2. Digestion and absorption of Lipids
• DIGESTION OF LIPIDS
• The major dietary lipids are triacylglycerol,
cholesterol and phospholipids.
• The lingual lipase from the mouth enters
stomach along with the food. It has an optimum
pH of 2.5 – 5. The enzyme therefore continues to
be active in the stomach.
• It acts on short chain triglycerides (SCT). SCTs are
present in milk, butter and ghee.
• Gastric lipase is acid stable, with an optimum pH
about 5.4. It is secreted by Chief cells. Up to 30%
digestion of triglycerides occurs in stomach.
3. • Digestion in Intestines
• Emulsification is a pre-requisite for digestion
of lipids. The lipids are dispersed into smaller
droplets; surface tension is reduced; and
surface area of droplets is increased.
This process is favored by:
1. Bile salts (detergent action)
2. Peristalsis (mechanical mixing)
3. Phospholipids.
4. Lipolytic Enzymes in Intestines
1. Pancreatic lipase with Co-Lipase
2. Cholesterol esterase
3. Phospholipase A2.
Digestion of Triacylglycerols
• Pancreatic lipase can easily hydrolyze the fatty
acids esterified to the 1st and 3rd carbon
atoms of glycerol forming 2-monoacylglycerol
and two molecules of fatty acid
5. • Then an isomerase shifts the ester bond from
position 2 to 1. The bond in the 1st position is
then hydrolyzed by the lipase to form free
glycerol and fatty acid
6. • Co-lipase
• The binding of co-lipase to the triacylglycerol
molecules at the oil water interface is
obligatory for the action of lipase.
• The co-lipase is secreted by the pancreas as an
inactive zymogen (molecular weight 11,000).
It is activated by trypsin.
7. ABSORPTION OF LIPIDS
• Absorption of Long Chain Fatty Acids
• Long chain fatty acids (chain length more than 14
carbons) are absorbed to the lymph and not directly to
the blood.
• The theory proposed by Bergstrom (Nobel Prize,
1982) has the following steps.
• Action of bile salts. The hydrophobic portions of bile
salts intercalate into the large aggregated lipid, with
the hydrophilic domains remaining at the surface. This
leads to breakdown of large aggregates into smaller
and smaller droplets.
• Thus the surface area for action of lipase is increased
8.
9. • Mixed Micelle Formation
• 1- The products of digestion, namely 2
monoacylglycerols, long chain fatty acids,
cholesterol and phospholipids are incorporated
into molecular aggregates to form mixed micelle.
The micelles are spherical particles with a
hydrophilic exterior and hydrophobic interior
core
• Due to their amphipathic nature, the bile salts
help to form micellar aggregates.
• 2- Micellar formation is essential for the
absorption of fat-soluble vitamins such as vitamin
A, D and K.
10. SEPARATION OF PLASMA LIPIDS
(a) Ultracentrifugation
• Pure fat is less dense than water. As the
proportion of lipid to protein in lipoprotein
complex increases, the density of the
molecule decreases.
• This property has been utilised in separation
of plasma lipids, the various lipoprotein
fractions, by ultracentrifugation.
11. (b) Electrophoresis
• Lipoproteins may be separated also according
to their electrophoretic properties and
identified more accurately using
immunoelectrophoresis.
• Fredrickson and others (1967) identified
lipoproteins into 4 groups by electrophoresis
as follows:
12. • HDL: Moves fastest and occupies position of α-
globulin-called α lipoproteins
• LDL: β-lipoproteins
• VLDL: (Pre-β or α2 lipoproteins) and
• Chylomicrons: Slowest moving and remains near the
origin.
13. • In mammals, principal Lipids that have
metabolic significance are as follows:
• Triacyl glycerol (TG): Also called Neutral fats
(NF)
• Phospholipids
• Steroids: Chief of which is cholesterol.
14. • Methods by which fatty acids are oxidised in
the body are as follows:
•β-oxidation: Principal method of oxidation
of FA.
• Other ancillary and specialised methods are:
• α-oxidation,
• ω-oxidation, and
• Peroxisomal FA oxidation.
15. β-OXIDATION
• Principal method by which FA is oxidised is
called β-oxidation.
• Several theories have been proposed to
explain the mechanism of the oxidation of FA
chains.
• The classical theory of β-oxidation was the
outcome of the work of Knoop.
16.
17.
18. Tissues in which β-Oxidation is carried
• The circulating FA
out:
en up by various
are tak
tissues and oxidised.
• Tissues like liver, heart, kidney, muscle, brain,
lungs, testes and adipose tissue have the
ability to oxidise long chain FA.
• In cardiac muscle, fatty acids are an important
fuel of respiration (80% of energy derived
from FA oxidation).
19. Enzymes Involved in β-Oxidation
• Several enzymes known collectively as FA-
oxidase system are found in the mitochondrial
matrix, adjacent to the respiratory chain,
which is found in the inner membrane.
• These enzymes catalyse the oxidation of FA to
acetyl-CoA.
20. Activation of FA:
• Fatty acids are in cytosol of the cell
(extramitochondrial).
• 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 (previously
called as thiokinases) catalyses the conversion
of a free fatty acid to an ‘active’ FA (acyl-CoA).
21. • The presence of inorganic pyrophosphatase
ensures that activation goes to completion by
facilitating the loss of additional high energy ~
P bond of PPi.
• Thus, in effect 2 ~ P bonds are expended
during activation of each FA molecule.
• Not only saturated FA but unsaturated FA and
–OH fatty acids are also activated by these
acyl-CoA synthetases.
22.
23. Location and types of Acyl-CoA
synthetases:
• The enzymes are found in the endoplasmic
reticulum and inside (for short-chain FA) and
outside (for long-chain FA) of the
mitochondria.
24. CARNITINE AND ITS ROLE IN FA
METABOLISM
• “Active” FA (acyl-CoA) are formed in cytosol,
whereas β-oxidation of FA occurs in
mitochondrial matrix.
• Acyl- CoA are impermeable to mitochondrial
membrane.
• Long chain activated FA penetrate the inner
mitochondrial membrane only in combination
with carnitine.
25. • Carnitine is chemically “β–OH–γ–trimethyl
ammonium butyrate”
26.
27. STEPS OF β-OXIDATION
• Once acyl-CoA is transported by carnitine in the
mitochondrial matrix, it undergoes β-oxidation by
Fatty acid oxidase complex.
1. Dehydrogenation: Removal of 2 H atoms:
• Removal of two hydrogen atoms from the 2 (α)
and 3 (β) carbon atoms is catalysed by the
enzyme acyl-CoA dehydrogenase, resulting in
formation of transenoyl-CoA (also called α, β-
unsaturated acyl-CoA).
• Hydrogen acceptor, i.e the co-enzyme for this
dehydrogenase is a flavo-protein.
28. 2. Hydration: Addition of one
molecule of H2O:
• One molecule of water is added to saturate
the double bond to form 3-OH acyl-CoA
(called also as β-OH acyl-CoA),
• The reaction is catalysed by the enzyme “Δ 2
enoyl - CoA hydratase” (also called as Enoyl
hydrolase)
29. 3. Dehydrogenation: Removal of 2 hydrogen
atoms:
• The 3 – OH – Acyl-CoA undergoes further
dehydrogenation on the 3 carbon, catalysed by
the enzyme 3–OH–acyl-CoA dehydrogenase, to
form the corresponding 3–ketoacyl-CoA (β-
ketoacyl-CoA).
• Hydrogen acceptor, i.e. coenzyme of this
dehydrogenase is NAD+.
• Reduced NAD when oxidised in respiratory chain
produces 3 ATP.
30. 4. Thiolytic cleavage:
Finally, 3-keto-acyl-CoA is split at the 2,3 position
by thiolase (“3 – keto acyl thiolase” or “acetyl–
CoA acyl transferase”), which catalyses a thiolytic
cleavage involving another molecule of CoA.
• End-products of this reaction:
The thiolytic cleavage results in formation of:
• One molecule of acetyl-CoA and
•An acyl-CoA molecule containing 2-carbons
less than the original acyl-CoA molecule, which
enters for oxidation by the enzyme acyl-CoA
dehydrogenase (reenters at step 1).
31. • In this way, a long-chain FA may be degraded
completely to “acetyl-CoA” (C-2 units).
• Acetyl-CoA can be oxidised to CO2 and H2O
and thus complete oxidation of FA is achieved.
• Thus, end-product of β-oxidation of a long-
chain FA will produce acetyl-CoA molecules (C-
2 units).
32.
33.
34.
35.
36. FATTY ACID SYNTHESIS
• Earlier it was believed that fatty acid synthesis
was reversal of fatty acid oxidation. But now it
is clear that there are three systems for fatty
acid synthesis.
• A. Extramitochondrial system:
This is a radically highly active system
responsible for de novo synthesis of palmitic
acid from 2-carbon unit acetyl-CoA.
37. B. Chain Elongation System
1. Microsomal system:
• A system present in microsomes which can
lengthen existing fatty acid chains. The
palmitic acid formed in cytosol is lengthened
to stearic acid and arachidonic acids.
2. Mitochondrial system:
• This system is mostly restricted to lengthening
of an existing fatty acid of moderate chain-
length.
38. A. Extramitochondrial (Cytoplasmic)
Synthesis of Fatty Acids: (De Novo
Synthesis)
• The synthesis takes place in cytosol. Starting
material is acetyl-CoA and synthesis always
ends in formation of palmitic acid.
39. Steps of FA Synthesis
• The starting material for the synthesis is
acetyl-CoA.
40. Formation of malonyl CoA from acetyl-CoA:
• In presence of the enzyme “acetyl-CoA carboxylase”,
the acetyl-CoA is converted to malonyl- CoA by “CO2-
fixation reaction”. Mn++ is required as a cofactor
and ATP provides the energy.
41.
42. • Acetyl-CoA carboxylase is a rate-limiting
enzyme. Citrate is an activator of the enzyme and
palmityl-CoA is inhibitor.
Subsequent steps:
• Once malonyl-CoA is synthesised, rest of fatty
acid synthesis reactions take place with FA
synthase complex.
• “Cys-SH” and “Pan-SH” may be considered as
two arms of the enzyme complex.
• “Cys-SH” is the acceptor of Acetyl-CoA whereas
“Pan-SH” takes up malonyl-CoA.
43. • 3. Condensation reaction:
• Now, the acetate attacks malonate to form
“aceto-acetyl-ACP”. The reaction is catalysed
by the enzyme “Keto-acyl synthase”
(condensing enzyme) and there is loss of one
molecule of CO2 (decarboxylation).
• The decarboxylation provides the extra
thermodynamic push to make the reaction
highly favourable.
44.
45. • First reaction (reduction): The keto-acyl group
is reduced to hydroxy group (–OH) to form “β-
OH butyryl-ACP” catalysed by the enzyme
“keto-acyl reductase”.
46. • Second reaction (dehydration): A molecule of
H2O is removed from “β-OH-butyryl-ACP” to
form “α, β-unsaturated butyryl-ACP” (also
called crotonyl-ACP), catalysed by the enzyme
“β-OH-acyl dehydratase”.
47. • Third reaction (reduction):
The third reduction is catalysed by “enoyl
reductase”using NADPH + H+, as a result the
double bond is saturated to form “butyryl-
ACP” (4 carbon).
48. 4. Continuation reaction:
• Now a fresh molecule of Malonyl-CoA is taken
up on to the free “Pan-SH” group of monomer
II and the sequence of events is repeated to
form a saturated six carbon fatty acid.
49. 5. Termination reaction:
• Palmityl-ACP is released as palmitic acid from
the enzyme complex by the enzyme
“thioesterase” (deacylase).
51. BIOSYNTHESIS OF CHOLESTEROL
• A number of established facts regarding
cholesterol biosynthesis are as follows
• Site of Synthesis
• Essentially all tissues form cholesterol.
• Liver is the major site of cholesterol
biosynthesis; also other tissues are active in
this regard, e.g. adrenal cortex, gonads, skin,
and intestine are most active.
• Low order of synthesis: Adipose tissue,
muscle, aorta and neural tissues.
52. Enzymes:
• Enzyme system involved in cholesterol
biosynthesis are associated with:
• Cytoplasmic particles “microsomes”
• Soluble fraction—cytosol.
Acetate
• ‘Active’ acetate (acetyl-CoA) is the starting
material and principal precursor.
• The entire carbon skeleton, all 27 C of
cholesterol in humans can be synthesised
from active acetate.
53. Steps of biosynthesis:
• Cholesterol biosynthesis can be thought of as occurring
in Five groups of reactions. They are:
I. Synthesis of mevalonate: A 6-C compound from
acetyl-CoA.
II.Formation of “Iso-Prenoid units” (C-5) from
Mevalonate:
By successive phosphorylations and followed by loss of
CO2.
• Note: The isoprenoid units are regarded as the
building blocks of the steroid nucleus.
III.Formation of Squalene: A 30-carbon aliphatic
chain, formed by condensation of six isoprenoid units.
IV.Cyclisation of Squalene to form Lanosterol.
V.Conversion of Lanosterol → to form cholesterol.
54. I. Synthesis of Mevalonate from
Acetyl-CoA
Consists of two steps:
• Formation of HMG-CoA: (β-OH-β-methyl
glutaryl-CoA):
• HMG-CoA can be formed in the cytosol from
acetyl-CoA in two steps catalysed by the
enzyme thiolase and HMG-CoA synthase.
55.
56. • In the next step, which is the rate-limiting
step, HMG-CoA is converted to Mevalonic acid
57. II. Formation of Isoprenoid Units
• Mevalonate is phosphorylated by ATP to form
several ‘active’ phosphorylated intermediates.
• Three such phosphorylated compounds are
formed and it is followed by decarboxylation
to form first “active” iso-prenoid unit: ”Iso-
pentenyl pyrophosphate” (5 C).
• Iso-pentenyl pyrophosphate undergoes
isomerisation to form another 5 C iso-prenoid
unit, called “3-3’-Dimethyl allyl
pyrophosphate”.
58. III. Formation of squalene:
• The pyrophosphorylated isoprenoid units
condense to form ultimately a 30-carbon aliphatic
chain called Squalene.
• IV. Cyclisation of squalene to form lanosterol:
• The formation of lanosterol from squalene takes
place in two steps:
• In the first step squalene-2,3-epoxide is formed
catalysed by the enzyme squalene mono-
oxygenase; which requires NADPH and molecular
O2.
• In the next step, an enzyme cyclase brings about
the cyclisation of squalene to form lanosterol.
59. V. Conversion of lanosterol to
cholesterol
• Main changes that are brought about are:
• Removal of three angular –CH3 groups. This
involves a series of reactions, mechanism of
demethylation is not properly known. CH3
group at C14 is first eliminated.
• Shift of double bond between C8 and C9 to C5
and C6, and Saturation of double bond in side
chain.
60.
61.
62.
63.
64.
65.
66. References
• Murray R, Rodwell V, Bender D, Kathleen M,
Botham P
, Weil A et al. Harper's Illustrated
Biochemistry. 28th Ed. Print-Hall; 2009
• Illustrated Biochemistry, 4th Ed, J Lippincot
Company,
• M N Chaterjea, Medical Biochemistry, 7th Ed,
Jaypee Brothers Medical Publishers, New
Delhi, 2007
• A text book of Biochemistry, Mushtaq Ahmad
