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C HAPTER 22: F ATTY A CID M ETABOLISM PRINCIPAL RXNS IN FATTY ACID OXIDATION on p 625!!! Principal rnxs in fatty acid synthesis in bacteria on p 636 . . . Fatty acids have four major physiological roles: o as fuel molecules o the building blocks of phospholipids and glycolipids o many proteins are modified by covalent attachment of fatty acids, which targets them to membrane locations o fatty acid derivatives serve as hormones and intracellular messengers Fatty acid degradation and synthesis mirror each other in their chemical reactions (p 618 for pathways) F.A. degradation – oxidative conversion of a fatty acid into acetyl CoA (activated acetyl groups) that citric acid cycle can process. Fatty acid – oxidated to introduce double bond; double bond hydrated to introduce hydroxyl group; alcohol oxidized to a ketone; last, fatty acid is cleaved by coenzyme A to yield acetyl CoA & a fatty acid chain 2 carbons shorter. F.A. synthesis – starts with activated acyl group and a malonyl unit; malonyl unit condenses with the acetyl unit to form four-carbon fragment; the carbonyl group is then reduced to a methylene group in 3 steps: reduction, dehydration, reduction – process repeats until C 16 fatty acid is synthesized. Triacylglycerols are highly concentrated energy stores (b/c they are reduced an anhydrous). Triacylglycerol storage is more efficient because of the amount of energy the oxidation of fatty acids can produce (if we stored most of our energy in glycogen, we would all weigh a lot more). Major site of tryacylglycerol accumulation is the cytoplasm of adipose cells (fat cells). Muscles also store triacylglycerols for its own energy needs. Dietary lipids are digested by pancreatic lipases. Most lipids are ingested as triacylglycerols and must be degraded to fatty acids for absorbption across the intestinal epithelium. Lipases – intestinal enzymes secreted by the pancreas – degrade triacylglycerols to free fatty acids and monoacylglycerol. Lipids are tricky because they aren’t water soluble. Lipids are wraped in a soluble container to make them available to the lipases in aqueous solution. Dietary lipids are transported in chylomicrons. In intestinal mucosal cells, triacylglycerols are resynthesized from fatty acids and monoacylglycerols & then packaged into lipoprotein transport particles called chylomicrons. Chylomicrons are composed mainly of triacylglycerols, with apolipoprotein b-48 as the main protein component (apolipoprotein – protein constituents of lipoprotein particles). Chylomicrons transport fat-soluble vitamins and cholesterol.
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