Lecture 23 Flashcards
What is the main challenge of lipid digestion and where are they digested?
Lipids are not soluble in our bloodstream but must be accessable by enzymes which are. The digestion occurs primarily in the small intestine but also in the stomach and mouth.
What do lipases do?
Lipases break down triglycerides (90% of lipids in diet) and phospholipids and are found in the mouth stomach and small intestine. The breakdown of this triacylglycerol leads to two fatty acids being cleaved off and leaves monoacylglycerol.
What do bile salts do? What are they made of?
Bile salts are made of cholesterol (can lead to gall stones) (and water, bile acids, electrolytes, phospholipids and bile pigments), they are synthesised in the liver and stored in the gall bladder as bile. it is secreted into the small intestine in response to cholecystokinin release. It acts to allow increase surface area of the lipid at the aqueous surface by forming micelles (a coat over the triacylglycerides due to the bile salts ampipathic nature) with the triacylglycerols.
How does lipase act against the triacylglycerol and then what happens?
The lipase will hydrolyse the triacylglycerol at the carbonyl carbon on the fatty acid chain (positions 1 and 3 of glycerol backbone). Smaller micelles form with the bile salts and are absorbed across the lumen.
What is fat malabsorption? How does it occur? What is a drug that works on this principle?
fat malabsorption leads to excess fat and fat soluble vitamins in the faeces, it is caused by conditions that interfere with bile or pancreatic lipase secretion. Xenical (orlistat) inhibits lipase in order to prevent fat uptake and help lower weight.
How are absorbed fatty acids transported?
Absorbed fatty acids are resynthesised back into triacylglycerols in the intestinal cells before being transported to other cells via lipoproteins (like chylomicrons) into lacteals.
What are lipoproteins and what classes do they have?
lipoproteins consist of a core of triacylglycerol, cholesterol esters, and then surrounded by phospholipids and apolipoproteins (to make it soluble). The four main classes are based on density from lowest to highest: chylomicrons (90% triacylglycerol), very low density (VLDL, 55% triacylglycerol), low density (LDL 12% triacylglycerol) and high density (HDL 6% triacylglycerol).
What do lipoproteins and apoproteins do? How are chylomicrons assembled and what do they give the plasma?
Lipoproteins solubilise lipids for transport in blood to tissues, provide a delivery system for shifting lipids in and out of cells, the apoproteins are for structural assembly, ligands for cell surface receptors and as enzyme cofactors. The chylomicrons are asembledvia triacylglycerols and other lipids combining with apoB (apoprotein) in the ER, they are then secreted from intestinal cells entering the bloodstream via lymphatic system and they are what give plasma a milky appearance after a fat rich meal.
What are the two major lipid transport pathways and what are the key points of lipoprotein lipase?
The two major lipid transport pathways are exogenous chylomicron pathway (dietary fat) and the endogenous VLDL/LDL pathway (endogenously synthesised fat, typically from glucose by the liver). Lipoprotein lipase is found on the endothelial surface of peripheral tissues and acts as a normal lipase. It is activated by apoCII and has highest activity in the heart, skeletal muscle and adipose tissue and defect can lead to elevated levels of chylomicrons and plasma triacylglycerol.
What is FH?
Familial hypercholesterolemia (Fh) is a common form of hyperlipidaemia (too many lipids in the blood), it leads to premature atherosclerosis (deposition of fatty material on the inner walls of arteries. and is cause be a defect in the LDL receptor gene. It is a dominant disorder and the LDL levels will be 2-3x higher than normal (treatable with statins).
