Flashcards in Chapter 11- Lipid and Amino Acid Metabolism Deck (58)
also cholesterol, cholesteryl esters, phospholipids, and free fatty acids.
lipid digestion after intake
gets to the small intestine without being broken down much at all. in the duodenum they are emulsified which creates a larger surface area of the lipid
mixing of two normally immiscible liquids, in this case fat and water. aided by bile (contains bile salts, pigments, and cholesterol)
secreted by liver and stored in gallbladder
pancreas and lipid digestion
pancreas secretes pancreatic lipase, colipase, and cholesterol esterase into the small intestine. these enzymes hydrolyze the lipid componenets to 2-monoacylglycerol, free fatty acids, and cholesterol.
during lipid digestion there is micelle formation...explain.
micelles are clusters of amphipathic lipids that are soluble in the aqueous environment of the intestinal lumen. formed at duodenum and travel all the way down to the ileum. at the end of the ileum bile salts are reabsorbed and recycled. any fat that remind will pass into the large intestine and will be in stool.
micelles diffuse to the brush border of the intestinal mucosal cells where they are absorbed into the mucosa and re-esterfied to form triacylglycerols and cholesteryl esters and packaged, along with certain apoproteins, fat-soluble vitamins, and other lipids, into chylomicrons.
chylomicrons leave intestine via lacteals (vessels of lymphatic system) and re-enter the bloodstream via the thoracic duct (long lymphatic vessel that empties into the left subclavian vein at the base of the neck)
*note: more water soluble short chain fatty acids can be absorbed by simple diffusion directly into the bloodstream.
what does a fall in insulin levels activate?
hormone-sensitive lipase (HSL, effective in adipose cells), which hydrolyzes triacylglycerols- yielding fatty acids and glycerol. once released it may be transported to liver for glycolysis or gluconeogenesis.
- HSL can also be activated by epinephrine and cortisol.
lipoprotein lipase (LPL)
necessary for metabolism of chylomicrons and very-low-density lipoproteins (VLDL). LPL is an enzyme that can release free fatty acids from triacylglycerols in these lipoproteins.
carrier protein that transports free fatty acids through the blood
how are triacylclycerol and cholesterol transported in the blood?
as lipoproteins: aggregates of apolipoproteins and lipids. (named according to their protein density)
ex: chylomicros (least dense, highest fat to protein ratio)
VLDL, very low density lipoprotein (slightly more dense)
IDL, intermediate density
LDL, low desnity
HDL, high density
transport dietyary triacylglycerols and cholesterol as micelles from intestine to tissues
highly soluble in lymphatic fluid and blood.
assembly occurs in intestinal lining.
transports triacylglycerols from liver to tissues
similar function to chylomicrons but produced and assembled in liver cells.
contain fatty acids that are synthesized from excess glucose or chylomicron remnants.
IDL (aka. VLDL remnants)
picks up cholesterol from HDL to become LDL. formed when a triacylglycerol is removed from VLDL.
some is absorbed with liver apolipoproteins and some is processed in bloodstream.
IDL exists as a transition particle between triacylglycerol transport (associated with chylomicrons and VLDL) and cholesterol transport (associated with LDL and HDL)
NOT healthy cholesterol: delivers cholesterol into cells.
majority of cholesterol measured in blood is LDL.
HEALTHY CHOLESTEROL: picks up cholesterol accumulating in blood vessels. Delivers cholesterol to liver and steroidogenic tissues.
synthesized in liver and intestines and released as dense, protein rich particles into the blood.
contains apolipoproteins that clean up excess cholesterol from blood vessels for excretion.
apolipoproteins (aka. apoproteins)
form the protein component of the lipoproteins. receport molecules and are involved in signaling
ex: apoE- permits uptake of chylomicron remnants and VLDL by the liver
plays major role in synthesis of cell membranes, steroid hormones, bile acids, and vitamin D.
De novo synthesis of cholesterol
de novo: occuring from the beginning.
occurs in the liver and is driven by acetyl-CoA and ATP
citrate shuttle, carries mitochondrial acetyl-CoA into cytoplasm. then mevalonic acid is synthesized in the smooth ER (rate-limiting step). then cholesterol is formed via 3-hydroxy-3-methylglutaryl (HMG) CoA reductase.
cholesterol synthesis regulation
1. high levels of cholesterol inhibit formation
2. insulin promotes cholesterol synthesis
3. control over de novo cholesterol synthesis is also dependent on regulation of HMG-CoA reductase gene expression in the cell.
2 enzymes involved in cholesterol transportation
Lecithin-cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP)
enzyme found in the bloodstream that is activated by HDL apoproteins. Adds fatty acid to cholesterol (producing cholesteryl esters, like the ones in HDL.
*LCAT catalyzes esterfication of cholesterol to form cholesteryl esters. CETP promotes transfer of cholestryl esters from HDL to IDL, forming LDL.
long-chain carboxylic acids. carboxyl carbon is carbon 1 and carbon 2 is referred to as the alpha-carbon.
2 important, essential fatty acids
1. a-linolenic acid
2. linoleic acid
*both polyunsaturated fatty acids as well as other acids are formed and help maintain membrane fluidity which is critical for proper functioning of the cell.
nomenclature of fatty acids
carbons: double bonds
omega (w) numbering system used for unsaturated fatty acids. w- describes the position of the last double bond relative to the end of the chain and identifies the major precursor fatty acid.
do not rely directly on coding of nucleic acid
ex: lipid and carbohydrate synthesis
fatty acid biosynthesis
humans can only synthesize one fatty acid (palmitic acid----16:0)
occurs in liver. essentially activation followed by bond formation, reduction, dehydration, and reduction.
products transported to adipose tissue for storage
primary end product: palmitic acid (palmitate)
2 major enzymes in fatty acid synthesis
acetyl-CoA carboxylase and fatty acid synthase
*both stimulated by insulin
what happens after a large meal?
acetyl-CoA accumulates in the mitochondrial matrix and needs to be moved to the cytosol for fatty acid biosynthesis.