Lipids

• Lipids are a group of heterogenous biomolecules that are insoluble in water and soluble in non-polar solvents.
• Lipids have various biological functions including energy source, insulation of vital organs, covering of nerve fibers, components of cell membrane, and chemical messenger (hormones).
• Fatty acids are monocarboxylic acids that contain long, unbranched hydrocarbon chains, usually 12 to 25 carbon length.
• The smallest fatty acid is butanoic acid (4 C).
• Saturated fatty acids have no double bonds (High MP).
• Monounsaturated fatty acids have one double bond.
• Polyunsaturated fatty acids have 2 or more double bonds.
• Essential fatty acids are fatty acids that mammals cannot synthesize and are essential in normal visual and central nervous system development.
• Saponifiable lipids produce fatty acid salts upon treatment with base and include glyceryl esters (triglycerides, glycerophospholipids), non-glyceryl esters (sphingophospholipids, waxes, glycolipids).
• Non-saponifiable lipids include steroids (cholesterol, bile salts, steroid hormones), eicosanoids (prostaglandins, thromboxanes, leukotrienes), terpenes, pheromones, fat-soluble vitamins.
• There are three stages of fatty acid catabolism: lipid mobilization from adipose tissues, fatty acid activation, and beta-oxidation of fatty acyl-CoA.
• The small intestine triggers the release of pancreatic lipases and bile stored in the gall bladder.
• The small intestine is where fat digestion begins.
• Chylomicrons are a plasma lipoprotein that transports lipids from the small intestine to tissues.
• Chylomicrons bring triglyceride to the liver and contain the highest percentage of triglycerides.
• The arrival of TAGs triggers the secretion of cholecystokinin.
• Low-density lipoprotein (LDL) is transported from the liver to other tissues.
• Beta-oxidation of fatty acyl-CoA is the third stage of fatty acid catabolism.
• Bile is a mixture of bile salts, bile acids and cholesterol that acts as emulsifiers.
• Emulsified fats are formed in the duodenum then acted upon by pancreatic lipase.
• High-density lipoprotein (HDL) scavenges cholesterol in the blood and brings it to the liver.
• Lipid mobilization from adipose tissues is the first stage of fatty acid catabolism.
• Fatty acid activation is the second stage of fatty acid catabolism.
• Lipid catabolism involves the digestion of dietary lipids, catabolism of fatty acids, and B-oxidation of even numbered saturated fatty acids.
• Lipid anabolism involves the biosynthesis of fatty acids, biosynthesis of TAGs, and biosynthesis of cholesterol.
• Lipid anabolism, or the synthesis of fatty acids, occurs in the cytosol and requires Acetyl CoA, Malonyl CoA, NADPH (provided by the pentose phosphate pathway), and the Fatty acid synthase complex.
• Chain cleavage (Release of acetyl CoA: 2 - C chain shorter) Beta - oxidation of even numbered saturated fatty acid •For each round of chain cleavage, the following are produced: 1 acetyl CoA, 1 FADH2, and 1 NADH.
• Ketone bodies are used by skeletal and cardiac muscles and also by the brain.
• Fatty acid synthase is a multifunctional enzyme complex with 7 enzymatic activities.
• The net ATP yield from FA oxidation is 108 ATPs.
• The net ATP yield from FA oxidation is 106 ATPs.
• From stearic acid (18:0), odd-chain fatty acids are oxidized the same way as even-chain fatty acids, resulting in proprionyl CoA as the final product.
• Acetyl CoA enters the TCA cycle when there’s a balance between fat and carbohydrate digestion.
• Ketogenesis, the formation of ketone bodies, occurs in liver mitochondria during starvation, when glucose is very low, high fat, low carb diet, and diabetic conditions where the body cannot adequately process glucose.
• From a C16 saturated fatty acid, 80 acetyl CoA, 7 FADH2, and 17.5 NADH are produced.
• Ketone bodies are an alternative fuel for the cells, are soluble in aqueous medium, and do not need to be incorporated in lipoproteins for transport to the peripheral tissues through the blood.
• TCA cycle intermediates (OAA in particular) can be provided for by oxidation of sugars.
• For the oxidation of a C16 fatty acyl CoA, 7 rounds of chain cleavage are required, resulting in 7 NADH produced and 8 acetyl CoA.
• The major regulatory point in cholesterol biosynthesis is HMG-CoA reductase.
• Cholesterol is a precursor of steroid hormones and bile salts, with 27 carbon atoms of cholesterol all derived from acetyl CoA.