Lipid transport Flashcards
describe the properties of free fatty acids
βtheyβre formed from triacylglycerides stored in adipose tissue
β it circulates bound to protein as a Na+ salt, particularly the albumin protein
β saturation occurs at about 2 mM of fatty acid molecules
β it enters the cell by simple diffusion
β the intracellular concentration of free fatty acids is kept low
why do free fatty acids (FFAs) need to travel bound to a protein?
βif they are unbound then they will act as detergents
what is a lipoprotein?
βbiochemical assembly whose purpose is to transport hydrophobic lipid molecules in water, in blood or extracellular fluid.
what is the structure of a lipoprotein?
βmembrane consists of phospholipids and cholesterol, and large apolipoproteins.
βIn the center, there are cholesterol esters and triacylglycerol.
what are the 5 lipoproteins?
βChylomicrons βVery Low-Density Lipoproteins (VLDLs) β Low-Density Lipoproteins (LDLs) - βIntermediate Density Lipoproteins (IDLs) β High-Density Lipoproteins (HDLs)
what are the compositions of the 5 lipoproteins?
βCHYLOMICRONS: most (90-95%) triglycerides, so the least dense
βVLDLs: mostly triglycerides (53%)
βLDLs: mostly (50%) cholesterol
β IDLs: intermediate in all (highest is triglycerides, 31%)
βHDLs: mostly (49%) protein
what are apoproteins or apolipoproteins?
βproteins that bind lipids together to form lipoproteins.
what are the functions of apoproteins?
βstructural functions (the backbone of the lipoproteins)
β to solubilise lipids (lipoproteins allow lipids to travel in aqueous solution)
β act as enzymes or enzyme cofactors
β tissue targeting
what is an example of apoproteins acting as enzymes or enzyme cofactors?
βAPO C2 activates lipoprotein lipase (LPL, breaks down fat in the form of triglycerides, so a lipoprotein expressing this apoprotein will lose its triglycerides)
β APO A1 activates Lecithin-Cholesterol AcylTransferase (LCAT, converts free cholesterol into cholesteryl esters)
what is an example of apoproteins involved in tissue targeting?
β APO B100 and APO E bind to the LDL receptor
β APO E binds to the HDL receptor
what are the apoproteins involved in the composition of the different lipoproteins?
βCHYLOMICRONS: B48, APO C2, C3 and E
βVLDL: B100, APO C1, C2, C3 and E
β LDL: B100
β IDL: B100, APO E
βHDL: APO A1, A2, C1, C3, D and E
how does the synthesis of chylomicrons happen?
β formed in the cells that line the gut.
βIn the lumen of the gut, triglycerides are broken down to fatty acids.
βthe triglycerides and monoacylglycerols are brought into the mucosal cell and reformed into triacylglycerides.
βThey then combine with other lipids and proteins in the cell to form chylomicrons.
how and why are chylomicrons delivered directly to the lymph system?
βChylomicrons are secreted into the lymphatics which carries them via the thoracic duct to the superior vena cava.
βBy this pathway, dietary fats avoid the direct delivery to the liver and instead are made available to the extrahepatic tissue.
where are digested proteins and carbohydrates delivered?
βdigested proteins and carbohydrates are released into the portal vein and delivered directly to the liver.
how does a nascent chylomicron become a mature chylomicron (in terms of apoproteins)?
βwhen first formed, the only apoprotein chylomicrons consist of are the ApoB48.
βas it circulates, it interacts with HDL and the HDL donates certain apoproteins to the nascent chylomicron, primarily Apo C2 and Apo E.
βthis forms a mature chylomicron.
βthis donation occurs in the SER.
what are chylomicrons for and how are they removed?
βtheyβre important for transporting exogenous (dietary) lipids from gut around the circulation
β they reflect meal composition (if a fatty meal is ingested, there will be many chylomicrons of that composition)
β their remnants are removed by the liver, with the help of Apo E
what do chylomicrons contain and what is their lifetime?
βthey contain fat soluble vitamins such as Vitamins A and E
βtheir lifetime in the circulation is about one hour (the triglycerideβs is about 5 minutes)
why do chylomicrons have a low density?
βthey have a low density due to a high number of triglycerides