P1:FPP 2ndRevisedPages Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN002C-64 May19,2001 20:39 Table of Contents (Subject Area: Biochemistry) Pages in the Article Authors Encyclopedia Richard E. McCarty Bioenergetics Pages 99-115 and Eric A. Johnson Stephen J. Benkovic Enzyme Mechanisms Pages 627-639 and Ann M. Valentine Food Colors Pericles Markakis Pages 105-120 Glycoconjugates and Eugene A. Davidson Pages 833-849 Carbohydrates Ion Transport Across George P. Hess Pages 99-108 Biological Membranes Lipoprotein/Cholesterol Alan D. Attie Pages 643-660 Metabolism Anna Seelig and Membrane Structure Pages 355-367 Joachim Seelig Natural Antioxidants Eric A. Decker Pages 335-342 In Foods Sankar Mitra_ Tapas Nucleic Acid Synthesis Pages 853-876 K. Hazra and Tadahide Maurice Eftink and Protein Folding Pages 179-190 Susan Pedigo Protein Structure Ivan Rayment Pages 191-218 Paul Schimmel and Protein Synthesis Pages 219-240 Rebecca W. Alexander Vitamins and David E. Metzler Pages 509-528 Coenzymes P1:FYDRevisedPages Qu:00,00,00,00 EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 Bioenergetics Richard E. McCarty Eric A. Johnson JohnsHopkinsUniversity I. CatabolicMetabolism:TheSynthesisofATP II. Photosynthesis III. OriginofMitochondriaandChloroplasts IV. IllustrationsoftheUsesofATP:IonTransport, Biosynthesis,andMotility V. ConcludingStatements GLOSSARY BIOENERGETICS,anamalgamationofthetermbiolog- icalenergetics,isthebranchofbiologyandbiochemistry (cid:2) Adenosine 5-triphosphate (ATP) The carrier of free thatisconcernedwithhoworganismsextractenergyfrom energyincells. theirenvironmentandwithhowenergyisusedtofuelthe Bioenergetics Thestudyofenergyrelationshipsinliving myriadoflife’sendergonicprocesses.Organismsmaybe systems. usefully divided into two broad groups with respect to Chloroplasts The sites of photosynthesis in green howtheysatisfytheirneedforenergy.Autotrophicorgan- plants. ismsconvertenergyfromnonorganicsourcessuchaslight Ion transport The movement of ions across biological orfromtheoxidationofinorganicmoleculestochemical membranes. energy.Asheterotrophicorganisms,animalsmustingest Metabolism Thetotalofallreactionsthatoccurincells. andbreakdowncomplexorganicmoleculestoprovidethe Catabolicmetabolismisgenerallydegradativeandex- energyforlife. ergonic,whereasanabolicmetabolismissyntheticand Interconversionsofformsofenergyarecommonplace requiresenergy. in the biological world. In photosynthesis, the electro- Mitochondria Sitesofoxidative(catabolic)metabolism magneticenergyoflightisconvertedtochemicalenergy, incells. largely in the form of carbohydrates, with high overall Photosynthesis Light-drivensynthesisoforganicmole- efficiency.Theenergyoflightisusedtodriveoxidation– culesfromcarbondioxideandwater. reductionreactionsthatcouldnottakeplaceinthedark. Plasma membrane The barrier between the inside of Light energy also powers the generation of a proton cellsandtheexternalmedium. electrochemicalpotentialacrossthegreenphotosynthetic 99 P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 100 Bioenergetics FIGURE 1 Central role of adenosine 5(cid:2)-triphosphate (ATP) in metabolism. Catabolic (degradative) metabolism is exergonic and provides the energy needed for the synthesis of ATP from adenosine 5(cid:2)-diphosphate (ADP) and inorganic phosphate (Pi). The exergonic hydrolysis of ATP in turn powers the endergonic processes of organisms. membrane.Thus,electricalworkisanintegralpartofpho- (ADP) and inorganic phosphate (P) is a strongly en- i tosynthesis. Chemical energy is used in all organisms to dergonic reaction that is coupled to exergonic reactions drivethesynthesisoflargeandsmallmolecules,motility such as the breakdown of glucose. ATP hydrolysis in at the microscopic and macroscopic levels, the genera- turnpowersmanyoflife’sprocesses.Thecentralroleof tion of electrochemical potentials of ions across cellular ATP in bioenergetics is illustrated in Fig. 1. Partial struc- membranes,andevenlightemissionasinfireflies. tures of several compounds that play important roles in Giventhediversityintheformsoflife,itmightbeex- metabolism are shown in Fig. 2. pectedthatorganismshaveevolvedmanymechanismsto Inthisarticle,theelementsofenergymetabolismwill deal with their need for energy. To some extent this ex- bediscussedwithemphasisonhoworganismssatisfytheir pectationisthecase,especiallyfororganismsthatlivein energeticrequirementsandonhowATPhydrolysisdrives extreme environments. However, the similarities among otherwiseunfavorablereactions. organismsintheirbioenergeticmechanismsareas,oreven more,strikingthanthedifferences.Forexample,thesugar glucose is catabolized (broken down) by a pathway that I. CATABOLIC METABOLISM: is the same in the enteric bacterium Escherichia coli as THE SYNTHESIS OF ATP it is in higher organisms. All organisms use adenosine 5(cid:2)-triphosphate(ATP)asacentralintermediateinenergy Metabolism may be defined as the total of all the chem- metabolism.ATPactsinawayasacurrencyoffreeen- ical reactions that occur in organisms. Green plants can ergy.ThesynthesisofATPfromadenosine5(cid:2)-diphosphate synthesize all the thousands of compounds they contain P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 Bioenergetics 101 ofglucosebyP isanunfavorablereaction,characterized i bya(cid:2)G(cid:2) ofabout4kcal/mol,atpH7.0and25◦C.(Note 0 that the biochemist’s standard state differs from that as usuallydefinedinthattheactivityofthehydrogenionis takenas10−7 M,orpH7.0,ratherthan1 M,orpH0.0. pH7.0ismuchclosertothepHinmostcells.)Thisprob- lemisneatlysolvedincellsbyusingATP,ratherthanP, i asthephosphoryldonor: Glucose+ATP←→Glucose6-phosphate+ADP. The(cid:2)G(cid:2) forthisreaction,whichiscatalyzedbytheen- 0 zymehexokinase,isapproximately−4kcal/mol.Thusthe phosphorylationofglucosebyATPisanenergeticallyfa- vorablereactionandisoneexampleofhowthechemical energyofATPmaybeusedtodriveotherwiseunfavorable reactions. Glucose 6-phosphate is then isomerized to form fruc- tose6-phosphate,whichinturnisphosphorylatedbyATP at the 1-position to form fructose 1,6-bisphosphate. It FIGURE2 Someimportantreactionsinmetabolism.Shownare seemsoddthatametabolicpathwayinvests2molofATP thephosphorylationofADPtoATP,NAD+,NADH,FAD,FADH2 in the initial steps of the pathway when ATP is an im- acetate, CoA, and acetyl CoA. For clarity, just the parts of the portantproductofthepathway.However,thisinvestment largermoleculesthatundergoreactionareshown.NAD+,nicoti- paysoffinlatersteps. namideadeninedinucleotide;NADH,nicotinamideadeninedinu- Fructose1,6-bisphosphateiscleavedtoformtwotriose cleotide(reducedform);FAD,flavinadeninedinucleotide;FADH2, flavin adenine dinucleotide (reduced form); CoA, coenzyme A; phosphatesthatarereadilyinterconvertible.Notethatthe AMP,adenosinemonophosphate. oxidation–reduction state of the triose phosphates is the sameasthatofglucose6-phosphateandthefructosephos- phates. All molecules are phosphorylated sugars. In the fromcarbondioxide,water,andinorganicnutrients.The nextstepofglycolysis,glyceraldehyde3-phosphateisox- discussion of the complicated topic of metabolism is idizedandphosphorylatedtoformasugaracidthatcon- somewhat simplified by separation of the subject into tainsaphosphorylgroupatpositions1and3.Theoxidiz- twoareas—catabolicandanabolicmetabolism.Catabolic ingagent,nicotinamideadeninedinucleotide(NAD+),is metabolismisdegradativeandisgenerallyexergonic.ATP aweakoxidant(E(cid:2),atpH7.0of−340mV).Theoxida- 0 is a product of catabolic metabolism. In contrast, an- tionofthealdehydegroupofglyceraldehyde3-phosphate abolic metabolism is synthetic and requires ATP. Fortu- to a carboxylate is a favorable reaction that drives both nately,therearerelativelyfewmajorpathwaysofenergy the oxidation and the phosphorylation. This is the only metabolism. oxidation–reductionreactioninglycolysis. The hydrolysis of acyl phosphates, such as that of position 1 of 1,3-bisphosphoglycerate, is characterized A. GlycolysisandFermentation by strongly negative (cid:2)G(cid:2) values. That for 1,3-bisphos- 0 Carbohydratesareamajorsourceofenergyfororganisms. phoglycerate is approximately −10 kcal/mol, which is Themajorpathwaybywhichcarbohydratesaredegraded significantlymorenegativethanthe(cid:2)G(cid:2) forthehydrol- 0 iscalledglycolysis.Starch,glycogen,andothercarbohy- ysisofATPtoADPandP.Thus,thetransferoftheacyl i dratesareconvertedtothesugarglucosebypathwaysthat phosphatefrom1,3-bisphosphoglyceratetoADPtoform willnotbeconsideredhere.Inglycolysis,glucose,asix- 3-phosphoglycerate and ATP is a spontaneous reaction. carbonsugar,isoxidizedandcleavedbyenzymesinthe Since two sugar acid bisphosphates are formed per glu- cytoplasm of cells to form two molecules of pyruvate, a cosemetabolized,thetwoATPinvestedinthebeginning three-carbon compound (see Figs. 3 and 4). The overall ofthepathwayhavebeenrecovered. reaction is exergonic and some of the energy released is In the next steps of glycolysis, the phosphate on the conservedbycouplingthesynthesisofATPtoglycolysis. 3-positionofthe3-phosphoglycerateistransferredtothe Before it may be metabolized, glucose must first be hydroxyl residue at position 2. Removal of the elements phosphorylated on the hydroxyl residue at position 6. ofwaterfrom2-phosphoglycerateresultsintheformation Underintracellularconditions,thedirectphosphorylation ofanenolicphosphatecompound,phospho(enol)pyruvate P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 102 Bioenergetics FIGURE 3 Schematic outline of carbohydrate metabolism. Glucose is oxidized to two molecules of pyruvate by glycolysisinthecytoplasm.Inmitochondria,pyruvateisoxidizedbymolecularoxygentoCO2andwater.Thesynthesis ofATPiscoupledtopyruvateoxidation. (PEP).ThefreeenergyofhydrolysisofPEPtoformthe B. OxidationofPyruvate:TheCitricAcidCycle enolformofpyruvateandP isontheorderof−4kcal/mol. i Inhigherorganisms,theoxidationofpyruvatetakesplace Inaqueoussolution,however,theenolformofpyruvateis in subcellular, membranous organelles known as mito- veryunstable.Thus,thehydrolysisofPEPtoformpyru- vate is a very exergonic reaction. The (cid:2)G(cid:2) for this re- chondria. Because mitochondria are responsible for the 0 synthesisofmostoftheATPinnonphotosynthetictissue, actionis−14.7kcal/mol,whichcorrespondstoanequi- they are often referred to as the powerhouses of cells. librium constant of 6.4×1010. PEP is thus an excellent Mitochondrial ATP synthesis is called oxidative phos- phosphoryl donor and the formation of pyruvate is cou- phorylationsinceitislinkedindirectlytooxidativereac- pled to ATP synthesis. Since two molecules of pyruvate tions.Inthecompleteoxidationofpyruvate,therearefive areformedperglucosecatabolized,twoATPareformed. oxidation–reductionreactions.Threeofthesereactionsare ThusthenetyieldofATPistwoperglucoseoxidizedto oxidativedecarboxylations.Theelectronacceptor(oxidiz- pyruvate. ingagent)forfourofthereactionsisNAD+;theoxidizing Insomeorganisms,glycolysisistheonlysourceofATP. agentforthefifthisflavinadeninedinucleotide,orFAD. Afamiliarexampleisyeastgrowingunderanaerobic(no Knowingtheoxidation–reductionpotentialsofthereac- oxygen)conditions.Inthiscase,glucoseissaidtobefer- tantsinanoxidation–reductionreactionpermitstheready mented and ethyl alcohol and carbon dioxide (CO ) are 2 calculationofthestandardfreeenergychangeforthere- the end products (Fig. 5). In contrast, all higher organisms action.Itmaybeshownthat cancompletelyoxidizepyruvatetoCO andwater,using 2 molecularoxygen as the terminalelectron acceptor. The (cid:2)G(cid:2) =−nF(cid:2)E(cid:2), (1) 0 0 conversion of glucose to pyruvate releases only a small fractionoftheenergyavailableinthecompleteoxidation wheren isthenumberofelectronstransferredinthere- of glucose. In aerobic organisms, more than 90% of the action,F isFaraday’sconstant(23,060cal/V-equivalent), ATPmadeduringglucosecatabolismresultsfromtheox- and (cid:2)E(cid:2) is the difference between the E(cid:2) value of the 0 0 idationofpyruvate. oxidizingagentandthatofthereducingagent. P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 Bioenergetics 103 FADH is−38kcal/mol.Thesetwostronglyexergonicre- 2 actionsprovidetheenergyfortheendergonicsynthesisof ATP. Thedetailsofcarbonmetabolisminthecitricacidcy- clearebeyondthescopeofthisarticle.Inbrief,pyruvate isfirstoxidativelydecarboxylatedtoyieldCO ,NADH, 2 andanacetylgroupattachedinanesterlinkagetoathiol onalargemolecule,knownascoenzymeA,orCoA.(See Fig. 2.) Acetyl CoA condenses with a four-carbon dicar- boxylic acid to form the tricarboxylic acid citrate. Free CoA is also a product (Fig. 6). A total of four oxidation– reductionreactions,twoofwhichareoxidativedecarboxy- lations,takeplace,whichresultsinthegenerationofthe three remaining NADH molecules and one molecule of FADH . The citric acid cycle is a true cycle. For each 2 two-carbonacetylmoietyoxidizedinthecycle,twoCO 2 moleculesareproducedandthefour-carbondicarboxylic acidwithwhichacetylCoAcondensesisregenerated. The mitochondrial inner membrane (Fig. 7) contains proteinsthatactinconcerttocatalyzeNADHandFADH 2 oxidationbymolecularoxygen.[Seereactions(2)and(3) above.]Thesereactionsarecarriedoutinmanysmallsteps byproteinsthatareintegraltothemembraneandthatun- dergooxidation–reduction.Theseproteinsmakeupwhat iscalledthemitochondrialelectrontransportchain.Com- ponents of the chain include iron proteins (cytochromes andiron–sulfurproteins),flavoproteins(proteinsthatcon- FIGURE4 Aviewofglycolysis.Glucose,asix-carbonsugar,is tainflavin),copper,andquinonebindingproteins. cleavedandoxidizedtotwomoleculesofpyruvate.Thereisthe TheoxidationofNADHandFADH bymolecularoxy- 2 netsynthesisoftwoATPperglucoseoxidizedandtwoNADHare geniscoupledinmitochondriatotheendergonicsynthesis alsoformed. of ATP from ADP and P. For many years the nature of i thecommonintermediatebetweenelectrontransportand ThereducedformofNAD+,NADH,isastrongreduc- ATPsynthesiswaselusive.PeterMitchell,whoreceived ingagent.The E(cid:2) atpH7.0oftheNAD+–NADHcouple aNobelPrizeinchemistryin1978forhisextraordinary 0 is−340mV,whichisequivalenttothatofmolecularhy- insights,suggestedthatthiscommonintermediatewasthe drogen.E isthepotentialwhentheconcentrationsofthe protonelectrochemicalpotential.Heproposedintheearly 0 oxidized and reduced species of an oxidation–reduction pair are equal. Reduced FAD, FADH , is a weaker re- 2 ductantthanNADH,withan E(cid:2) (pH7.0)ofabout0V.In 0 contrast,molecularoxygenisapotentoxidizingagentand fullyreducedoxygen,water,isaverypoorreducingagent. TheE(cid:2) (pH7.0)fortheoxygen–watercoupleis+815mV. 0 TheoxidationofNADHandFADH resultsinthere- 2 ductionofoxygentowater: H++NADH+ 1O →NAD++H O (2) 2 2 2 and FADH + 1O →FAD+H O. (3) 2 2 2 2 In both cases two electrons are transferred to oxygen, FIGURE 5 Fates of pyruvate. In yeasts under anaerobic con- ditions, pyruvate is decarboxylated and reduced by the NADH so that the n in Eq. (1) is equal to 2. Under standard formedbyglycolysistoethanol.Inanaerobicmuscle,theNADH conditions, the oxidation of 1 mol of NADH by oxygen liberates close to 53 kcal, whereas the (cid:2)G(cid:2)0 for that of gisepnreersaetendt,bpyyrgulyvacotelyissiscoremdpulceeteslypyorxuidvaizteedtotolaCcOtic2aacnidd.wWahteern.O2 P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 104 Bioenergetics FIGURE6 Aviewoftheoxidationofpyruvate.TheoxidationofpyruvategeneratesthreeCO2,fourNADH,andone FADH2.TheoxidationofNADHandFADH2bythemitochondrialelectrontransportchainisexergonicandprovides mostoftheenergyforATPsynthesis. 1960s that electron transport through the mitochondrial potentialisexergonicandistheimmediatesourceofen- chain is obligatorily linked to the movement of protons ergy for ATP synthesis. The proton-linked synthesis of acrosstheinnermembraneofthemitochondrion.Inthis ATPiscatalyzedbyacomplexenzymecalledATPsyn- way, part of the energy liberated by oxidative electron thase.Remarkablysimilarenzymesarelocatedinthecou- transfer is conserved in the form of the proton electro- pling membranes of bacteria, mitochondria, and chloro- chemical potential. This potential, (cid:2)µH+, is the sum of plasts, the intracellular sites of photosynthesis in higher contributions from the activity gradient and that of the plants.Eventhoughthereactionthattheycatalyzeseems electricalgradient: relatively straightforward (see Fig. 2), the ATP synthases (cid:2) (cid:3) (cid:4) (cid:2)µH+ = RT ln [H+]a [H+]b +F(cid:2)ϕ, (4) caobnotuatin20apmoliynpimeputmideofch8aidnisff.erent proteins and a total of where Risthegasconstant;T,theabsolutetemperature; ATPisformedintheaqueousspaceboundedbythemi- aandb,theaqueousspacesboundedbythemembrane;F, tochondrial inner membrane. This space is known as the Faraday’sconstant;and(cid:2)ϕ,themembranepotential.As matrix (see Fig. 7). Most of the ATP generated within mi- Mitchellsuggested,themitochondrialinnermembraneis tochondriaisexportedtothecytoplasmwhereitisusedto poorly permeated by charged molecules, including pro- driveenergy-dependentreactions.TheADPandP formed i tons. The membrane thus provides an insulating layer inthecytoplasmmustthenbetakenupbythemitochon- between the two aqueous phases it separates. Thus the dria.Theinnermembranecontainsspecificproteinsthat transport of protons across the membrane generates an mediate the export of ATP and the import of ADP and electrochemicalpotential.Inthecaseofmitochondria,the P. One transporter catalyzes counterexchange transport i membrane potential is the predominant component of the ofATPoutofthematrixwithADPinthecytoplasminto electrochemicaloftheproton.Thetotal(cid:2)µH+ inactively the matrix (Fig. 8). At physiological pH, ATP bears four respiringmitochondriaisontheorderof−200mV,ifone negative charges, and ADP, three. Thus, the one-to-one usestheconventionthattheinsidespaceboundedbythe exchangetransportofATPwithADPcreatesamembrane membraneisnegative. potentialthatisoppositeinsignofthatcreatedbyelectron- ElectrontransportfromNADHandFADH tooxygen transport-drivenprotontranslocation.ATP/ADPtransport 2 providestheenergyforthegenerationoftheelectrochem- costs energy and the direction of transport is poised by icalpotentialoftheproton.Theflowofprotonsdownthis the proton membrane potential. In addition, phosphate P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 Bioenergetics 105 FIGURE7 Diagramsofthestructuresofmitochondriaandchloroplasts.Theinnermembraneofmitochondriaand the thylakoid membrane of chloroplasts contain the electron transport chains and ATP synthases. Note that the orientationoftheinnermembraneisoppositethatofthethylakoidmembrane. uptakeintomitochondriaiscoupledtotheelectrochemical O inthecompleteoxidationofglucose.ThesixthO is 2 2 proton potential. The phosphate translocator (see Fig. 8) reducedtowaterbyelectronsfromtheNADHformedby catalyzes the counterexchange transport of H PO2− and theoxidationoftriosephosphateinglycolysis. 2 4 hydroxideanion(OH−).TheoutwardmovementofOH− Fermentation,oranaerobicglycolysis,yieldsbut2mol causes acidification of the matrix, whereas the direction ofATPper1molofglucosecatabolized.Incontrast,com- of proton transport driven by electron transport is out of pleteoxidationofglucosetoCO andwateryieldsabout 2 themitochondrialmatrixandresultsinanincreaseinthe 15timesmoreATP.Thus,itisunderstandablewhyyeasts pHofthematrix. andsomebacteriaconsumemoreglucoseunderanaerobic InthetotaloxidationofglucosetoCO andwater,six conditionsthanwhenoxygenispresent. 2 CO are released and six O are reduced to water. For In animals, glucose is normally completely oxidized. 2 2 eachpyruvateoxidized,fourNADHandoneFADH are Duringstrenuousexercise,however,thedemandforoxy- 2 generated.Sincetwomoleculesofpyruvatearederivedby genbymuscletissuescanoutstripitssupplyandthetis- meansofglycolysisfromonemoleculeofglucose,atotal sue may become anaerobic. Muscle contraction requires ofeightNADHandtwoFADH areformedbypyruvate ATP,andrapidbreakdownofglucoseanditsstoragepoly- 2 oxidation. Four electrons are required for the reduction mer,glycogen,takesplaceunderanaerobiosis.Glycolysis ofO totwomoleculesofH O.Thus,pyruvateoxidation wouldstopquicklyiftheNADHproducedbytheoxida- 2 2 accountsforthereductionoffiveofthesixmoleculesof tionoftriosephosphatewerenotconvertedbacktoNAD+. In muscle cells under O -limited conditions, pyruvate is 2 reduced by NADH to lactic acid (see Fig. 5), a source of muscle cramps during exercise. At rest, lactic acid is converted back to glucose in the liver and kidneys and returnedtomuscletissueswhereitstoredintheformof glycogen. C. OxidationofFatsandOils, MajorMetabolicFuels Fatsandoilsareubiquitousbiologicalmoleculesthatare major energy reserves in animals and developing plants. Fats and oils are esters of glycerol, a three-carbon com- poundwithhydroxylgroupsonallthreecarbons,andcar- FIGURE 8 ATP, ADP, and Pi transport in mitochondria. ATP is boxylic acids with long hydrocarbon chains. The most formedinsidemitochondria.MostoftheATPisexportedtothe common fats and oils contain fatty acids with straight cytoplasmwhereitiscleavedtoADPandPi.Themitochondrial chainswithanevennumberofcarbonatoms.Mostoften, innermembranecontainsspecificproteinsthatmediatenotonly thetotalnumberofcarbonsinafattyacidinatriglyceride ATPreleasecoupledtoADPuptake,butalsoPiuptakelinkedto hydroxideion(OH−)release. rangesfrom14to18.Thedifferencebetweenafatandan P1:FYDRevisedPages EncyclopediaofPhysicalScienceandTechnology EN002H-54 May17,2001 20:22 106 Bioenergetics oilissimplymeltingtemperature.Oilsareliquidatroom temperature,whereasfatsaresolid.Familiarexamplesare oliveoilandbutter. Themostsignificantreasonforthisdifferenceinmelt- ing temperatures between fats and oils is the degree of unsaturation (double bonds) of the fatty acids they con- tain.Theintroductionofdoublebondsintoahydrocarbon chain causes perturbations in the structure of the chain that decrease its ability to pack the chains closely into asolidstructure.Oliveoilcontainsfarmoreunsaturated fatty acids than butter does and is thus a liquid at room temperatureandeveninthecold. Regardless of the physical properties of triglycerides, they are the long-term energy reserves of higher organ- isms. Consider the fact that the complete oxidation of triglycerides to CO and water yields 9 kcal/g, whereas 2 that of the carbohydrate storage polymers, starch and glycogen, yields just 4 kcal/g. When it is also remem- beredthatfatsandoilsshunwater,butglycogenandstarch aremorehydrophilic,triglycerideshaveanadditionalad- vantageovertheglucosepolymersasdepositsofpotential freeenergy.Ashydrophobicmoieties,fatsandoilsrequire less intracellular space than that required by the glucose polymers. Thefirststepinthebreakdownoftriglycerides(Fig.9)is theirconversionbyhydrolysistotheircomponents,glyc- erolandfattyacids.Glycerolisacloserelativeofthethree- carboncompoundsinvolvedinthecatabolismofglucose and may be completely oxidized to CO and water by 2 glycolysisandthetricarboxylicacidcycle. ThefattyacidsarefirstconvertedtoCoAderivativesat theexpenseofthehydrolysisofATPandthentransported into mitochondria where they are broken down sequen- tially,twocarbonunitsatatime,byapathwayknownas β-oxidation (see Fig. 9). The fatty acyl CoA derivatives undergooxidationatthecarbonthatisβ tothecarboxyl carbonfromthatofasaturatedcarbon–carbonbondtothat of an oxo-saturated carbon bond. Enzymes that contain FIGURE9 Oxidationoffattyacids.Fatsandoilsarehydrolyzedto FADoruseNAD+astheelectronacceptorscatalyzethese formglycerolandfattyacids.CoAderivativesofthefattyacidsare reactions.Asisthecaseintheoxidationofcarbohydrates, oxidizedinmitochondriabyNAD+andFADtoβ-oxo-derivatives. the NADH and FADH generated by the β-oxidation of CoAcleavesthesederivativestoyieldacetylCoAandafattyacid 2 fatty acids are converted to their oxidized forms by the CoAmoleculethatistwocarbonsshorter.Theprocesscontinues untilthefattyacidhasbeencompletelyconvertedtoacetylCoA. mitochondrial electron transport chain, which results in theformationofATPbyoxidativephosphorylation. TheacetylmoietyisoxidizedinthecitricacidcycletoCO2 and water.Thecompleteoxidationofafattyacidofaboutthesame Onceβ-oxidationiscomplete,theterminaltwocarbons molecularweightofglucoseyieldsfourtimesmoreATPthanthat of the fatty acid chain are then released as acetyl CoA. ofglucose. Oxidation and cleavage of the fatty acid continue until itisentirelyconvertedtoacetylCoA.Theconversionof D. CatabolismofProteinsandAminoAcids a saturated fatty acid with 18 carbon atoms to 9 acetyl CoAproduces8NADHand8FADH .TheacetylCoAis Inadditiontocontainingcarbohydratesandfats,dietsmay 2 burnedbythecitricacidcycletogeneratemoreATP.The berichinproteins.Thecatabolismofproteinsresultsinthe highcaloriccontentoffatspaysofftocellsintheyieldof generationoftheircomponentparts,aminoacids.When ATP. thedietaryaminoacidrequirementsofanindividualare