Part A: What is a Lipid, Anyway? - UCLA Chemistry and Biochemistry
14
Know Your Inner : All about Lipids Have you ever thought understanding lipids was a simple task…until you tried to tackle the exam questions? This tutorial will mainly serve as a guided outline to comprehending and completing the toughest and most intensive questions about lipids. Part A: What is a Lipid, Anyway? By definition, a lipid is “an [organic] molecule of biological origin that is soluble in solvents of low polarity and insoluble in solvents of high polarity.” 1 At first glance, there seem to be many complex parts to this definition, so let’s break it down: First, a lipid is an organic molecule, meaning the molecule must contain carbon. Furthermore, lipids are biomolecules, meaning that they derive from biological origins (ie. are synthesized by a living organism). As such, at its most base level, the molecular formula for lipids will contain carbons, hydrogens, and oxygens. Secondly, lipids are soluble in solvents of low polarity, which simply means that lipids are soluble in nonpolar solvents. This solubility property is a result of the large nonpolar regions that are indicative of the lipid molecule. HOW CAN YOU DISTINGUISH A LARGE NONPOLAR REGION? Long hydrocarbon chains in lipid molecules develop the largely nonpolar regional property of lipids. Due to carbon and hydrogen forming nonpolar bonds due to their low electronegative difference (as opposed to Oxygen‐Hydrogen, which have a high electronegative difference and are thus polar), a large hydrocarbon chain or ring creates a nonpolar region. For example: PHOSPHOLIPID NONPOLAR REGION Another example: STEROIDS FATTY
Transcript of Part A: What is a Lipid, Anyway? - UCLA Chemistry and Biochemistry
KnowYourInner:AllaboutLipids
Haveyoueverthoughtunderstandinglipidswasasimpletask…untilyoutriedtotackletheexamquestions?Thistutorialwillmainlyserveasaguidedoutlinetocomprehendingandcompletingthetoughestandmostintensivequestionsaboutlipids.
PartA:WhatisaLipid,Anyway?Bydefinition,alipidis“an[organic]moleculeofbiologicaloriginthatissolubleinsolventsoflowpolarityandinsolubleinsolventsofhighpolarity.”1
Atfirstglance,thereseemtobemanycomplexpartstothisdefinition,solet’sbreakitdown:
First,alipidisanorganicmolecule,meaningthemoleculemustcontaincarbon.Furthermore,lipidsarebiomolecules,meaningthattheyderivefrombiologicalorigins(ie.aresynthesizedbyalivingorganism).Assuch,atitsmostbaselevel,themolecularformulaforlipidswillcontaincarbons,hydrogens,andoxygens.
Secondly,lipidsaresolubleinsolventsoflowpolarity,whichsimplymeansthatlipidsaresolubleinnonpolarsolvents.Thissolubilitypropertyisaresultofthelargenonpolarregionsthatareindicativeofthelipidmolecule.
HOWCANYOUDISTINGUISHALARGENONPOLARREGION?
Longhydrocarbonchainsinlipidmoleculesdevelopthelargelynonpolarregionalpropertyoflipids.Duetocarbonandhydrogenformingnonpolarbondsduetotheirlowelectronegativedifference(asopposedtoOxygen‐Hydrogen,whichhaveahighelectronegativedifferenceandarethuspolar),alargehydrocarbonchainorringcreatesanonpolarregion.
Forexample:PHOSPHOLIPID
NONPOLARREGION
Anotherexample:STEROIDS
FATTY
Thelargehydrocarbonrings,methylgroups,andhydrocarbonchainsallconsistofnonpolarC‐Hbondsduetoitslowelectronegativedifference,andassuchcontributedtoitsnon‐polarity.
Whenfinalsdayapproaches,itwillbeimportanttounderstandwhatareasofthelipidmoleculecontributetoitsnon‐polarity,sotakeheed!
What’sthesignificanceofthenon‐polarity/lowpolarity?
Themostsimplistic(andnotentirelychemicallytrue)rule‐of‐thumbisthat“likedissolveslike”,whichindicatesthatasolutewilldissolveinasolventofidenticalpolarproperty,buttwosubstancesofopposingpolarpropertieswillnotinteract.Forexample,anonpolarmolecule(ie.afattyacid)willnotdissolveinapolarsubstance(ie.water),andviceversa.
Assuch,thirdly,lipidsareinsolubleinwater(apolarmoleculeduetothehighelectronegativeO‐Hbonddifference),fornon‐polarmoleculesareinsolubleinpolarmolecules.Thus,thepresenceofanonpolarmolecule(orregionofmolecule)meansthattheregionormoleculeishydrophobic(Greek,“waterfearing”),andproducesahydrophobiceffect,“whichcausesthepolarendstobeorientedoutwardstowardsthesolventandthenonpolarregionsorientedinwardsawayfromthepolarsolvent.”2,whenincontactwithapolarsolvent(ie.solvent),makingthenonpolarmoleculeisinsolubleinwater.Thistermisinterchangeablewithlipophilic(Greek,“fatloving”),indicatedthatnonpolarlipidsaresolubleinothernonpolarsolvents.
Thus,polarmoleculesareconsideredtobehydrophilic(“waterloving)andlipophobic(“waterfearing”),duetotheabilityofhighlypolarbondsbeingsolubleinpolarsolvents,andinsolubleinnonpolarsolvents(ie.fats,lipids).
However,therearecategoriesoflipidsthatarebothhydrophobicandhydrophilic.Theselipidsarecalledamphiphiles,moleculesthatcontainbothlargeareasofpolarityandnonpolarity,andarethusbothhydrophilicandlipophilic.Phospholipidshavebothalonghydrophobichydrocarbontail,andahydrophilichead,givingphospholipidsintegralbiologicalproperties,whichwillbelaterdiscussed.
SUMMARY,PLEASE!:Lipidshavelargenonpolarregionsderivingfromnonpolarhydrocarbonbonds(whichhavelowelectronegativedifference).Thus,lipidsarelargelyinsolubleinpolarsolvents(water),andaresolubleinnonpolarmolecules.Assuch,thenonpolarregionsarehydrophobic/lipophilic,andthepolarregionsarehydrophilic/lipophobic.
Part2:WhataretheDifferentkindsofLipids?Thereare8majorclassificationsoflipids(with7ofthembeingimportantforthepurposeofChem14C)
1. FattyAcids
2. Waxes
3. Triacylglycerides
4. Phospholipids
5. Prostaglandins
6. Steroids
7. LipophilicVitamins
8. Terpenes(plant‐basedlipids)
Part3:HowintheworlddoItelltheselipidsapart?Thissectionwillbethemostinvaluabletoyoufortheexam.Mostexamquestionsfrompreviousyearscenteronhowtodistinguishandclassifytypesoflipids,andtheimportantdefinitionsbehindsaidmolecules.
FattyAcids:
Fattyacidsarelipidsthatconsistof(1)Carboxylicacid(‐COOH)witha(2)long,un‐branchedhydrocarbonchain.
Example:Arachidicacid(C20H40O2)
CARBOXYLICACID(‐COOH)LONGUNBRANCHEDHYDROCARBONCHAIN(C‐H)
Fattyacidscommonlyhavethefollowingcharacteristics(besuretolookforthesepropertiesontheexam)
‐ Evennumberofcarbons(ex.arachidicacidhas20carbons).Themostcommonfattyacidsconsistof12‐20carbons,whilethemostbiologicallyimportantfattyacidshave18carbons(forexample,stearic,oleic,andlinoleicacids)
‐ Actasabasic“buildingblock”formorecomplexlipidmolecules(actasaprecursortootherlipids).Forexample,thelonghydrocarbonchainprovidesanessentialnonpolarregionforthehydrophobictailofphospholipids;waxes,triacylglycerides,andphospholipidshaveatleastonefattyacidasanattachment.
‐ LowpolarityduetothelowelectronegativedifferenceintheC‐Hbondsofthelonghydrocarbonchain,producingahydrophobiceffect.
Furthermore,fattyacidscanbedividedintotwosubcategories:saturatedandunsaturated.ThisismaincategorizedbythepresenceofC=Cbondsinthehydrocarbonchain.SaturatedcarbonshavefullC‐Hbondsinthehydrocarbonchain,andassuchhavenonC=Cbonds.Unsaturatedcarbons,however,haveatleastoneC=Cbond(monounsaturated,morethanoneispolyunsaturated)inthechaintoreplacetwohydrogenC‐Hbonds.
Withinunsaturatedfattyacids,cis‐C=Cbonds(Carbonbondsareonthesameside)aremuchmoreprevalentthantrans‐C=Cbonds(carbonbondsonoppositesides)innature.“Trans‐fats”arenotaseasilymetabolizedbycatalyticenzymesascis‐fats,andareleftuncatalyzedandcandamagehealth(plaquebuildup).
Waxes:
Waxesare(1)estersthathaveattached(2)afattyacidand(3)along‐chainalcohol.Thesynthesisofwaxesusuallyderivesfromthedehydrationsynthesisofafattyacidandalong‐chainalcoholgroup.
Duetothelowpolaritycausedbythetwolonghydrocarbonchainsoneithersideoftheesterlinkage,waxesarehighlyinsolubleinpolarsolventsandarehydrophobic;extremelyrepellenttowardwater.Asaresult,themajorbiologicalfunctionwaxesaretoactasawaterbarrier(ie.birdfeathersarecoatedinwaxlipids,whichminimizeswettingofthefeathers,leavesarecoatedinwaxtopreventtheevaporationofwater,damagingplantnourishment.
Myricyl cerotate Present in beeswax, carnauba
wax
ESTER
Long‐chainalcohol(‐OH)
Fattyacid
Triacylglycerols:
Triacyglycerols,ortriacylglycerides,arelipidssimilartothatofwaxes(inthattheycontainesterandanalcohol‐likefunctiongroup),butconsistof(1)atriester,(2)afattyacid,and(3)glycerol,orglycerin,athreecarbonalcoholstructure:
Glycerolandthefattyacidundergodehydrationsynthesis,whichinturncreatesthetriacylglycerolmolecule.
Where“R”indicatesthelong,un‐branchedhydrophobictailofhydrocarbonsthatarerepresentativeofthefattyacid.
Triacylglycerolshavethefollowingimportantcharacteristics
‐ Varyingmeltingpoints.Ifsolidatroomtemperature,thenthelipidisconsideredafat,ifliquidatroomtemperature,oil.
‐ Themostabundantnaturallysynthesizedlipid.‐ Itsmainbiologicalfunctionisforenergystorage(ie.consumptionoffatsprovidesaslowburninglong‐
termstorageofenergythatcanbemetabolized).‐ Withtheadditionofwaterinahydrolysisreaction,the“breaking”offatsderivedfromanimalsyields
soaps,whicharehydrophilicCO2‐groupsattachedtoahydrophobichydrocarbonchain.
Glycerol Fatty acids
Triacylglycerol
HOWDOSOAPSWORK?
1. Thepolar,hydrophilicheadofsoap(CO2‐)isattractedtothepositivelychargedendsofwater,whilethe
hydrophobichydrocarbontailcanavoidthewater.2. Thishydrophobic,lipophilictailthencanattachtodirts,fattyacids,andotherlipids,duetotheirlow
polarity.3. Thesetailssurroundthe“dirts”inspherescalledmicelles,whichisolatethedirtandremovethem
frominteractionwiththewatermolecules.4. Finally,whenthewaterisremoved,themicellesencapsulatingthedirtarecarriedaway,leavingthe
areafreeofthedirts.
Phospholipids:
Aphospholipidsimilarlyconsistsof(1)glycerol,three‐carbonalcoholstructureswhichthenformsan(2)triesterwith(3)twofattyacidsand(4)onephosphategroup
Genericexample:
Where“R”representsthelongun‐branchedhydrocarbonchainrepresentativeofafattyacid.
Phospholipidshavethefollowingkeyproperties:
‐ Secondmostabundantnaturallysynthesizedlipid‐ Itsmajorbiologicalfunctionistoformaintegralphospholipidbilayerinthecellmembraneofliving
organisms,acriticalcomponentthatallowsfortheselectivediffusionofionsandmoleculesthroughthebarrier
HOWDOESTHEPHOSPHOLIPIDBILAYERWORK?
Twophospholipidsfaceapartfromeachother,thepolar,hydrophilicheadofthephosphategroupfacingoutwards,andthehydrophobic,hydrocarbonchainonthefattyacidsfacinginwards.Thiscreatesthehydrophobiceffectofthetails,avoidingwater.Thisbilayerofthecellmembraneallowsforthecelltofreelymovethroughwater,yetmaintaintheinternalstructureofthecell
Phosphategroupester
2Fattyacidesters
Prostoglandins:
Prostoglandinsarelipidmoleculesthatcontaina(1)prostanoicacidskeleton,whichconsistofa(2)cyclopentaneringattachedtoaupperchainofa(3)fattyacidwithsevencarbons,andalower(4)long,unbranchedchainofhydrocarbons,with8carbons.
Ex.ProstaglandinF2alpha
Thenomenclatureofprostaglandinsisdependentonthestereochemistryofthelipid,basedonthenumberofOH,C+C,andC=Ogroupsinthemolecule.
Majorcharacteristicsofprostaglandinsinclude:
‐ Majorbiologicalfunctiontoactasaregulatorandsignalmolecule(regulationofhormones,inflammation,calciummovement;controlofcellulargrowth;constrictionofsmoothmusclecells;regulationofplateletgrowth;spinalneuronsensitization)
‐ Prostoglandinsoftensynthesizeatwoundsitestoregulateandsignalaninflammatoryresponse,leadingtoinflammation.
‐ Shorthalf‐lifefromorigination,about5minutesorless.‐ Whileeachstructurehasthesameprostanoicacidskeleton,thedifferingstereochemistryofeach
prostaglandinderivesvastlydifferingbiologicalfunctionsforeachlipid.
Prostaglandin F2α (PGF2α
)
Upperchainoffattyacids
Lowerchainofhydrocarbons
Cyclopentane
ring
Steroids:
Steroidmoleculesconsistofabasic(1)4‐carbonringskeletonformingaflatcore.
Examplesofsteroidsinclude:cholesterol,testosterone,estrone,cortisone,aldosterone,corticoidhormones,bileacids,andcholicacids.
Whileallsteroidshaveaverysimilarcarbonringskeletalstructure,thevariousfunctiongroupattachmentsgivesteroidsaverywiderangeoffunctions.
Abbreviatedlistofbiologicalfunctions:
1. Hormones:chemicallyregulatethebodya. Testosteroneandestrone,sexhormones,regulatethesexualdevelopmentoforganismsb. Corticoidsteroids/hormones,regulatethemetabolicprocessesofthebodyc. Cortisone:regulatesthebodyinflammatoryresponse,glucosemetabolismd. Digestive:emulsifiesfatsinintestinestoaidinthedigestiveprocess,integralcomponentofbile
acid.
Biosynthesisofsteroidsisalongandcomplexprocess,startingfromAcetylCoenzymeA;variousenzymaticstepsformintermediatesteroidmolecules,formingcholesterolattheend.
Cholesterolistheprecursortoallvertebratesteroids,themostbasicsteroidbiologicallyproducedbylivingorganisms.
LipophilicVitamins:
First,vitaminsareorganiccompounds(containingcarbon,hydrogen),thatarenotclassifiedasfats,proteins,orcarbohydrates,thatarerequiredforthegrowth,regulation,andmaintenanceofanimals.Thisbroaddefinitionofvitaminsthusencompassescompoundswithawidevarietyofstructuresandfunctions.
Therearetwomajorlipophilic(andthushydrophobic)vitamins:VitaminEandVitaminA(retinol)
VitaminEisamixtureofstereoisomers,mainlyconsistingofamoleculecalledalpha‐tocopherol
Withahydrophobictailcreatedbythelonghydrocarbontail.Itsmainfunctionistoprotectcellsagainstfreeradicals,unstableionswithanopenvalencethatcancauseoxidativedamagetothecells.
VitaminA(retinol)containsalonghydrophobicnonpolarhydrocarbonchain,withitsmainfunctionbeingusedinphoton‐harvestingproteins,essentialtothedevelopmentofvision.
Theeasiestmethodofdistinguishinglipophilicvitaminsfromotherlipidcategoriesisthattheyhaveastructureunlikethatofotherlipids,andcanwidelyvary.Whenindoubt,Useamethodofeliminationtorejectthepossibilityofthemoleculebeinganyotherkindoflipidfirst.
α-Tocopherol Hydrophobic antioxidant
vitamin
Part4:Finally,whatshouldIpracticethemostformytestonlipids?Thefollowingisanabbreviatedlistoftopicsyououghttorememberforthefinalexam.Thebestrecommendationistodoalloftheproblemsfrompreviousfinalexams,astheywillbemostindicativeoftherigorandextentinwhichitdelvesintolipids.
‐Moleculeswiththelargesthydrocarbonchainshavelowerpolarity,andarethusmostsolublewithfats,andinsolublewithwater.
‐Whenaskedwhichmoleculewitheasilypassthroughahydrophobicmembrane,lookforthemoleculewhichexhibitsthelowestpolarity(lowestelectronegativedifferenceinbonds,diploemoments,etc.);rememberthesimple“likedissolvelike”rule.
‐Knowinsideandoutthekeyfundamentalcharacteristicsofeachcategoryoflipid(asoutlineineachbox),andthecommonnamesofafewmoleculesofeachcategory.
‐Ifdescribingthestructurecangetconfusingforyouattimes,learninghowtodrawthebasicstructureofeachlipidwouldbeinvaluableinyouranalysisofthelipidanditsfunctionalproperties.
‐Knowthemostbasispropertiesoflipids(insolubleinwater,solubleinnonpolarsolvents,organicbiologicalmolecule)
‐Finally,whenindoubtaboutdrawingthestructureoflipidcategories,drawthemostgenericexample.
Suggestedpracticereview:Problem16,FinalExamPartA,Fall2012;Problem13,FinalExamPartA,Winter2012.Allproblemscanbefoundonhttp://www.chem.ucla.edu/harding,underPastExams.
Goodluck,anddigdeep,theanswerismostdefinitelyinyourgut.
Works Cited
Brandt, Mark. "Introduction to Lipids." Rose-Hulman.edu. Rose-Hulman Institute of Technology, 2011. Web. 9 June 2012. <http://www.rose-hulman.edu/~brandt/Chem330/Lipid_properties.pdf>. Web.
“Fatty Baby”. Picture from http://musicallmorning2.files.wordpress.com/2009/03/fattybaby.jpg. Google Images, 9 June 2012. Web.
Hardinger, Steven. "Biomolecules Survey 2: Lipids." Chemistry 14C, Lecture 1. UCLA, Young CS50, Los Angeles. 4 June 2012. Lecture. (Majority of information from this tutorial derives from this lecture)
Hardinger, Steven. Chemistry 14C: Organic Molecular Structures and Interactions : PowerPoint Lectures for Chemistry 14C with PowerPoint CD. Plymouth, MI: Hayden-McNeil, 2012. Print. (All chemical diagrams, pictures, and figures are taken from Prof. Hardinger’s Powerpoint Slides provided with the Course Supplement.)
1,2 Hardinger, Steven. “Illustrated Glossary of Organic Chemistry: A product of the Institute for Reduction of Cognitive Entropy in Organic Chemistry”. Organic Chemistry at UCLA. http://www.chem.ucla.edu/harding/ 2012. Web.
