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Lipids (In Water (Monolayer - when only a small amount of fatty acid is…
Lipids
In Water
Monolayer - when only a small amount of fatty acid is added, a monolayer forms on the surface of the water with hydrophobic facing up.
Micelle - a monolayer that forms a sphere with the hydrophobic tails facing inwards. Like a single-layered vesicle.
Bilayers - glycerophospholipids and sphingolipids prefer to form bilayers, with two lipids facing each other. Their pairs of fatty acid chains do not fit nicely into a micelle.
Chylomicrons - phospholipid monolayer with cholesterol and apolipoproteins. Apolipoproteins play a role in activating lipase on endothelial cell surfaces.
Catabolism
Triacylglycerols start to be hydrolysed in the stomach by lipases but the lipase becomes more active once the solution reaches the duodenum.
Products of lipase hydrolysis (monoacylglycerol and fatty acids) interact with bile salts which induces them to form mixed micelles. Mixed micelles are far more soluble.
Components of mixed micelles are taken up by small intestine epithelial cells and re condense once inside the cell. triacylglycerols, cholesteorl, proteins etc combine to form chylomicrons.
Chylomicrons leave epithelial cells into lymphatic ducts and into bloodstream. Then are taken out of bloodstream and broken down by lipase in endothelial cells then transported into nearby liver, muscle, adipose.
eg Muscle cells, fatty acids diffuse in from capillaries. Immediately activated by linking as thioesters to CoA by acyl-CoA synthetase and ATP in cytosol. Labels FA's to be taken to mitochondria.
Only short-chain acyl-CoA can go directly into mitochondria. Otherwise, acyl-CoA is used to bump the CoA off of carnitine to form acylcarnitine by carnitine acyl transferase I.
Acylcarnitine translocase in inner mitochondrial membrane then transports an acylcarnitine into matrix and L-carnitine out of matrix (coupled transport). Carnitine acyltransferase transfers fatty acid from carnitine to CoA. Can be inhibited by Malonyl-CoA.
- Double bond created between alpha and beta carbons. Creates FADH2. Enzyme is Acyl-CoA dehydrogenase.
- Double bond is rehydrated by enoyl-CoA hydratase.
- Beta-carbon oxidation to ketone by L-hydroxylacyl-CoA-dehydrogenase, converts NAD+ to NADH. 4. CoASH comes in and bumps alpha carbon off to bond with beta-c - cycle restarts. Acetyl CoA created can be used for TCA.
Types
Glycerol-derived
Triglycerides - main energy storage of our body in adipose tissue. Usually 3 different fatty acids. Yield large amount of energy.
Glycerophospholipids
Phosphatidic Acid - 2 fatty acids. A phosphate group replaces one fatty acid at C3 of glycerol. Can have different head groups eg Pi.
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Phosphatidyl ethanolamine - an ethanolamine group esterified to the phosphate group of phosphatidic acid.
Sphingosine-based
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Ceramide - a non-phosphorylated sphingosine with 1 fatty acid attached. Important for eukaryote membranes.
Glycosphingolipids - ceramides with sugar chains attached where the phosphate would be. Cell outer leaflet, glycocalyx.
Sphingomyelins - a ceramide with a phosphate group and a phosphorylcholine attached. Important in brain & nerve tissue.
Steroids
Cholesterol - almost exclusively hydrophobic except for -OH group on top. Precursor for other steroids.
Terpenes - 2 or more isoprene units. Hormones/pheromones most commonly found in essential oils, plants etc. Triterpenes squalene and lanosterol are important precursors of cholesterol.
In Adipose Tissue
During starvation, glucagon, adrenaline and adrenocortotropic hormone trigger phosphorylation and activation of triacylglycerol and activation of lipase.
Adipose cell - large lipid globule surrounded by cytoplasm and flattened nucleus. Serves as a buffer for fatty acid levels.