Very low density lipoproteins arenot as dense as chylomicrons. These lipoproteins are based on proteincomponents not as much lipids. Both vldl and ldl are known as bad cholesterolssince their serum concentration levels are responsible for different heart andartery diseases like strokes. HDL are about half and half when it comes to thelipid and protein ratio.
They are good cholesterols since they to lower therate of artery diseases. High?density lipoproteins (HDLs) willcontain a different apolipoprotein form, Apolipoprotein A which is differentthan those of low density. These proteins are just about half lipid and halfprotein by weight. Phospholipids and cholesterol esters are the most importantlipid components. HDL can be sometimes referred to as “good cholesterol”because a higher ratio of HDL to LDL corresponds to a lower rate of coronaryartery disease.In summary, these triacylglycerolsfrom the diet are digested by lipase and associate with bile salts into mixedmicelles.
The free fatty acids are absorbed by the cells in the smallintestine, from which they are transported via the lymph system to the liver. Fromthe liver, they are released as apolipoproteins in the circulation, which canbe used for carrying fatty acids and cholesterol to the cells throughout thebody.Triacylglycerols in chylomicrons and LDLs will circulatethrough the blood system; the former carries dietary lipids while the lattercarries lipids that are synthesized by the liver. These triacylglycerols are usedas substrates for cellular lipases, which hydrolyze them to make fatty acidsand glycerol in several different steps. Many carrier proteins transport thelipids into the cell in different pathways. There are many different carriers thatexist for different chain?length lipids.
Energy production from triacylglycerolsstarts with their hydrolysis into free fatty acids and glycerol. When analyzingthis adipose (fat?storing)tissue, this hydrolysis will be carried out by a cellular lipase, whichcatalyzes the hydrolysis reaction to release the free fatty acids and glycerol.Afterwards the fatty acid is taken through the bloodstream and liver will absorbthe glycerol. This can be sent to the glycolytic pathway by glycerol kinase andglycerol three phosphate dehydrogenase. This enzyme can be used as energy viaglucose or the TCA cycle via intermediates. In target tissues, fatty acids arebroken down through the ?? oxidationpathway that releases 2?carbon units in succession.
For example,palmitic acid has 16 carbons. Its initial oxidation produces eightacetyl?Coenzyme A (CoA) molecules, eight reduced FAD molecules, and eight NADHmolecules. The fatty acid is first activated atthe outer mitochondrial surface by conjugating it with CoA, then transported through the innermitochondrial membrane to the matrix, and then, for each 2?carbon unit, brokendown by successive dehydrogenation,water addition, dehydrogenation, and hydrolysis reactions. The first reaction will involve thecatalization by the isoforms ofacyl-CoA dehydrogenase (AD) on the inner-mitochondrial membrane. This reactionwill result in trans double bond, different from naturally occurringunsaturated fatty acids. Analogous to succinate dehydrogenase reaction in thecitric acid cycle; the electrons from bound FAD transferred directly to theelectron- transport chain via electron-transferring flavoprotein (ETF) which is catalyzed bytwo isoforms of enoyl-CoA hydratase. Next, water adds across the double bond yieldingalcohol, analogous to fumarase reaction in the citric acid cyclewith the same stereospecificity. Following this reaction, the one is catalyzed by b-hydroxyacyl-CoAdehydrogenase which uses NAD cofactoras the hydride acceptor.
In this reaction, only L-isomers of hydroxyacyl CoAact as substrates which is analogous to malate dehydrogenase reaction inthe citric acid cycle. The next reaction is catalyzed by acyl-CoA acetyltransferase(thiolase) via covalent mechanism: The carbonyl carbon in b-ketoacyl-CoAis electrophilic, active site thiolate acts as nucleophile and releasesacetyl-CoA, and the terminal sulfur in CoA-SH acts as nucleophile andpicks up the fatty acid chain from the enzyme. This will result in the netreaction of thiolysis of carbon-carbon bond.
