Plasma Lipoproteins

1. Plasma Lipoproteins • What are plasma lipoproteins? • Spherical macromolecular complexes of: • Lipids + specific proteins (apo-proteins) • They includes: • Chylomicron • Very low density lipoproteins (VLDL) • Low density lipoproteins (LDL) • High density lipoproteins (HDL)

2. How can lipoproteins Differ? • They differ according to: • Composition of lipids to proteins • Size • Density

3. Functions of Lipoproteins • Transport lipids in plasma by the protein portion (keep lipids soluble) • Transporting their lipid content to & from tissues • N.B. In humans, the transport system is less perfect than in other animals cholesterol deposition in tissues atherosclerosis

4. Composition of Plasma Lipoproteins • Neutral core (TAG, exogenous or de novo, cholesterol esters) • Amphipathic apolipoprotein • Phospholipids • Cholesterol

6. Size & Density • Chylomicrons: largest in size, lowest in density, highest in lipids & lowest in proteins • VLDL & LDL: denser, higher ratio of protein to lipid than chylomicrons • HDL: densest • N.B. lipoprotein particles constantly interchange lipids & proteins with each other making them variable • N.B. lipoproteins can be separated by electrophoresis (mobility) or by ultracentrifugation (density)

7. Apolipoproteins • The apolipoproteins associated with lipoprotein particles have a number of diverse functions • Proteins in nature • Function: • 1.has recognition site for cell-surface receptors • 2.serve as activators or coenzyme for lipoprotein metabolism • 3.important for lipoprotein function • 4.Some are freely transferred between lipoproteins

8. Classes of apolipoproteins • A, B, C, D, E are major classes • Subclasses: apo A-1, apo C-II • N.B. function of all apolipoproteins are not yet known

9. Metabolism of Chylomicrons CM • Site: intestinal mucosal cells • Function: carry dietary TAG, chol., fat-soluble vit., chol. esters & lipids made in intestinal cells to peripheral tissues. • Synthesis of apolipoproteins : • Apo B-48 is unique to CM • Synthesized in rough endoplasmic reticulum (RER) • Named 48 because 48% of n-terminal protein is coded by apo B gene

10. Composition of CM Apoprotein • Apo B-48 • Apo C-II from HDL • Apo E from HDL • Assembly of CM: • Requires microsomal TAG transfer protein to load apo B-48 with lipids (TAG, Chol. phospholipids) • Synthesis occurs in ER golgi  packed in secretory vesicles fused with plasma membrane releasing lipoproteins lymphatic system blood

11. Modification of Nascent CM • Intestinal mucosal cells produce nascent CM • Called nascent because it is functionally incomplete • Once reaches plasma, CM particles receive apo E & apo C-II from HDL • Apo C-II is activator of lipoprotein lipase that degrades TAG in CM

12. Formation of CM Remnant • As CM circulates & its TAG degrades by lipoprotein lipase, apo C is returned to HDL  remnant CM  liver where its cell membrane can recognize apo E (receptor) to take up CM remnant by endocytosis  degradation releasing AA, cholesterol & FA

13. Lipoprotein lipase • Lipoprotein lipase is extra-cellular enzyme of capillary walls of adipose tissue, cardiac & skeletal muscle, not found in liver • Activated by apo C-II • Hydrolyzes TAG in CM  FA + glycerol • FA either stored by adipose tissues or generate energy by muscle • Glycerol  liver for lipid synthesis, glycolysis or gluconeogenesis

14. Type 1 hyperlipoproteinemia or familial lipoprotein Lipase deficiency • Patients deficient in lipoprotein lipase or apo C-II • Accumulation of CM in plasma • N.B. lipoprotein lipase is stimulated by insulin

16. Metabolism of VLDL • Site of production: Liver • Composition: predominately TAG, apo B-100& obtain apo C-II & apo E from circulating HDL • Function: carry TAG from liver to peripheral tissues • apo C-II is required for activation of lipoprotein lipase

17. Modification of circulating VLDL • TAG is degraded from VLDL by lipoprotein lipase  particles decrease in size & gets denser • Apo C & E return to HDL • TAG goes to HDL & Cholesterol ester (CE) from HDL goes to VLDL (exchange mechanism by CE transfer protein) fig18.18

18. Production of LDL from VLDL • VLDL in plasma  LDL + IDL (VLDL remnant) • IDL can be taken up by cells via receptor-mediated endocytosis using apo E (apo E isoforms: E2, E3, E4) • Apo E2 binds poorly to receptors

19. Metabolism of VLDL • In peripheral tissues, VLDL-TAG are digested by LPL, and VLDL is converted to IDL. • IDL returns to the liver, is taken up by endocytosis, and is degraded by lysosomal enzymes. IDL can also further degradation, forming LDL. • LDL reacts with receptors with receptors on various cells and is digested by lysosomal enzymes.

22. A beta lipoproteinemiaHypolipoproteinemia • Defect in TAG transfer protein • Inability to load apo B with lipid • No chylomicrons or VLDLs • TAG accumulates in liver & intestine

23. Familial Type III hyperlipoproteinemia • Also called familial dysbetalipoproteinemia • Patients are deficient in apo E2 •  Clearance of CM remnant & IDL • Hypercholesterolemia

24. Metabolism of LDL • Composition: 50% cholesterol & CE, apo 100 • Function: provide cholesterol to peripheral tissues or return it to liver • Mechanism of LDL uptake: by cell-surface membrane LDL receptors that recognize apo B-100 (LDL) or apo E (VLDL) • These receptor are called apo B-100/apo E receptors

25. The primary function of LDL particles is to provide cholesterol to the peripheral tissues (or return it to the liver). • LDL receptors are negatively charged glycoproteins that are clustered in pits on cell membranes. The intracellular side of the pit is coated with the protein clathrin, which stabilizes the shape of the pit • After binding, the LDL-receptor complex is internalized by endocytosis

26. The pH of the endosome falls (due to the proton-pumping activity of endosomal ATPase), which allows separation of the LDL from its receptor. • The receptors then migrate to one side of the endosome, whereas the LDLs stay free within the lumen of the vesicle. [Note: This structure is called CURLCompartmentfor Uncoupling for Receptor and Ligand]

27. The receptors can be recycled, whereas the lipoprotein remnants in the vesicle are transferred to lysosomes and degraded by lysosomal (hydrolytic) enzymes, releasing free cholesterol, amino acids, fatty acids, and phospholipids. • These compounds can be reutilized by the cell.

28. LDL Receptors • -ve charged cell membrane glycoprotein • Intracellular side is coated with protein clathrin to stabilize the receptor • LDL receptor deficiency   plasma LDL & cholesterol  type II hyperlipidemia (familial hypercholest.) • T3 cause +ve effect on LDL binding to its receptors • Hypothyroidism  hypercholesterolemia

29. LDL Receptors (continue) • Vesicle containing LDL loses its clathrin & fuses with other vesicles  large vesicles  endosomes •  pH of endosomes due to proton pump ATPase CURL LDL + free receptor • N.B.CURL is Compartment for Uncoupling of Receptor & Ligand • Receptor can be recycled (fig 18.20)

32. Effect of endocytosed cholesterol on cellular cholesterol homeostasis • The chylomicron remnant-, IDL-, and LDL-derived cholesterol affects cellular cholesterol content in several ways. • First, HMG CoA reductase is inhibited by high cholesterol, as a result of which, de novo cholesterol synthesis decreases. • Second, synthesis of new LDL receptor protein is reduced by decreasing the expression of the LDL receptor gene, thus limiting further entry of LDL cholesterol into cells

33. Effect of endocytosed cholesterol on cellular cholesterol homeostasis • Third, if the cholesterol is not required immediately for some structural or synthetic purpose, it is esterified by acyl CoA cholesterol acyltransferase (ACAT) transfers a fatty acid from a fatty acyl CoA derivative to cholesterol, producing a cholesteryl ester that can be stored in the cell. • The activity of ACAT is enhanced in the presence of increased intracellular cholesterol.

34. Macrophage Scavenger receptors • In addition to the highly specific and regulated receptor-mediated pathway for LDL uptake, macrophages possess high levels of scavenger receptor activity. • These receptors, known as scavenger receptor class A (SR-A), can bind a broad range of ligands, and mediate the endocytosis of chemically modified LDL.

35. Macrophage Scavenger receptors • Chemical modifications that convert circulating LDL into ligands that can be recognized by SR-A receptors include oxidation of the lipid components and apolipoprotein B. • Unlike the LDL receptor, the scavenger receptor is not down-regulated in response to increased intracellular cholesterol. • Cholesteryl esters accumulate in macrophages and cause their transformation into "foam" cells, which participate in the formation of atherosclerotic plaque

36. Macrophage Scavenger receptors • Receptor: scavenger receptor class A (SR-A) • Ligand: oxidized LDL (lipid + apo B) • SR-A is not down-regulated to increased intracellular cholesterol  cholesterol in macrophages  foam cells  atherosclerosis plaque (fig 18.22) • Lipoprotein (a) in heart disease: • Is identical to LDL & linked to apo B-100 • It increases the risk for coronary heart disease

38. Metabolism of HDL (good Cholesterol carrier) • Heterogeneous, secreted into blood from liver & intestine • Structure: nascent HDL contains PL, apo A,C & E fig.18.23 N.B PL solubilize cholesterol • Function:1. reservoir of apolipoproteins apo C-II VLDL & chylomicrons, apo E is required for receptor-mediated endocytosis of IDLs & chylomicron remnants • 2. activator of lipoprotein lipase • 3. Uptake of cholesterol from other lipoproteins & cell membrane

39. 4.Esterification of Chol. in HDL by plasma phosphatidylcholine : chol.acyltransferase (PCAT) [also known as LCAT L for lecithin] • LCAT binds to nascent HDL & is activated by apo A-1 • 5.Selective transfer of chol. from peripheral cells to HDL & from HDL to liver for bile acid & hormone synthesis mediated by scavenger receptor class B-1 (SR-B1) • key for chol. Homeostasis,  plasma HDL atherosclerosis

40. Metabolism of HDL (good Cholesterol carrier) • HDL is a reservoir of apolipoproteins: • HDL particles serve as a circulating reservoir of apo C ll (the apolipoprotein that is transferred to VLDL and chylomicrons, and is an activator of lipoprotein lipase), • and apo E (the apolipoprotein required for the receptor-mediated endocytosis of IDLs and chylomicron remnants • Apo C ll and Apo E are transferred back to HDL following digestion of TAG of chylomicrons and VLDL.

41. HDL uptake of unesterified cholesterol: • Nascent HDL are disk-shaped particles containing primarily phospholipid (largely phosphatidylcholine) and apolipoproteins A, C, and E. They are rapidly converted to spherical particles as they accumulate cholesterol. • [Note: HDL particles are excellent acceptors of unesterified cholesterol (both from other lipoproteins particles and from cell membranes) as a result of their high concentration of phospholipids, which are important solubilizers of cholesterol.]

42. Esterification of cholesterol: • When cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme phosphatidylcholine:cholesterol acyltransferase (PCAT, also known as LCAT) • This enzyme is synthesized by the liver. • PCAT binds to nascent HDLs, and is activated by apo A-1 • PCAT transfers the fatty acid from carbon 2 of phosphatidylcholine to cholesterol. This produces a hydrphobic cholesteryl ester. As cholesterol esters accumulate in the core of lipoprotein

43. HDL transfers cholesterol esters to other lipoproteins in exchange for various lipids. • Cholesterol ester transfer protein (CETP) mediates this exchange • HDL and other lipoproteins carry the cholesterol esters back to the liver.

45. Activity • Schematic diagram of lipogenesis and lipolysis • Schematic diagram of ketogenesis and ketolysis • Schematic diagram cholesterol synthesis • Discuss types and functions of plasma lipoproteins.