Lipoproteins I 19-2-2021_060347.pptbbnrx

Lipoprotina

1. Lipoproteins: structure and function Prof Kunzang Chosdol Department of Biochemistry

2. Objectives List different classes of lipoproteins Distinguish between endogenous and exogenous lipid transport Describe the structure and function of different classes of lipoproteins Function of different apolipoproteins Enumerate different methods for lipoprotein separation

3. Definition: Lipoproteins – Spherical macromolecular complexes of lipids and proteins that are essential for transport of cholesterol, triglycerides, and fat-soluble vitamins Core of hydrophobic lipids ( cholesterol ester & Triacylglycerol ) Shell of proteins ( apolipoproteins ) and polar lipids (phospholipids and free cholesterol)

4. General structure of plasma lipoprotein A shell of hydrophilic lipids (phospholipids, unesterified cholesterol) and proteins (called apolipoproteins ) that interact with body fluids A core of hydrophobic lipids (triglycerides and cholesteryl esters)

5. Lipoproteins help in the transport of triacylglycerol and cholesterol through blood stream between various tissues Functions of lipoproteins

6. Types of lipoproteins The plasma lipoproteins are divided into five major classes based on their relative density Chylomicrons : Very low- density lipoproteins ( VLDL): Intermediate -density lipoproteins ( IDL): Low-density lipoproteins ( LDL): High-density lipoproteins ( HDL): Each lipoprotein class comprises a family of particles that vary in density, size, and protein composition

7. Classification of L ipoproteins Chylomicrons are the most lipid-rich and therefore least dense lipoprotein particles, whereas HDLs have the least lipid and are therefore the most dense lipoproteins Lipoprotein fraction Major Lipids Density Apoproteins Electrophoretic Mobility Chylomicrons Dietary Triglycerides (Intestine) < 0.95 A (I , II , IV), B-48, C (I,II,III), E Remains at origin VLDL Endogenous Triglycerides (Liver) 0.95-1.006 B-100, C, E Pre beta IDL Cholesteryl ester, TG (VLDL) 1.006-1.019 B-100, E Slow pre beta LDL Cholesterol , Cholesteryl ester (VLDL) 1.019-1.063 B-100 Beta band HDL Cholesteryl ester, Phospholipid (Peripheral tissues) 1.063-1.210 A (I, II, IV), C, D, E Fast alpha band

8. Lipoproteins: comparative sizes, protein and lipid content Intechopen.com

9. chylomicrons , derived from intestinal absorption of triacylglycerol and other lipids VLDL, derived from the liver for the export of triacylglycerol (4) HDL, involved in cholesterol transport and also in VLDL and chylomicron metabolism (3) LDL, representing a final stage in the catabolism of VLDL Lipid compositions of lipoproteins

10. Harrison’s 17 th ed. Cofactors for LPL: Apo C-II Phospholipids Transport of lipids 80% of the dietary lipids are delivered to extra-hepatic tissues

11. There are three stages in the chylomicron's "lifecycle": Nascent, Mature and Remnant Nascent chylomicron : In the intestinal lumen, triglycerides are emulsified by bile and hydrolyzed by lipase Absorbed into the enterocyte , where they are re- esterified to form triglycerides The triglycerides are then combined with phospholipids, cholesteryl esters, and apolipoprotein B-48 to form a nascent chylomicron Chylomicron synthesis and secretion Nascent chylomicron is exocytosed  into the lymphatic vessels of the small intestine, and secreted into the bloodstream at the  thoracic duct's connection with the left  subclavian vein

12. Movement of dietary fatty acids to the endoplasmic reticulum of enterocyte doi :  10.1146/ annurev-physiol -021909-135801 Exit of prechylomicrons from the endoplasmic reticulum

13. Chylomicrons HDL donates  Apo-C2 and Apo-E to the nascent chylomicron Once triglyceride stores are distributed, the chylomicron returns Apo-C2 to the HDL  (but keeps Apo-E), and, thus, becomes a chylomicron remnant Apo-C2 is the coenzyme for lipoprotein lipase (LPL)

14. Chylomicrons: Apo B-48, Apo C-II, Apo E

15. Very low density lipoproteins (VLDL) and intermediate density lipoproteins (IDL): Apo B-100, Apo C-II and Apo E

16. VLDL assembly in liver Harper’s Illustrated Biochemistry, 30 th ed. MTP-microsomal TG transfer protein COPII- vesicle coat protein (ER to Golgi) PLD- phospholipaseD PA- phosphotidic acid ADP- ribosylation factor-1 Synthesis of VLDL2 in ER lumen: Apo B-100 is synthesized on polyribosomes and is lipidated with PL by MTP as it enters the ER lumen TG transferred into the ER lumen with the aid of MTP and interacts with apoB-100 forming VLDL2 Synthesis of VLDL1 in Golgi lumen: VLDL2 are translocated to the golgi in COPII vesicles where they fuse with TG-rich particles to form VLDL1 PA produced by activation of PLD by ARF-1 is incorporated into the VLDL1 and/or VLDL2 BothVLDL1 and VLDL2 may be secreted into the blood Insulin inhibits VLDL secretion

17. The formation and secretion of (B) VLDL lipoproteins by a hepatic cell (A) Chylomicrons by an intestinal cell Harper’s Illustrated Biochemistry, 30 th ed.

18. VLDL - LDL

19. LDL formation

20. The liver and many extrahepatic tissues express the LDL (apoB-100, E) receptor (specific for apo B-100 and also takes up lipoproteins rich in apo E) Approximately 30% of LDL is degraded in extrahepatic tissues and 70% in the liver A positive correlation exists between the incidence of atherosclerosis and the plasma concentration of LDL cholesterol The LDL receptor is defective in familial hypercholesterolemia, a genetic condition in which blood LDL cholesterol levels are increased , causing premature atherosclerosis Low density lipoproteins: Apo B-100

21. Major carrier of cholesterol in blood Core has hundreds of cholesterol ester molecules : responsible for transport of cholesterol and regulation of cholesterol homeostasis LDL level is regulated by uptake mediated by LDL receptors on extrahepatic and hepatic cells Many sub-classes ; sd (small dense) LDL having altered composition (decreased phospholipids, free cholesterol and cholesterol ester with similar TG content) - associated with higher risk of Cardio-vascular diseases (CVD) Ox- LDL ( oxidatively modified LDL ): A particle derived from circulating LDL having peroxides/their degradation products generated within the LDL molecule or elsewhere in the body Low density lipoproteins: Apo B-100 TG

22. oxLDL uptake by scavenger receptors leads to the accumulation of cholesterol within the foam cells of atherosclerotic lesions oxLDL leads to ApoB misfolding which tends to form fibrillar aggregates in atherosclerosic lesions Lipoprotein (a) : Similar to low-density lipoprotein ( LDL ), but containing an additional protein, apo (a) Apo(a) is synthesized in the liver attached to Apo B-100 via a disulfide bond. High levels of Apo(a) are associated with an increased risk of atherosclerosis Apo (a) is an inhibitor of fibrinolysis and can also enhance the uptake of lipoproteins by macrophages – both could increase the risk of atherosclerosis

23. High Density Lipoproteins (HDL): (Apo A-I, A-II, Apo C, Apo-D and Apo-E) ABCA 1, ATP-binding cassette transporter A1; ABCG1 , ATP-binding cassette transporter G1 LCAT, lecithin:cholesterol acyltransferase ; CETP, cholesteryl ester transfer protein SR-B1, scavenger receptor B1

24. HDL is synthesized and secreted from both liver and intestine Apo C and apo E are synthesized in the liver and transferred from liver HDL to intestinal HDL in the plasma A major function of HDL is to act as a repository for the apo C and apo E required in the metabolism of chylomicrons and VLDL Nascent HDL consists of discoid phospholipid bilayers containing apo A and free cholesterol LCAT ( lecithin:cholesterol acyltransferase ) and the LCAT activator apo A-I— bind to the discoidal particles, and the surface phospholipid and free cholesterol are converted into cholesteryl esters and lysolecithin High Density Lipoproteins (HDL): (Apo A-I, A-II, Apo C, Apo-D and Apo-E)

25. The nonpolar cholesteryl esters move into the hydrophobic interior of the bilayer , whereas lysolecithin is transferred to plasma albumin Thus, a nonpolar core is generated , forming a spherical, pseudomicellar HDL covered by a surface film of polar lipids and apolipoproteins This aids the removal of excess unesterified cholesterol from lipoproteins and tissues Patients with a deficiency of the plasma enzyme lecithin:cholesterol acyltransferase ( LCAT ) and in the plasma of patients with obstructive jaundice –found to have discoid phospholipid bilayers similar to Nascent HDL in the plasma High Density Lipoproteins (HDL): (Apo A-I, A-II, Apo C, Apo-D and Apo-E)

26. The class B scavenger receptor B1 (SR-B1) has been identified as an HDL receptor with a dual role in HDL metabolism In the liver and in steroidogenic tissues - it binds HDL via apo A-I, and cholesteryl ester is selectively delivered to the cells, the HDL including apo A-I is not taken up In other tissues , SR-B1 mediates the acceptance of cholesterol effluxed from the cells by HDL, which then transports it to the liver for excretion via the bile (either as cholesterol or after conversion to bile acids) in the process known as reverse cholesterol transport High Density Lipoproteins (HDL): (Apo A-I, A-II, Apo C, Apo-D and Apo-E)

27. Metabolism of HDL - in reverse cholesterol transport LCAT , lecithin:cholesterol acyltransferase; SR-B1 , scavenger receptor B1; ABCA 1, ATP-binding cassette transporter A1; ABCG1 , ATP-binding cassette transporter G1 HDL is synthesized from both liver and intestine Apo -C and apo E synthesized in liver are tansferred to intestinal HDL in plasma ABCA1 transport intracellular free cholesterol and phospholipids to pre- β HDL to form nascent HDL SR-BI - selective uptake of CE, phospholipids and triglycerides ABCG1 cooperate with ABCA1 to complete the reverse cholesterol transport

28. Apolipoproteins Apolipoproteins And their functions

29. Apolipoprotein C : freely exchanges between lipoprotein particles Apo C-II is a co-factor for lipoprotein lipase (LPL) and thus stimulates triglyceride hydrolysis. Apo C-III is an inhibitor of LPL . inhibits the interaction of triglyceride rich lipoproteins with their receptors Apolipoprotein E : synthesized in liver Associated with chylomicrons, chylomicron remnants, VLDL, IDL, and a subgroup of HDL particles There are three common genetic variants of Apo E (Apo E2, E3, and E4) Apo E2 homozygotes can develop familial dysbetalipoproteinemia Apo E4 is associated with an increased risk of Alzheimer’s disease and atherosclerosis Apolipoproteins

30. Name Predominant lipoprotein Molecular weight ( kda ) Plasma concentration Role Apo (a) Lp(a) Variable 0.2-200 Unknown Apo A-I HDL 28.3 90-160 ACAT activation, structural Apo A-II HDL 17 25-45 Structural Apo A-IV HDL 45 10-20 Structural, absorption Apo A-V VLDL , HDL     TRL metabolism Apo B-100 LDL , VLDL 512 50-150 Structural, LDL-R binding Apo B-48 Chylomicrons 241 0-100 Structural Apo C-I Chylomicrons 6.63 5-6 TRL Metabolism Apo C-II Chylomicrons , VLDL 8.84 3-5 LPL Activation Apo C-III Chylomicrons , VLDL 8.76 10-14 LPL Inhibition Apo D HDL 33 4-7 LCAT Apo E Chylomicrons remnants, IDL 34 2.8 LDL-R, apo E receptor binding

31. Properties Chylomicron VLDL IDL LDL HDL Density (g/mL) <0.95 0.950-1.006 1.006-1.019 1.019-1.063 1.063-1.210 Diameter (nm) 75-1200 30-80 15-35 18-25 7.5-20 Components (% dry weight): Protein   2   8   15   22   33-57 ii. Triacylglycerol 86 55 31 6 5-10 iii. Free cholesterol 2 7 7 8 3-7 iv. Cholesterol esters 3 12 23 42 14-21 v. Phospholipids 7 18 22 22 19-29 vi. Apoprotein composition A-IV , B-48 C-I,C-II, C-III, E B-100, C-I, C-II, C-III, E B-100, C-I,C-II, C-III, E B-100, E A-I, A-II, C-I, C-II, C-III, D, E vii. Half –life: 20 minutes 4-6 hrs Few minutes 2-3 days 6 days Electrophoretic Mobility Remains at origin Pre beta Slow pre beta Beta band Fast alpha band

32. To be continued……..