1. Dr. Eman Shaat Professor of Medical Biochemistry and Molecular Biology
Cardio Vascular System lipoprotein metabolism lecture 1 ( 43 slides)

2. Relatively water insoluble
Introduction
Lipid compounds:
Relatively water insoluble
Therefore, they are transported in plasma (aqueous) as Lipoproteins
For triacylglycerol transport (TG-rich):
Chylomicrons: lipids of dietary origin (exogenous)
VLDL: lipids of endogenous (hepatic) synthesis
For cholesterol transport (cholesterol-rich):
LDL
HDL

3. Function of lipoproteins
Serve to transport lipids and lipid-soluble compounds between tissues and organs
Substrates for energy metabolism (TG)
Essential components for cells (PL, C)
Precursors for hormones (C)
Lipid soluble vitamins (D, E, K, A)
Precursors for bile acids (C)

4. Classification of plasma lipoproteins
Lipoproteins differ in the ratio of protein to lipids, & in the particular apoproteins & lipids that they contain.
They are classified based on their density:
Chylomicron (CM) (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)

5. Plasma lipoproteins
All lipids in plasma are transported in the form of lipoproteins.
Lipoproteins solubilize lipids in plasma.
The structure of lipoprotein is:
Central core formed of non-polar lipids:
triacylglycerols (TG; TAG) and
cholesterol esters (CE).
Outer layer contains polar lipids:
phospholipids (PL).
non-esterified cholesterol (C).
proteins (apoproteins).

6. Hydrophobic lipids
Amphiphilic lipids

7. Important enzymes and proteins involved in lipoprotein metabolism
Enzymes (LPL, HL, LCAT, CETP).
Apolipoproteins (A, B, C, D, E).
Functions of apolipoproteins:
Structural and transport function
Enzymatic function
Ligands for receptors

8. Types of apolipoproteins
Apo Lp (a) E
CI,II,III
B100
B48
AI & AII
In liver
In liver & Intestine
synthesized in the liver
synthesized in the liver.
synthesized in the intestine
In Lp(a)
in VLDL, HDL, chylomicron
CII lipoprotein lipase
In LDL &
VLDL
In chylom
AI in HDL
LCAT
Peripheral
Peripheral Transferable
Integral
Attached to B100; impairs fibrinolysis; atherogenic
Helps in receptor recognition
HDL chylomicron and VLDL
Chylomic
HDL + +

9. Lipoprotein classes TG TG, PL, C TG, cholesterol cholesterol
Source
Apo Proteins
Protein/lipid %
Main lipid component
Diameter(nm)
Function CM
Intest.
(A I,II,III,IV)
(B48)
(C I,II,III)
(E)
1/99 % TG
Dietary:
FFA  Adipose/muscle
CE  Liver via remnants
CM remnants
6/94
TG, PL, C
45-150
VLDL
liver
(B100)
(CI,II,III)
7/93
30-90
Synthesized:
FFA adipose/muscle
CE  LDL
IDL
11/89
TG, cholesterol
25-35
LDL
Blood (VLDL)
B100
21/79
cholesterol
20-25
CE to liver (70%) and peripheral cells (30%)
HDL
1,2,3
Liver, intes., VLDL, CM
(A1,II,IV)
(D in HDL2,3)
PL, cholesterol
HDL1: 32/68
HDL2: 33/67
HDL3: 57/43
preβHDL: 70/30
HDL1: (only apo-E)
HDL2: 10-20
HDL3: 5-10
preβHDL:<5
supplies apo CII, E to chylomicrons and VLDL; mediates reverse cholesterol transport
Albumin/FFA
Adipose tissue
Free fatty acids
99/1

10. A A
apo-E receptor

11. Metabolism of CM I- Synthesis of nascent chylomicrons
Nascent chylomicrons are formed in the intestinal mucosa and pass to the chyle and then to the blood stream through the thoracic duct.
Triglycerides are the predominant lipid in chylomicrons and form 90% of the total lipid present in them. ( Exogenous ; dietary triglycerides )
Its protein content is about 1%.
Apo-A and apo-B48 are component of the nascent cylomicrons.
II- conversion of nascent chylomicrons to mature chylomicrons
After entering the blood stream, the chylomicron particles transfer apo-A to HDL and acquire apo-C from HDL.

12. Metabolism of CM III- Degradation of chylomicrons
Apo-CII activates lipoprotein lipase (LPL= clearing factor)
LPL located in the capillaries of adipose and other peripheral tissues as cardiac and skeletal muscles and lactating mammary gland.
LPL Hydrolyze triglyceride (TG) in the core of CM and VLDL to free fatty acids and glycerol.
Insulin enhances the synthesis of this enzyme and heparin increases its activity.
Lipoprotein lipase (LPL)

13. Metabolism of CM III- Degradation of chylomicrons
2. Triglycerides in chylomicrons are hydrolyzed into glycerol and FFA.
The fatty acids are released and oxidized in muscles or stored as triglycerides in adipose tissue. They are transported bound to plasma albumin.
Glycerol is taken by the liver cells mainly (due to high activity of glycerol kinase).
IV- Formation of chylomicron remnants
1. As triglycerides are removed from the hydrophobic core of chylomicrons, the surface area of cylomicrons decreases.
2. The more hydrophilic surface components (apo-C, unesterified cholesterol and phospholipids) are transferred to HDL.
The remaining particles are termed chylomicron remnants.

14. Metabolism of CM V- Fate of chylomicron remnants
They are triglycerides poor particles and consist mainly of cholesterol esters, apo-B48, apo-E.
They interact with receptors on liver cells and are taken by endocytosis, where they are catabolized within the hepatic lysosomes and the products (amino acids, fatty acids, cholesterol) are released into the cytosol and reutilized by the hepatic cells.
remnants are taken up by liver cells due to apoE recognition sites.

15. 70%
HDL
30%
apo-B/E receptor

16. Metabolism of VLDLs
Assembled and secreted by liver directly into blood as nascent form.
Carry lipids (hepatic origin; endogenous TG) from liver to peripheral tissues.
Mature VLDLs: contain Apo B-100 plus Apo C-II and Apo E (both from HDL). ApoC-II is required for activation of lipoprotein lipase.
Lipoprotein lipase is required to degrade TG into glycerol and fatty acids.
As TG is degraded, VLDLs become
Smaller in size
More dense
Apo C back to HDL

17. CE HDL Metabolism of VLDLs VLDL TG
Exchange of TG with cholesterol ester (HDL) by cholesterol ester transfer protein (CETP)
End products: IDL (returns Apo E to HDL) and become LDL
Most core lipid in LDL is cholesterol ester.
ApoB100 is only apolipoprotein in the surface.
VLDL
HDL TG CE
Choleteryl Ester Transfer Protein (CETP)

18. Summary of CM, VLDL, LDL LDL VLDL Chylomicron
Synthesized in small intestine
Transport dietary lipids
98% lipid, large sized, lowest density
Apo B-48: Receptor binding
Apo C-II: Lipoprotein lipase activator
Apo E: Remnant receptor binding
VLDL
Synthesized in liver.
Transport endogenous TG.
90% lipid, 10% protein.
Apo B-100
Receptor binding
Apo C-II
LPL activator
Apo E
Remnant receptor binding
LDL
Synthesized from IDL.
Cholesterol transport.
78% lipid, 58% cholesterol & CE.
Apo B-100
Receptor binding

19. LDL receptor
Also named as apoB-100/apoE receptors. because of its ability to recognize particles containing both apos B and E (apo E>> apo B-100)
LDL receptors exist in the liver and in most peripheral tissues
The complexes of LDL and receptor are taken into the cells by endocytosis, where LDL is degraded but the receptors are recycled
LDL cholesterol levels are positively related to risk of cardiovascular disease
Therefore, cholesterol in LDL has been called “bad cholesterol”

20. Fates of cholesterol intracellular.
Non-hepatic cells obtain cholesterol from the plasma LDL rather than by synthesizing it.
LDL uptake occurs by receptor mediated mechanism.
Receptor-LDL complex fuses with lysosomes.
cholesterol esters are hydrolyzed by lysosomal acid lipase to free cholesterol that is either.
Used unesterified for biosynthesis of cell membrane.
Reesterified for storage inside the cell by ACAT.
Synthesis of steroids in specified tissues

21. Fates of cholesterol intracellular.
Apo-B100
Apo-B100/E receptor
C, CE> TG
LDL TG
Membrane C C HL
ACAT
FA+Glycerol CE
Storage
Steroids
Liver

22. Hepatic Lipase (HL) Bound to the surface of liver cells.
Hydrolyzes TG → FFA + glycerol
Unlike LPL, HL does not react readily with CM or VLDL but is concerned with TG hydrolysis in VLDL remnants and HDL metabolism

23. Coordinate control of cholesterol uptake and synthesis
Increased uptake of LDL-cholesterol results in:
inhibition of HMG-CoA reductase (reduced cholesterol synthesis).
stimulation of acyl CoA:cholesterol acyl transferase (ACAT)
increased cholesterol storage
decreased synthesis of LDL-receptors
“down-regulation”
decreased LDL uptake

24. High density lipoprotein (HDL)

25. Functions of HDL Transfers proteins to other lipoproteins
Picks up lipids from other lipoproteins
Picks up cholesterol from cell membranes
Converts cholesterol to cholesterol esters via the LCAT reaction
HDL can transfer cholesterol ester into the liver:
directly or
through other lipoproteins
“reverse cholesterol transport)
High blood levels of HDL ("good" cholesterol) correlate with low incidence of atherosclerosis.

26. Functions of HDL Function:
Transports C from peripheral tissues to liver (reverse C transport).
Only HDL can pick up cholesterol from peripheral tissues
Only the liver can degrade cholesterol
HDL acts as a circulating store for apo-C & E required in the metabolism of CM & VLDL.

27. HDL: structure
These are the smallest and the most dense lipoprotein particles.
They contain large amount of proteins (50-60%) and very little TG, while the predominant lipid is phospholipid.
Synthesized de novo in the liver and small intestine, primarily as protein-rich disc-shaped particles.
Newly formed HDL are nearly devoid of C & CE.
Apoproteins : apoA, apoC, apoD and apoE.
Apo C- & E- are synthesized in liver and transferred from liver HDL to intestinal HDL when the latter enter the plasma.
apoA-I is synthesized by liver & intestine, which is secreted in a lipid-poor form and then immediately recruits additional PLs and free cholesterol (C) via the ABCA1 pathway, forming nascent HDL.

28. HDL: synthesis
Nascent HDL consists of discoid PL bilayers containing apo E & free cholesterol (C).
On entering the circulation:
apo-A, the major HDL apoprotein is acquired from CM
apo-C is transferred to CM and VLDL.
LCAT and its activator apo-A1, bind to the discoidal particles, and the surface PL & free C are converted to CE & lysolecithin.
The non-polar CE move into the hydrophobic interior of the bilayer, whereas lysolecithin is transferred to plasma albumin. Thus, a spherical HDL is formed.

29. HDL metabolism Liver C + FA C C CE CE C C CM VLDL Bile acids
Apo-A CM
VLDL
Liver
Apo-C
Apo-E
Bile acids
LCAT
Nascent HDL
C + FA C
Apo-A C CE
Peripheral tissue CE C
LCAT C
Other lipoproteins
HDL receptor
Mature HDL

30. HDL metabolism
Apo-AI activates LCAT (lecithin – cholesterol acyl transferase) which is present on the HDL surface. Free cholesterol, present on the HDL surface is esterified by LCAT.
Formation of cholesterol esters in lipoproteins

31. CETP

32. Class B scavenger receptor B1 (SR-B1)
In liver and in steroidogenic tissues: it binds HDL via apo-A1, then CE is selectively delivered to the cells (the particle itself, including apo-A1 is not taken up).
In the tissues on the other hand: SR-B1 mediates the acceptance of cholesterol effluxed from the cells by HDL, which then transports it to the liver for excretion.

33. 2nd mechanism for reverse C transport ATP-binding cassette transporters
It involves the ATP-binding cassette transporters A1 & G1 (ABCA1 & ABCG1).
They are transporter proteins that couple hydrolysis of ATP to the binding of a substrate, enabling it to be transported across the membrane.
ABCG1: transports C from cells to HDL.
ABCA1: preferentially promotes efflux to poorly lipidated particles such as preβ-HDL, which are then converted to discoidal HDL then to HDL3 and finally to HDL2.

34. HDL cycle: interchange of HDL2 & HDL3
HDL3 generated from discoidal HDL by the action of LCAT, accepts cholesterol from the tissues via SR-B1 and C is then esterified by LCAT, increasing the size of particles to form the less dense HDL2.
HDL2 is formed due to the action of cholesterol ester transfer protein (CETP). Exchanges CE with CM & VLDL remnants; gets TG in return. By transferring CE out of HDL, product inhibition of CE on LCAT is eliminated allowing more cholesterol removal from peripheral tissues. When C efflux followed by esterification by LCAT is effective, HDL2 will be high.
HDL3 is then reformed, either after selective delivery of CE to the liver via the SR-B1 or by hydrolysis of HDL2 PL & TG by hepatic lipase (its activity is increased by androgens and decreased by estrogens → higher conc. of plasma HDL2 in women) and endothelial lipase.
Free apo-A1 is released by these processes and forms Preβ-HDL after associating with a minimum amount of PL & C. Surplus apo-A1 is destroyed by kidney.

35. HDL metabolism

36. HDL: types
Pre-β-HDL (<5nm): Hydrolyzed CM & VLDL donate PL & apo-A1. This induces cholesterol efflux leading to discoidal HDL.
Discoidal HDL (nascent HDL): newly secreted HDL from intestine with additional C & PL. Contains Apo-A1. Does not contain apo C or E; synthesized in liver & transferred to HDL.
HDL3 (5-10nm):formed due to the action of LCAT.
HDL2 (10-20nm): formed due to the action of cholesterol ester transfer protein (CETP).
CE transferred to CM & VLDL remnants is degraded to bile acids.
HDLc (HDL1) (20-25nm): is found in blood of diet-induced hypercholesterolic animals. It is rich in C, and its sole apolipoprotein is apo-E.

37. Liver C C C TG CE Intest CM Apo-A1 SR-B1 ABCA1 HDL R PL & C preβHDL
Peripheral tissue C
Discoid HDL
ABCG1 C
LCAT + A1
SR-B1
HDL3 TG
VLDL
IDL
LDL
LCAT
CETP HL EL
HDL2
(↓↓ CE)
(↑↑TG) CE

38. Reverse Cholesterol Transport (RCT)
The process whereby excess cholesterol in peripheral cells, especially foam cells, is returned to the liver for degradation and excretion.

39. HDL summary HDL ACAT LCAT Cholesterol ester formation Site
Intracellular
Extracellular in HDL (in blood)
Acyl donor
Acyl Co-A
Lecithin
Function
Storage of cholesterol
Transport of cholesterol from tissues to HDL then to the liver
HDL
Synthesized in liver and intestine.
Reservoir of apoproteins.
Reverse cholesterol transport.
Liver is the only organ capable of metabolizing and excreting cholesterol.
52% protein, 48% lipid, 35% C & CE
Apo A: Activates LCAT
Apo C: Activates LPL
Apo E: Remnant receptor binding
Cholesterol ester formation

40. Lipoprotein (a) [Lp(a)]
Lp(a) When present, is attached to apo-B100 by a disulfide bond.
in some normal population, there is no detectable level of Lp(a) in serum. High blood level is linked to susceptibility for heart attack at a younger age.
Lp(a) is similar in structure to plasminogen. So, it interferes with plasminogen activation and impairs fibrinolysis. This leads to thrombosis & MI.

41. Separation of Lipoproteins
Why do we have different types of lipoproteins?
They differ in lipid and protein composition
and therefore. So, they differ in
Size and density
Electrophoretic mobility
1- Ultracentrifugation:
Lipoproteins are separated according to their density.
The higher the protein content the higher the density of the particles.
The main fractions separated are: CM, VLDL, LDL, HDL & Free fatty acids-albumin complex.

42. Separation of Plasma lipoproteins
2- Electrophoresis:
- It depends on the charge and molecular weights of various particles.
- Plasma lipoproteins are separated
according to their mobility in electric field into:
1.  lipoproteins. ( HDL)
2. Pre -lipoproteins. (VLDL)
3.  lipoproteins. (LDL)
4. Chylomicrons. (immobilized fraction)
[Free fatty acids-albumin complex is not considered typical lipoprotein fraction]

43. - + Thank you & good Luck Dr. Eman Shaat Chlomicrons
VLDL
LDL
HDL
Albumin+FFA
Ultracentrifugation fractions
Electrophoresis fractions
Non-mobile lipoproteins
B-Lipoproteins
Pre-B Lipoproteins
α-Lipoproteins +
Thank you & good Luck
Dr. Eman Shaat