Biochemistry - Blood Lipoproteins 2023 PDF

Objective A Lipoproteins - Introduction • TGs, cholesterol and cholesterol esters (CEs) are too hydrophobic to travel in a free form in the blood. • They are packaged into the hydrophobic core of an aggregate of phospholipid, cholesterol and amphipathic proteins. • Aggregate known as a lipoprotein,...

Objective A Lipoproteins - Introduction • TGs, cholesterol and cholesterol esters (CEs) are too hydrophobic to travel in a free form in the blood. • They are packaged into the hydrophobic core of an aggregate of phospholipid, cholesterol and amphipathic proteins. • Aggregate known as a lipoprotein, part of a class of lipoproteins that circulate in the blood. VLDL • Grouped according to density Less fat, greater protein content = higher density More fat, less protein = lower density 3 3 Objective A Lipoprotein Density Depends on Content… Classed as: chylomicrons, VLDL, IDL (i.e., VLDL remnants), LDL and HDL 4 4 Objective A …and Function Driven by Apoproteins 32.3 Lipoprotein-associated Phospholipase A2 55 Objective A Chylomicrons • Intestinal epithelial cells make chylomicrons from dietary materials: • • • • • TGs which are contained within shell (largest component) Cholesterol which partitions with the phospholipids Cholesterol esters, which partitions within the shell Fat soluble vitamins that partition within the hydrophobic core Proteins on the surface of the particle: dictate function / recognition • carry dietary TGs from the intestine to peripheral tissues, especially muscle and adipose tissue • deliver cholesterol and phospholipids to the liver. • Major proteins contained are: • • • • ApoB-48 Apo CII (binds to lipoprotein lipase) Apo E (promotes recycling by liver) Apo A (lipoprotein associated lipase A2) 6 6 Objective B Introduction: Digestion and Uptake of Dietary Lipid • Most dietary lipid is triglycerides (TGs) and some phospholipids, cholesterol, cholesterol esters and fat soluble vitamins • Dietary fat molecules are very hydrophobic and do not mix with the water medium. Very little digestion in saliva and stomach. • The “solubility” of lipid in the intestine is facilitated by bile salts (BS) and “detergent” molecules: amphipathic molecules that emulsify dietary TGs and other lipids into micelles (microdroplets or chylomicrons). • Bile salts: gall bladder (see Cholesterol lecture) • “Detergent” molecules formed from TGs and PLs. • If bile salts and detergent molecules aren’t present, fats will clump together to minimize water contact, form large droplets, clog the digestive tract and be eliminated fairly intact (steatorrhea). • Small and medium chain fatty acids (8-10C) are soluble enough to be absorbed directly and can move into the portal vein without bile salt involvement. 7 7 Objective B Pancreatic Lipase • A water soluble enzyme that removes fatty acids from TGs • Uses colipase, a small cofactor protein • Both bind to emulsified TGs at the lipid-water interface • Hydrolysis occurs at C-1 and C-3 to form 2 free FAs and 2-monoacylglycerol • 2-MG far more soluble than TG • The lipase products are amphipathic and form mixed micelles with bile salts • micelles are tiny microdroplets emulsified with bile salts • Pancreas also produces esterases, which remove FAs esterified with other compounds (e.g., cholesterol esters) 8 8 Objective B Pancreatic Phospholipase A2 • Dietary phospholipids are hydrolyzed by pancreatic phospholipase A2 to form lysophospholipid and a free fatty acid. • Lysophospholipid is a powerful detergent that aids in the emulsifying action of bile salts 9 9 Objective B Cholesterol and Cholesterol Esters • Esterified cholesterol is more common in the diet than free cholesterol. It is more hydrophobic and easier to store in tissues. • Cholesterol esterase hydrolyzes the ester bond, producing free cholesterol and a free acyl compound. • Free cholesterol is easier to transport from the intestinal lumen into the intestinal epithelial cells. 10 10 Objective B Fat Uptake and Resynthesis of TG in Epithelial Cells • Micelles are microscopic • Interact with enterocyte membrane • Move into cell by simple diffusion and also by specific transporters Micelles broken apart in to components Once inside the enterocyte: • Two free FAs and one 2-monoacylglycerol recombine into TG • enzymatic process on smooth ER • FAs are first activated by acetyl CoA, add sequentially to form TG (more in later lecture) • Reformed TGs are packaged into chylomicrons for transport Process similar to VLDL formation in the liver 11 11 Objective B Proteins Associate with the Surface of the Chylomicron VLDL • apoB-48 is the major apoprotein associated with chylomicrons • apoB-48 is a smaller version of the apoprotein B-100, which is synthesized in the liver and is the major protein of VLDL • apoB proteins are added to the chylomicrons/VLDLs as they are made in the tissue ER 12 12 Objective B Addition of Lipid and TGs • microsomal triglyceride transfer protein (MTP) transfers lipid and TG across ER membrane and into the ApoB particle as it is made in the ER lumen. • MTP binds to B-lipoproteins (chylomicron and VLDL) and is activated. • Abetalipoproteinemia : disease where MTP is missing – chylomicrons and VLDLs aren’t formed. 13 13 Objective B Chylomicron Formation: Release into the Lymph System • TG resynthesized in smooth ER, incorporated into lipoprotein structure. • ApoB-48 made in rough ER. • Golgi combines ApoB-48 with lipoprotein, finalizes chylomicron synthesis. • “Nascent” (newly formed) chylomicrons excreted by exocytosis into the lymph fluid. • Lymph fluid empties into the blood at the thoracic duct – blood flows to heart to supply cardiac cells. • Additional processing of chylomicrons in the blood (later). 14 14 Objective A Very Low Density Lipoproteins - VLDLs • VLDLs carry endogenously synthesized TGs, cholesterol and cholesterol esters from the liver to tissues, especially TGs to muscle and adipose tissue • Smaller than chylomicrons, slightly higher density • Made by the liver in a process very similar to assembly of chylomicrons in intestinal epithelial cells – except released directly to the blood. • Major difference is the presence of apo B100 rather than apo B48 15 15 Objective D Fate of Chylomicrons (and VLDLs) Nascent chylomicrons enter the blood (after lymph) with only Apo-B48 and VLDLs with only Apo-B100 They pick up other apoproteins from HDL particles: primarily Apo-CII and Apo-E Apo-CII binds to lipoprotein lipase which holds the chylomicron and digests the triglyceride to free fats and imports the fat into the cell. Recall: muscle cell LPL has much higher affinity for Apo-C than adipose tissue. Allows muscle to utilize fat even when concentration of chylomicrons and VLDLs are low. The fat content of the chylomicron decreases substantially, and the density increases, forming a “chylomicron remnant. Apo-E binds to receptors on liver cells, stimulating endocytosis and degradation of the remnant. 16 16 Objective A Low Density Lipoproteins - LDLs • LDL made from VLDL that has had its TG removed. • LDL smaller and more dense than VLDL due to low amount of TG. • Exposes attached proteins differently • LDL are rich in cholesterol and cholesterol esters • Delivers cholesterol and cholesterol esters to all peripheral tissues • Primary cholesterol transport mechanism. 17 17 Objective E The LDL Receptor • All blood lipoproteins have receptors on cells – best characterized is the LDL receptor • LDL receptor recognizes apoE and apoB100 • Binds LDL, VLDL, IDL (liver synthesized cholesterol) • chylomicron remnants (dietary cholesterol) • Its job is to bind lipoproteins and bring them into the cell by endocytosis • For LDLs, that means delivering cholesterol and cholesterol esters 18 18 Objective F Cholesterol and the LDL Receptor • LDL receptor delivers cholesterol and CE to cell • Increases cholesterol in cell • But…increased cholesterol tightly regulates its own metabolism: • LDL receptor under control of SREBP/SRE • (see cholesterol lecture) • inhibits LDL receptor synthesis • controls endogenous cholesterol synthesis • Activates storage of excess cholesterol: • Re-esterified to form cholesterol ester droplets in the cytosol 19 19 Objective F Endocytosis of LDLs • Receptor-mediated process. • Endosomes fuse with lysosomes, lipoprotein degraded to basic constituents: cholesterol esters are hydrolyzed to free cholesterol • Some cholesterol moves to ER, most cholesterol is re-esterified to keep free cholesterol level low inside cell. • Receptors recycled via Golgi to Plasma Membrane 20 20 Note: IDL à LDL pretty subtle, depends on decrease in TGs Objective G Fate of VLDLs and LDLs • As VLDLs lose TGs, they are converted into Intermediate Density Lipoproteins (IDLs), which are smaller and have a higher protein content, thus a higher density. • Some IDL is taken up by the liver, some is converted to LDL and some give up remaining cholesterol to nascent HDL • Note role of receptor-mediated endocytosis for importing lipoproteins into the cell…LDL recptor binds LDL particles, liver receptor binds ApoE on various lipoprotein species. Note: LDL particles in sick / damaged cells exposed to ROS, become oxidized Macrophages seek out oxidized LDLs, consume them – but become more active, make more ROS àmore OxLDL Become foam cells that burst and dump contents à recruits more macrophages, etc. 21 21 Objective A High Density Lipoproteins (HDL) • Smallest blood lipoprotein with the highest protein content, thus the highest density • Nascent HDLs are synthesized and released from the liver and intestinal cells. • Can also be formed from Chylomicron / VLDL remnants • HDLs pick up cholesterol from tissues: • esterify it by LCAT and transport cholesterol esters to IDL transforming IDL to LDL • OR…carry CEs back to the liver. • Also pick up apolipoproteins released from other lipoproteins or act as a reservoir to donate apoproteins to other lipoproteins. 22 22 Objective H HDL in Reverse Cholesterol Transport REVERSE CHOLESTEROL TRANSPORT • Nascent HDL are synthesized in liver and intestine • Upon release, HDLs interact with cells taking up excess cholesterol from the outside face of the plasma membrane • Once taken up by HDL, the cholesterol is esterified by LCAT (lecithin-cholesterol acyl transferase) and moved into the HDL interior. • Cholesterol esters can be transferred to VLDL (moving them to IDLs) or transported back to the liver directly by HDL uptake by the liver. PROTEIN RESERVOIR FOR NASCENT VLDL / Chylomicrons • In the blood, HDLs also serve as a protein reservoir, providing apoproteins apoC-I,II and III and apoE to nascent VLDLs and nascent chylomicrons 23 23 Sample Question Why does intracellular cholesterol regulate the number of LDL receptors on the cell surface? a. To control the number of LDL particles that are synthesized by the cell. b. To control the entry of cholesterol into the cell via LDL c. To control the entry of cholesterol precursors into the cell via LDL d. To control the release of cholesterol from the cell via LDL. 24 24 Sample Question How does intracellular cholesterol regulate the number of LDL receptors on the cell surface? Cholesterol affects endocytosis and exocytosis of the LDL receptor Cholesterol in the plasma membrane inhibits the LDL receptor activity Cholesterol levels control SREBP Cleavage Activating Protein (SCAP) activity Cholesterol uses LDL receptors in the cell as a storage device. 25 25 Sample Question Familial hypercholesterolemia is a disease that involves defective LDL cellular receptors, such that the cells do not take up LDL at a normal rate. Which of the following would NOT be a result of this disease? a. b. c. d. increased blood LDL levels increased synthesis of cholesterol in cells fatty deposits in coronary arteries and elsewhere increased endocytosis of LDLs. 26 26 Thank You! 27 27