Biochemistry-II - Fatty Acid Oxidation - Quick Guides | Biochemistry

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Study GuideBiochemistry-II–Fatty Acid Oxidation1. Dietary Fat Absorption1.1 How Dietary Fats Are Broken DownMost of the fat we eat is in the form oftriacylglycerols (TAGs).To use this fat for energy, the body must first break it down.This process begins withhydrolysis, a reaction carried out by enzymes calledlipases.Lipases split one triacylglycerol molecule into:•Three free fatty acids•One glycerol moleculeTheglycerolis water-soluble, so it is easily absorbed by intestinal cells. Later, it can recombine withfatty acids inside the cells.1.2 Why Fat Digestion Is More ComplicatedFatty acids arenot very soluble in water.Because of this:•Fat molecules stick together•They formlarge lipid droplets•Digestive enzymes (which are water-soluble) cannot easily reach them

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Study GuideTo solve this problem, fats must be broken intosmaller dropletswith a larger surface area. Thisprocess is calledemulsification.1.3 Role of Bile Salts in Fat DigestionThe molecules responsible for emulsification are calledbile salts (bile acids).Where do bile salts come from?•They aremade in the liver•Stored in thegall bladder•Released into theduodenum (first part of the small intestine)Why are bile salts effective?•They are made fromcholesterol•They act likedetergents•One end ishydrophobic(attracts fat)•The other end ischarged and water-solubleAt a certain concentration, called thecritical micelle concentration (CMC)—usually2–5 mM—bilesalts group together to formmicelles.

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Study Guide1.4 Mixed Micelles and Fat AbsorptionFigure 1In the intestine, bile salts combine with:•Fatty acids•Triacylglycerols•CholesterolThis structure is called amixed micelle.Structure of a mixed micelle:•Bile salts form the outer edge•Some bile salts are scattered inside•Lipids form abilayer in the center

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Study GuideMixed micelles are essential because:•They greatly increase the surface area for digestion•They help fatty acids move through the watery intestinal environmentImportant note:Fat-soluble vitaminsA, D, E, and Kcannot be absorbed without bile salts.1.5 Action of Pancreatic LipaseThe main digestive enzyme for fats ispancreatic lipase.•It works on the surface of mixed micelles•It needs a helper protein calledcolipase•Colipase helps lipase bind to the micelle surfaceLipase breaks fats intofree fatty acids, which are slightly soluble in water.What happens next?•Most fatty acids are absorbed by cells of thesmall intestine•Any remaining fatty acids may be used bybacteria in the large intestine•Bile salts are reabsorbedin the final third of the small intestine1.6 Bile Acids, Fiber, and Cholesterol ControlBile acids are reused many times—oftenfive or more cyclesbetween the liver and intestine.How does fiber help?•Soluble fiber(like oat bran) binds bile acids•Fiber cannot be absorbed•Bound bile acids are lost in fecesBecause bile acids are made fromcholesterol, losing them forces the body to:•Use more cholesterol to make new bile acids•This lowersblood cholesterol levels•Reduces the risk ofheart disease

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Study GuideHowever, fiber cannot cancel out a very high-fat, high-cholesterol diet.Eating large amounts of fatty meat and expecting fiber alone to fix it is unrealistic.1.7Lipids in the BloodstreamTransport of Lipids in BloodDifferent lipids travel in the blood in different ways:•Free fatty acidsbind toserum albumin•Other lipids are carried inlipoproteinsWhat are lipoproteins?•Spherical protein-lipid complexes•Proteins on their surface are calledapolipoproteins•Lipoproteins are classified bydensityTypes of LipoproteinsChylomicrons•Largest and least dense•Over95% lipid•Carrydietary fat•Contain mostly triacylglycerols•Include several apolipoproteinsVery-Low-Density Lipoproteins (VLDL)•Slightly denser than chylomicrons•90–95% lipid•Carry fats made in theliver•Rich inApolipoprotein BLow-Density Lipoproteins (LDL)

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Study Guide•About85% lipid•High incholesterol•Deliver cholesterol to cells•High LDL levels are linked toheart disease•Often called“bad cholesterol”High-Density Lipoproteins (HDL)•About50% protein•ContainApolipoprotein A•Carry cholesterol away from tissues•Help reduce heart disease risk•Known as“good cholesterol”Summary of Fat Digestion and Transport•Dietary triacylglycerols are digested bylipase•Bile salts formmixed micelles•Fatty acids are absorbed in the small intestine•Lipids travel through thelymph system to the liver•They enter the bloodstream aslipoproteins•These lipoproteins deliver fats and cholesterol to tissues1.8Lipids in Body TissuesUse of Lipids by Cells•Chylomicronscarry dietary fats•LDLcarries liver-made fats•Cellular lipases break triacylglycerols into:oFatty acidsoGlycerolSpecialcarrier proteinstransport fatty acids into cells.Different carriers exist fordifferent chain-length fatty acids.

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Study Guide2.Hydrolysis of Triacylglycerols

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Study Guide2.1 Starting Energy Production from FatsThe body stores most of its fat astriacylglycerols (TAGs).To use this stored fat for energy, the first and most important step ishydrolysis.Hydrolysis breaks one triacylglycerol molecule into:•Free fatty acids•GlycerolThis reaction is catalyzed by an enzyme calledlipase.

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Study Guide2.2Hydrolysis in Adipose (Fat-Storing) TissueInadipose tissue, which stores fat, acellular lipasecarries out the hydrolysis of triacylglycerols.As a result:•Fatty acidsare released into the bloodstream•Glycerolis released separatelyThis step allows stored fat to be mobilized whenever the body needs energy.2.3 Transport of Fatty Acids and GlycerolOnce released:•Free fatty acidsarenot very soluble in water, so they travel through the bloodattached toserum albumin•Glycerolis water-soluble and travels freely to theliverThis separation ensures that each molecule reaches the tissue where it can be used most effectively.2.4 Fate of Glycerol in the LiverIn the liver, glycerol enters energy-producing pathways with the help oftwo key enzymes:1.Glycerol kinaseoConverts glycerol into glycerol-3-phosphate2.Glycerol-3-phosphate dehydrogenaseoConverts glycerol-3-phosphate into glyceraldehyde-3-phosphate2.5 Metabolic Uses of Glyceraldehyde-3-PhosphateGlyceraldehyde-3-phosphate is a very versatile molecule. It can:•Enter theglycolytic pathwayto produce energy•Be used to formglucosethrough gluconeogenesis•Be converted intophosphoenolpyruvate (PEP)and then used to supply intermediates forthetricarboxylic acid (TCA) cycle

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Study GuideThis flexibility allows glycerol from fat breakdown to support multiple metabolic needs.Key Takeaways•Triacylglycerols are broken down bylipase•This releasesfatty acidsandglycerol•Fatty acids travel in blood bound toserum albumin•Glycerol goes to theliver•In the liver, glycerol entersglycolysis, gluconeogenesis, or the TCA cycleTogether, these steps connect fat storage directly to the body’s energy-producing pathways.3.β-Oxidation of Fatty Acids3.1Overview: What Isβ-Oxidation?β-Oxidation is the main pathway by whichfatty acids are broken down to produce energy.This process occurs intarget tissuessuch as muscle and liver cells.Duringβ-oxidation:•Fatty acids are broken downtwo carbons at a time•Each cycle releases a2-carbon unit called acetyl-CoA•These acetyl-CoA molecules enter theTCA (citric acid) cycleto produce energy3.2Structure and Transport of Fatty Acids

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