidation.
β -oxidation takes place in mitochondrion. Several enzymes known
collectively as FA-oxidase system are found in the mitochondrial matrix.
These enzymes catalyze the oxidation of fatty acid to acetyl-CoA.

29. Activation of Fatty Acid:
• Fatty acids are in cytosol of the cell. Fatty acids must be first
activated so that they participate in metabolic pathway. The
activation requires energy, which is provided by ATP. In presence of
ATP and coenzyme A, the enzyme acyl-CoA synthetase (thiokinase)
catalyzes the conversion of a free fatty acid to an active fatty acid
(Acyl-CoA).
• The acyl-CoA synthetase enzyme is found in the endoplasmic
reticulum and inside and outside of mitochondria.
• Active fatty acid (acyl-CoA) are formed in the cytosol, where as β –
oxidation of fatty acid occurs in mitochondrial matrix. Acyl-CoA is
impermeable to mitochondrial membrane. Long chain activated fatty
acid penetrate the inner mitochondrial membrane only in
combination with carnitine (β –hydroxy ϒ-trimethyl ammonium
butyrate). It is considered as a carrier molecule.

31. • Carnitine next carries the long chain fatty acyl group across the
inner mitochondrial membrane to the mitochondrial matrix.
• Acyl-CoA molecule diffuse through the outer mitochondrial
membrane to enter the outer mitochondrial compartment where
carnitine palmitoyl transferase I of the outer mitochondrial
membrane transfers acyl group of acyl-CoA to carnitine,
forming acylcarnitine. This is transported inward across the
inner membrane by an inner protein, carnitine- acylcarnitine
translocase, in exchange of free carnitine brought out from the
matrix (Carnitine- acylcarnitine antiport).
• Carnitine palmitoyl transferase II on the matrix surface of the
inner membrane then transfers the acyl group of acyl carnitine
to coenzyme A of the matrix, producing acyl-CoA and free
carnitine.

33. β –Oxidation reaction.
Four β –Oxidation
reactions are repeated
cyclically in the
mitochondrial matrix,
each cycle shortening the
fatty acid chain of acyl-
CoA by releasing two
carboxy terminal carbons
of the acyl chain as
acetyl-CoA.

34. This impedes entry of acetyl-CoA into Krebs cycle.
Acetyl-CoA in liver mitochondria is converted then to
ketone bodies, acetoacetate & b-hydroxybutyrate.
Glucose-6-phosphatase
glucose-6-P glucose
Gluconeogenesis Glycolysis
pyruvate
fatty acids
acetyl CoA ketone bodies
cholesterol
oxaloacetate citrate
Krebs Cycle
During fasting
or carbohydrate
starvation,
oxaloacetate is
depleted in
liver due to
gluconeogenesis.

35. KETONE BODIES: AN ALTERNATE FUEL FOR CELLS
Liver mitochondria have the capacity to convert acetyl CoA derived from
fatty acid oxidation into ketone bodies.
The compounds categorized as ketone bodies are acetoacetate, 3-
hydroxybutyrate (also called β-hydroxybutyrate), and acetone.
Acetoacetate and 3-hydroxybutyrate are transported in the blood to the
peripheral tissues. There they can be reconverted to acetyl CoA, which can
be oxidized by the TCA cycle. Ketone bodies are important sources of
energy for the peripheral tissues because
• 1) They are soluble in aqueous solution and, therefore, do not need to
be incorporated into lipoproteins or carried by albumin as do the other
lipids.
• 2) They are produced in the liver during periods when the amount of
acetyl CoA present exceeds the oxidative capacity of the liver; and
• 3) They are used in proportion to their concentration in the blood by
extrahepatic tissues, such as the skeletal and cardiac muscle and renal
cortex. Even the brain can use ketone bodies to help meet its energy
needs if the blood levels raise sufficiently; thus, ketone bodies spare
glucose. This is particularly important during prolonged periods of
fasting.

36. • Disorders of fatty acid oxidation present with the general picture of hypoketosis (due to
decreased availability of acetyl CoA) and hypoglycemia (due to increased reliance on
glucose for energy).
• During a fast, the liver is flooded with fatty acids mobilized from adipose tissue. The
resulting elevated hepatic acetyl CoA produced primarily by fatty acid degradation
inhibits pyruvate dehydrogenase and activates pyruvate carboxylase. The OAA thus
produced is used by the liver for gluconeogenesis rather than for the TCA cycle.
Therefore, acetyl CoA is channelled into ketone body synthesis. This pushes acetyl CoA
away from gluconeogenesis and into ketogenesis.

37. Use of ketone bodies by the peripheral tissues:
ketolysis
Although the liver constantly synthesizes low levels of ketone
bodies, their production becomes much more significant during
fasting when ketone bodies are needed to provide energy to the
peripheral tissues.
3- Hydroxybutyrate is oxidized to acetoacetate by 3-hydroxy
butyrate dehydrogenase, producing NADH (Figure 16.23). Aceto
acetate is then provided with a CoA molecule taken from
succinyl CoA by succinyl CoA:acetoacetate CoA transferase
(thiophorase). This reaction is reversible, but the product,
acetoacetyl CoA, is actively removed by its conversion to two
acetyl CoA. Extrahepatic tissues, including the brain but
excluding cells lacking mitochondria (for example, red blood
cells), efficiently oxidize acetoacetate and 3-hydroxybutyrate in
this manner. In contrast, although the liver actively produces
ketone bodies, it lacks thiophorase and, therefore, is unable to
use ketone bodies as fuel.

40. Triacylglycerol synthesis and degradation
Biosynthesis of Triacylglycerols
• Triacylglycerol is the principle lipid or fat depots in the body and also
constitute the major dietary lipids. These are synthesized mostly by
microsomal enzymes of hepatocytes, adipocytes, mammocytes,
enterocytes and muscle fibres in the following ways:-
Glycerophosphate pathway
• Substrates required for the synthesis of Triacylglycerol are:
(a) glycerol-3 phosphate
(b) Acyl-CoA
1. Fatty acid thiokinases use ATP and Co-A to thioesterify fatty acids to
acyl-CoA molecules.
2. Glycerol 3-phosphate is produced in two alternative ways.
(i) In hepatocytes and mammocytes, glycerol kinase uses
ATP to phosphorylate glycerol.
(ii) In adipocytes, enterocytes and skeletal muscles, glycerol
3-phosphate dehydrogenase uses NADH to reduce dihydroxyacetone
phosphate from glycolysis to glycerol 3-phosphate.

41. 3. Glycerol 3-phosphate acyltransferase transfers an acyl
group, mostly from a saturated acyl-CoA to glycerol 3-
phosphate to change the latter to lysophosphatidic acid.
4. Lysophosphatidate acyltransferase transfers an acyl
group from an acyl-CoA to lysophosphatidic acid, changing
the latter to phosphatidic acid.
5. Phosphatidate phosphohydrolase hydrolyzes the
phosphoester bond in phosphatidic acid to release Pi and
1,2-diacylglycerol.
6. Diacylglycerol acyltransferase transfers an acyl group,
mostly from acyl-CoA to 1,2 diacylglycerol to produce
Triacylglycerol.

42. • Intestinal monoacylglycerol shunt
In enterocytes, (a) monoacylglycerol acyltransferase transfers an
acyl group, from an acyl-CoA to 2-monoacylglycerol produced
by fat digestion, to change it to 1,2 diacylglycerol.
(b) diacylglycerol acyltransferase next transfers
another acyl group from an acyl-CoA to 1,2-diacylglycerol to
produce Triacylglycerol.

56. Steatorrhoea and malabsorption syndrome
Normally more than 95% of ingested lipid is
absorbed. When a large fraction is excreted in
the feces, it is called steatorrhea. Measurement
of fecal lipid with adequate lipid intake is a
sensitive indicator of lipid malabsorption.
Malabsorption can result from impairment in
lipolysis (Table 12-6), micelle formation (Table
12-7), absorption, chylomicron formation, or
transport of chylomicrons via the lymph to
blood.

59. Fatty Liver
TG and FA synthesized or taken up by the liver, are
rapidly incorporated into lipoproteins and then mobilized
to extrahepatic tissues through the plasma mainly as
VLDL. However, under the following conditions mentioned
below, the liver gets loaded with too much lipids, leading
to fibrosis of hepatocytes, cirrhosis and hepatic failure. The
amount of lipids in the liver at any given time is the
resultant of several influences, some acting in conjunction
with and some in opposition to other. Normal liver
contains about 4% as total lipids, three fourths of which is
phospholipids and one-fourth as neutral fats

61. a) Factors that tend to increase the fat content of liver are:
 Influx of dietary lipids.
• Plasma free fatty acids may rise due to a high- fat diet. The liver
then takes up far more fatty acids and produces far more TG
from them than what it can incorporate into lipoproteins like
VLDL for mobilization to extrahepatic tissue tissues. This may
accumulate too much fat in the liver.
 Synthesis of FA from carbohydrates and proteins.
• Plasma free fatty acids may rise due to an increased adipose
tissue lipolysis during starvation and diabetes.
 Mobilization of FA from depot to liver.
 Administration of puromycin or poisoning with CCl4, CHCl3, lead,
phosphorus or arsenic may cause fatty liver by blocking the
synthesis of apolipoprotein B-100 in the liver and consequently
reducing VLDL synthesis.

69. Hypercholesterolemia
• Familial hypercholesterolemia (FH) is an inborn error
of metabolism due to a defective LDL-receptor
protein. Five classes of mutations have been
identified consisting of more than 150 different
alleles. The defects in the receptor function can be
grouped into five types:
• 1. The receptor may not be synthesized at all;
• 2. The receptor may not be transported to the
surface;
• 3. The receptor may fail to bind LDL;
• 4. The receptor may fail to cluster in coated pits; or
• 5. The receptor may fail to release LDL in the
endosome.

77. ELECTRON TRANSPORT CHAIN
Energy-rich molecules, such as glucose, are metabolized
by a series of oxidation reactions ultimately yielding CO2
and water. The metabolic intermediates of these
reactions donate electrons to specific coenzymes—
nicotinamide adenine dinucleotide (NAD+) and flavin
adenine dinucleotide (FAD)—to form the energy-rich
reduced coenzymes, NADH and FADH2.

78. • These reduced coenzymes can, in turn, each donate a
pair of electrons to a specialized set of electron
carriers, collectively called the electron transport
chain. As electrons are passed down the electron
transport chain, they lose much of their free energy.
Part of this energy can be captured and stored by the
production of ATP from ADP and inorganic
phosphate (Pi). This process is called oxidative
phosphorylation. The remainder of the free energy not
trapped as ATP is used to drive ancillary reactions
such as Ca2+ transport into mitochondria, and to
generate heat.

80. A. Mitochondrion
The electron transport chain is present in the inner mitochondrial membrane and
is the final common pathway by which electrons derived from different fuels of
the body flow to oxygen. Electron transport and ATP synthesis by oxidative
phosphorylation proceed continuously in all tissues that contain mitochondria.
1. Membranes of the mitochondrion: The components of the electron
transport chain are located in the inner membrane. Although the outer
membrane contains special pores, making it freely permeable to most ions and
small molecules, the inner mitochondrial membrane is a specialized structure that
is impermeable to most small ions, including H+, Na+, and K+, and small molecules
such as ATP, ADP, pyruvate, and other metabolites important to mitochondrial
function. Specialized carriers or transport systems are required to move ions or
molecules across this membrane.
The inner mitochondrial membrane is unusually rich in protein, half of which is
directly involved in electron transport and oxidative phosphorylation. The inner
mitochondrial membrane is highly convoluted. The convolutions, called cristae,
serve to greatly increase the surface area of the membrane.
2. Matrix of the mitochondrion: This gel-like solution in the interior of
mitochondria is 50% protein. These molecules include the enzymes responsible
for the oxidation of pyruvate, amino acids, fatty acids (by beta-oxidation), and
those of the tricarboxylic acid (TCA) cycle. The synthesis of glucose, urea, and
heme occur partially in the matrix of mitochondria. In addition, the matrix
contains NAD+ and FAD (the oxidized forms of the two coenzymes that are
required as hydrogen acceptors) and ADP and Pi, which are used to produce ATP.

81. B. Organization of the electron transport chain
The inner mitochondrial membrane can be disrupted into five separate protein
complexes, called Complexes I, II, III, IV, and V.
Complexes I–IV each contains part of the electron transport chain Each complex
accepts or donates electrons to relatively mobile electron carriers, such as
coenzyme Q and cytochrome c.
Each carrier in the electron transport chain can receive electrons from an electron
donor, and can subsequently donate electrons to the next carrier in the chain. The
electrons ultimately combine with oxygen and protons to form water. This
requirement for oxygen makes the electron transport process the respiratory
chain, which accounts for the greatest portion of the body’s use of oxygen.
Complex V is a protein complex that contains a domain (Fo) that spans the inner
mitochondrial membrane, and a domain (F1) that appears as a sphere that
protrudes into the mitochondrial matrix. Complex V catalyzes ATP synthesis and so
is referred to as ATP synthase.
C. Reactions of the electron transport chain
With the exception of coenzyme Q, all members of this chain are proteins. These
may function as enzymes as is the case with the dehydrogenases, may contain iron
as part of an iron–sulphur center, may be coordinated with a porphyrin ring as in
the cytochromes, or may contain copper as does the cytochrome a + a3 complex.

82. 1. Formation of NADH: NAD+ is reduced to NADH by
dehydrogenases that remove two hydrogen atoms from their substrate.
Both electrons but only one proton (that is, a hydride ion, :H–) are
transferred to the NAD+, forming NADH plus a free proton, H+.
2. NADH dehydrogenase: The free proton plus the hydride ion
carried by NADH are next transferred to NADH dehydrogenase, a protein
complex (Complex I) embedded in the inner mitochondrial membrane.
Complex- II has a tightly bound molecule of flavin mono nucleotide (FMN, a
coenzyme structurally related to FAD, that accepts the two hydrogen atoms
(2e– + 2H+), becoming FMNH2. NADH dehydrogenase also contains iron
atoms paired with sulfur atoms to make iron–sulfur centers. These are
necessary for the transfer of the hydrogen atoms to the next member of
the chain, coenzyme Q (ubiquinone).
3. Coenzyme Q: Coenzyme Q (CoQ) is a quinone derivative with a long,
hydrophobic isoprenoid tail. It is also called ubiquinone because it is
ubiquitous in biologic systems. CoQ is a mobile carrier and can accept
hydrogen atoms both from FMNH2, produced on NADH dehydrogenase
(Complex I), and from FADH2, produced on succinate dehydrogenase
(Complex II), Glycerophosphate dehydrogenase and acyl CoA
dehydrogenase CoQ transfers electrons to Complex III. CoQ, then, links the
flavoproteins to the cytochromes.