1. METABOLISM OF LIPIDS:
DIGESTION OF LIPIDS. TRANSPORT
FORMS OF LIPIDS

2. PHYSIOLOGICAL ROLE OF LIPIDS
 Energetic role (fuel
molecules)
 Components of membranes
(structural role)
 Precursors for many
hormones (steroids)
 Signal molecules
(prostaglandins)
 Protective role (lipids
surround important organs)
 Enzyme cofactors (vitamin K)
 Electron carriers (ubiquinone)
 Insulation against
temperature extremes

3. TRIACYLGLYCEROLS ARE HIGHLY
CONCENTRATED ENERGY STORES
•Triacylglycerols (TGs) and glycogen -
two major forms of stored energy
TGs which are more efficient energy
stores because:
(1) They are stored in an anhydrous form
(2) Their fatty acids are more reduced
than monosaccharides.

4. • 1 g of triacylglycerols stores more than six times
as much energy as a 1 g of glycogen
• Glycogen reserves are depleted in 12 to 24 hours
after eating, triacylglycerols within several
weeks.
•Fat breakdown
about 50 % of energy in liver, kidney and
skeletal muscles up to 95 % of energy cardiac
muscle
•Fats are the major source of energy for:
fasting animal organism in diabetes

5. • Fatty acids and glycerol -
substances that are directly used
as a fuel by mammalian organisms.
• Fatty acids (FA) and glycerol for
metabolic fuels are obtained
from triacylglycerols:
(1) In the diet
(2) Stored in adipocytes (fat
storage cells)
• Free fatty acids occur only in
trace amounts in cells
•For supplying of fatty acids as a fuel for organism, the
triacylglycerols have to be digested

6. DIGESTION OF DIETARY LIPIDS
Lipids in diet:
•
triacylglycerols
• phospholipids
Digestion – in small intestine.
• cholesterol
Enzyme – pancreatic lipase.
Lipase catalyzes hydrolysis at the C1 and C3 positions of
TGs producing free fatty acids and 2-monoacylglycerol.

7. Colipase – protein which is present in the intestine and helps
bind the water-soluble lipase to the lipid substrates.
Colipase also activates lipase.
Bile salts (salts of bile acids) are required for lipids digestion.
Bile salts are synthesized in the
liver from cholesterol.
Taurocholate and glycocholate - the most abundant bile salts.
Amphipathic: hydrophilic (blue) and hydrophobic (black)

8. TGs are water insoluble and lipase is water soluble.
Digestion of TGs takes place at lipid-water interfaces.
Rate of digestion depends on the surface area of the
interface.
Bile salts are amphipathic, they act as detergent
emulsifying the lipid drops and increasing the surface area
of the interface.

9. Bile salts also activates the lipase.
Inadequate production of bile salts results in
steatorrhea.

10. Dietary phospholipids are degraded by
phospholipases
Phospholipases are synthesized in the pancreas.
Major phospholipase is phospholipase A2 (catalyses the
hydrolysis of ester bond at C2 of glycerophospholipids
and lysophosphoglycerides are formed).
Lysophospho-
glycerides are
absorbed and in
the intestinal
cells are
reesterified
back to glycero-
phospholipids.

11. Lysophosphoglycerides can act as detergent
and therefore in high concentration can
disrupt cellular membranes.
Lysophosphoglyceride is normally present in
cells in low concentration.
Snake venom contain
phospholipase A2 and
causes the lysis of
erythrocytes
membranes.

12. Dietary cholesterol
• Most dietary cholesterol is unesterified
• Cholesteryl esters are hydrolyzed in the intestine by
an intestinal esterase
• Free cholesterol is solublized by bile-salt micelles for
absorption
• After absorption in the intestinal cells cholesterol
react with acyl-CoA to form cholesteryl ester.

13. ABSORPTION OF DIETARY LIPIDS
Lipid absorption – passive diffusion process.
2-monoacylglycerols, fatty acids,
lysophosphoglycerides, free cholesterol form
micelles with bile salts.

14. Micelles migrate to the microvilli and lipids diffuse
into the cells.
Bile acids are actively absorbed and transferred
to the liver via portal vein.
Bile salts can circulate through intestine and liver
several time per day.

15. In the intestinal cells the fatty acids are converted to fatty
acyl CoA molecules.
Three of these molecules can combine with glycerol, or two with
monoacylglycerol, to form a triacylglycerols.
O
CH2 OH CH2 O C R1
O O
CH O C R2 + R1 CO SCoA CH O C R2 + HSCoA
1.
CH2 OH CH2 OH
O O
CH2 O C R1 CH2 O C R1
O O
2.
CH O C R2 + R3 CO SCoA CH O C R2 + HSCoA
O
CH2 OH CH2 O C R3
1-st reaction is catalyzed by monoacylglycerol acyltransferase
2-nd reaction is catalyzed by diacylglycerol acyltransferase

16. TRANSPORT FORMS OF LIPIDS
• TGs, cholesterol and cholesterol esters are insoluble in water
and cannot be transported in blood or lymph as free molecules
• These lipids assemble
with phospholipids and
apoproteins
(apolipoproteins) to
form spherical
particles called
lipoprotein
Structure:
Hydrophobic core:
-TGs,
-cholesteryl esters
Hydrophilic surfaces:
-cholesterol,
-phospholipids,
-apolipoproteins

17. The main classes of lipoproteins
1.Chylomicrons.
2.Very low density lipoproteins (VLDL).
3.Intermediate density lipoproteins (IDL).
4.Low density lipoproteins (LDL).
5.High density lipoproteins (HDL).

18. Chylomicrons
• are the largest lipoproteins (180 to 500 nm in diameter)
• are synthesized in the ER of intestinal cells
• contain 85 % of TGs (it is the main transport form of dietary TGs).
• apoprotein B-48 (apo B-48) is the main protein component
• deliver TGs from the intestine (via lymph and blood) to tissues (muscle
for energy, adipose for storage).
• bind to membrane-bound lipoprotein lipase (at adipose tissue and
muscle), where the triacylglycerols are again degraded into free fatty
acids and monoacylglycerol for transport into the tissue
• are present in blood only after feeding
Lymphatic
exocytosis vessel

19. VLDL
• are formed in the liver
• contain 50 % of TGs and 22 % of cholesterol
• two lipoproteins — apo B-100 and apo E
• the main transport form of TGs synthesized in the organism (liver)
• deliver the TGs from liver to peripheral tissue (muscle for energy,
adipose for storage)
• bind to membrane-bound lipoprotein lipases (triacylglycerols are again
degraded into free fatty acids and monoacylglycerol)
triacylglycerol
cholesteryl esters
Apo B
Apo E
phospholipids
cholesterol

20. Lipoproteinlipase – enzyme which is located within
capillaries of muscles and adipose tissue
Function: hydrolyses of TGs of chylomicrons and VLDL.
Formed free fatty acids and glycerol pass into the cells
Chylomicrons and VLDL which gave up TGs are called remnants
of chylomicrons and remnants of VLDL
Remnants are rich in cholesterol esters
Remnants of chylomicrons are captured by liver
Remnants of VLDL are also called intermediate density
lipoproteins (IDL)
Fate of the IDL:
- some are taken by the liver
- others are degraded to the
low density lipoproteins (LDL) (by the removal of more
triacylglycerol)

21. LDL
LDL are formed in the blood from IDL and in liver from IDL
(enzyme – liver lipase)
LDL are enriched in
cholesterol and
cholesteryl esters
(contain about 50 % of
cholesterol)
Protein component - apo
B-100
LDL is the major
carrier of cholesterol
(transport cholesterol
to peripheral tissue)

22. Cells of all organs have LDL receptors
Receptors for LDL are localized in specialized regions called
coated pits, which contain a specialized protein called clathrin
Apo B-100 on the surface of an LDL binds to the receptor
Receptor-LDL complex enters the cell by endocytosis.
Endocytic vesicle is formed

23. LDL uptake by receptor-mediated endocytosis

24. Familial hypercholesterolemia
 congenital disease when LDL receptor are not synthesized (mutation at a
single autosomal locus)
 the concentration of cholesterol in blood markedly increases
 severe atherosclerosis is developed (deposition of cholesterol in arteries)
 nodules of cholesterol called xanthomas are prominent in skin and tendons
 most homozygotes die of coronary artery disease in childhood
 the disease in heterozygotes (1 in 500 people) has a milder and more
variable clinical course
atherosclerosis
xanthomas

25. HDL
 are formed in the liver and partially in small intestine
 contain the great amount of proteins (about 40 %)
 pick up the
cholesterol from
peripheral tissue,
chylomicrons and
VLDL
 enzyme
acyltransferase in
HDL esterifies
cholesterols,
convert it to
cholesterol esters
and transport to
the liver

26. High serum levels of cholesterol
cause disease and death by
contributing to development of
atherosclerosis
Cholesterol which is present in the
form of the LDL is so-called "bad
cholesterol."
Cholesterol in the
form of HDL is
referred to as "good
cholesterol”
HDL functions as a
shuttle that moves
cholesterol
throughout the body

27. LDL/HDL Ratio
The ratio of cholesterol in the form of LDL to that in the
form of HDL can be used to evaluate susceptibility to
the development of atherosclerosis
For a
healthy
person,
the LDL/
HDL ratio
is 3.5

28. Transport Forms of Lipids

29. LIPID METABOLISM:
MOBILIZATION OF
TRIACYLGLYCEROLS;
OXIDATION OF
GLYCEROL

30. Storage and Mobilization of
Fatty Acids (FA)
• TGs are delivered to adipose
tissue in the form of
chylomicrones and VLDL,
hydrolyzed by lipoprotein
lipase into fatty acids and
glycerol, which are taken up
by adipocytes.
• Then fatty acids are
reesterified to TGs.
• TGs are stored in adipocytes.
• To supply energy demands
fatty acids and glycerol are
released – mobilisation of adipocyte
TGs.

31. At low carbohydrate and insulin concentrations (during
fasting), TG hydrolysis is stimulated by epinephrine,
norepinephrine, glucagon, and adrenocorticotropic
hormone.
TG
hydro-
lysis is
inhibited
by insulin
in fed
state

32. •Lipolysis - hydrolysis of
triacylglycerols by lipases.
•A hormone-sensitive lipase
converts TGs to free fatty
acids and monoacylglycerol
•Monoacylglycerol is
hydrolyzed to fatty acid
and glycerol or by a
hormone-sensitive lipase or
by more specific and more
active monoacylglycerol
lipase

33. Transport of Fatty Acids and Glycerol
• Fatty acids and glycerol diffuse
through the adipocyte membrane and
enter bloodstream.
• Glycerol is transported via the blood
in free state and oxidized or converted
to glucose in liver.
• Fatty acids are traveled bound to
albumin.
• In heart, skeletal muscles and liver
they are oxidized with energy release.

34. Oxidation of Glycerol
Glycerol is absorbed by the liver.
Steps: phosphorylation, oxidation and isomerisation.
Glyceraldehyde 3-phosphate is an intermediate in:
 glycolytic pathway
 gluconeogenic pathways

35. Isomerase

36. LIPID
METABOLISM:
FATTY ACID
OXIDATION

37. Stages of fatty acid oxidation
(1) Activation of fatty acids takes place
on the outer mitochondrial membrane
(2) Transport into the mitochondria
(3) Degradation to two-carbon
fragments (as acetyl CoA) in the
mitochondrial matrix (β-oxidation
pathway)

38. (1) Activation of Fatty Acids
• Fatty acids are converted to CoA thioesters by
acyl-CoA synthetase (ATP dependent)
• The PPi released is hydrolyzed by a
pyrophosphatase to 2 Pi
• Two phosphoanhydride bonds (two ATP equivalents)
are consumed to activate one fatty acid to a
thioester

39. (2) Transport of Fatty Acyl CoA into Mitochondria
• The carnitine shuttle
system.
• Fatty acyl CoA is first
converted to acylcarnitine
(enzyme carnitine
acyltransferase I (bound to
the outer mitochondrial
membrane).
• Acylcarnitine enters the
mitochondria by a
translocase.
• The acyl group is transferred
back to CoA (enzyme -
carnitine acyltransferase II).

40. (3) The Reactions of β oxidation
• The β-oxidation pathway (β-carbon atom
(C3) is oxidized) degrades fatty acids two
carbons at a time
α
β

41. 1. Oxidation of acyl
CoA by an acyl CoA
dehydrogenase to
give an enoyl CoA
Coenzyme - FAD

42. 2. Hydration of the
double bond between
C-2 and C-3 by enoyl
CoA hydratase with
the 3-hydroxyacyl
CoA (β-hydroxyacyl
CoA) formation

43. 3. Oxidation of
3-hydroxyacyl CoA to
3-ketoacyl CoA by
3-hydroxyacyl CoA
dehydrogenase
Coenzyme – NAD+

44. 4. Cleavage of
3-ketoacyl CoA by
the thiol group of
a second molecule
of CoA with the
formation of
acetyl CoA and an
acyl CoA
shortened by two
carbon atoms.
Enzyme -
β -ketothiolase.

45. The shortened acyl
CoA then
undergoes another
cycle of oxidation
The number of
cycles: n/2-1,
where n – the
number of carbon
atoms

46. β-Oxidation of
Fatty acyl CoA
saturated fatty
acids

47. • One round of β oxidation: 4 enzyme steps
produce acetyl CoA from fatty acyl CoA
• Each round generates one molecule each of:
FADH2
NADH
Acetyl CoA
Fatty acyl CoA (2 carbons shorter each round)
Fates of the products of β-oxidation:
- NADH and FADH2 - are
used in ETC - acetyl CoA - enters the
citric acid cycle - acyl CoA –
undergoes the next cycle of oxidation

48. ATP Generation from Fatty Acid Oxidation
Net yield of ATP per one oxidized palmitate
Palmitate (C15H31COOH) - 7 cycles – n/2-1
• The balanced equation for oxidizing one palmitoyl
CoA by seven cycles of b oxidation
Palmitoyl CoA + 7 HS-CoA + 7 FAD+ + 7 NAD+ + 7 H2O
8 Acetyl CoA + 7FADH2 + 7 NADH + 7 H+
ATP generated
8 acetyl CoA 10x8=80
7 FADH2 7x1.5=10.5
7 NADH 7x2.5=17.5
108 ATP
ATP expended to activate palmitate -2
Net yield: 106 ATP

49. LIPID METABOLISM:
FATTY ACID
OXIDATION

50. β-OXIDATION OF ODD-CHAIN FATTY ACIDS
• Odd-chain fatty acids
occur in bacteria and
microorganisms
• Final cleavage product is
propionyl CoA rather
than acetyl CoA
• Three enzymes convert
propionyl CoA to succinyl
CoA (citric acid cycle
intermediate)

51. Propionyl CoA Is Converted into Succinyl CoA
1. Propionyl CoA is carboxylated to yield the D
isomer of methylmalonyl CoA.
The hydrolysis of an ATP is required.
Enzyme: propionyl CoA carboxylase
Coenzyme: biotin

52. 2. The D isomer of methylmalonyl CoA is
racemized to the L isomer
Enzyme: methylmalonyl-CoA racemase

53. 3. L isomer of methylmalonyl CoA is converted
into succinyl CoA by an intramolecular
rearrangement
Enzyme: methylmalonyl CoA mutase
Coenzyme: vitamin B12 (cobalamin)

54. OXIDATION OF FATTY ACIDS IN
PEROXISOMES
Peroxisomes - organelles containing
enzyme catalase, which catalyzes
the dismutation of hydrogen
peroxide into water and molecular
oxygen Acyl CoA
dehydrogenase
transfers electrons
to O2 to yield H2O2
instead of capturing
the high-energy
electrons by ETC,
as occurs in
mitochondrial β-
oxidation.