生物化學(xué)課件

AnOverviewofMetabolism代謝的定義生物代謝是指生物活體與外界環(huán)境不斷進(jìn)行的物質(zhì)（包括氣體、液體和固體）交換過程。合成代謝一般是指將簡單的小分子物質(zhì)轉(zhuǎn)變成復(fù)雜的大分子物質(zhì)的過程。分解代謝則是將復(fù)雜的大分子物質(zhì)轉(zhuǎn)變成小分子物質(zhì)的過程。糖、脂和蛋白質(zhì)的合成代謝途徑各不相同，但是它們的分解代謝途徑則有共同之處，即糖、脂和蛋白質(zhì)經(jīng)過一系列分解反應(yīng)后都生成了酮酸并進(jìn)入三羧酸循環(huán)，最后被氧化成CO2和H2O。◆◆新陳代謝的類型：新陳代謝

合成代謝（同化作用）分解代謝（異化作用）由小分子合成大分子需要能量將大分子分解成小分子釋放能量1.TheRolesofmetabolismMetabolismisahighlycoordinatedanddirectedcellactivity,inwhichmanymultienzymesystemscooperatetoaccomplishfourfunctions:(l)toobtainchemicalenergybycapturingsolarenergyorbydegradingenergy-richnutrientsfromtheenvironment;(2)toconvertnutrientmoleculesintothecell'sowncharacteristicmolecules,includingmacromolecularprecursors;(3)topolymerizemonomericprecursorsintoproteins,nucleicacids,lipids,polysaccharides,andothercellcomponents;(4)tosynthesizeanddegradebiomoleculesrequiredinspecializedcellularfunctions.TheenergysupplyanddemandinHeterotrophs:

theATP-ADPcycle2.TheGeneralFeaturesofmetabolismOccursinlinear,branchedorcircularpathways;Highlyinterconnected(“EveryroadleadstoRome”).Highlyregulatedtoachievethebesteconomy(“Balancedsupplyanddemand”).Thenumberofreactionsislarge(over1000)andthenumberoftypesofreactionsisrelativelysmall.（見課本p5~15）Wellconservedduringevolution:reflectingtheunityofthelifephenomena(“whathappensinbacteriahappensinhumanbeing”).DegradationisconvergentandenergyreleasingSynthesisisdivergentandenergyconsumingThecitricacidcycle乙酰輔酶A草酰乙酸檸檬酸異戊烯焦磷酸3.Thingsthatwillbecovered1).Generalprinciplesforbioenergetics.

2).Oxidativedegradationoffuels(glycolysis,b-oxidation,ureacycle,a-ketoacidoxidation, citricacidcycle),generatingNADH,FADH2,ATP,andCO2.

3).OxidationofNADHandFADH2byO2andgenerationofATPandH2O(respiratorychains,ATPsynthase).

4).Photosynthsis(photophosphorylationandcarbonfixation).

5).Regulationofmetabolism.4.Understanding

Metabolism:aretrospect4.1

Sugardegradation(glycolysis)andsynthesis1897,EduardBuchner,Cell-freefermentation(NobelPrizein1907).OttoFritzMeyerhof,conversionofglucosetolacticacidinmuscleandbacktoglucoseinliver(NobelPrizein1922).SirArthurHardenandHansvonEuler-Chelpin,involvementofenzymes,coenzymes,andphosphorylatedintermediates(NobelPrizein1929).CarlFerdinandCoriandGertyTheresaCori,formationofglucose-1-phosphatefromglycogenandpyrophosphatebytheactionofphosphorylase(NobelPrizein1947).LuisF.Leloir,UDP-glucoseistheprecursorforglycogensynthesis(NobelPrizein1970).Thewholeglycolysispathway(conversionofglucosetopyruvate)wasrevealedby1940.4.2Completeoxidationoffuels:fromtwo-carbonunitstoCO2AlbertvonSzent-Gyorgyi,plantacids,fumaricacid(反丁烯二酸),malicacid(蘋果酸)werenotconsumed,butactascatalystsforthecellularcombustionprocess(NobelPrizein1937).FritzAlbertLipmann,roleofco-enzymeAandATP(NobelPrizein1953);SirHansAdolfKrebs,Acetyl-CoAandcitricacidcycle(檸檬酸循環(huán))forcompleteoxidationtwo-carbonunits(NobelPrizein1953).4.3SynthesisofATPusingenergyreleasedfromfueloxidationOttoHeinrichWarburg,involvementofiron-containingcytochromes(細(xì)胞色素)

incellularrespiration(NobelPrizein1931).PeterD.Mitchell,proposedthechemiosmotictheory（化學(xué)滲透學(xué)說）torelateelectronflowtoATPsynthesisinallorganisms(NobelPrizein1978).PaulD.Boyer,JohnE.Walker,enzymaticmechanismforATPsynthesis(NobelPrizein1997).4.4LipiddegradationandsynthesisKonradBlochandFeodorLynen,pathwaysforcholesterolandfattyacidsynthesis(NobelPrizein1964).MichaelS.BrownandJosephL.Goldstein,regulationofcholesterolbiosynthesis(NobelPrizein1985).4.5RegulationofMetabolismEarlW.Sutherland,Jr.,cAMPasthesecondmessagerforhormonestoregulatecellmetabolism(NobelPrizein1971).AlfredG.GilmanandMartinRodbell,involvementofmembraneG-proteinsinsignaltransductionofhormones(NobelPrizein1994).EdmondH.FischerandEdwinG.Krebs,regulationofenzymaticactivitybyreversiblephosphorylation(NobelPrizein1992).4.6PhotosynthesisHansFischer,constitutionofchlorophyll(葉綠素)

anditssimilaritytoheme(NobelPrizein1930).MelvinCalvin,CalvincycleforCO2assimilation(NobelPrizein1961).JohannDeisenhofer,RobertHuber,andHartmutMichel,3-DstructureofaPhotosyntheticreactioncenterfromapurplebacterium(NobelPrizein1988).4.7DNA,RNAandProteinsynthesisSeveroOchoaandArthurKornberg,enzymaticsynthesisofRNAandDNA(NobelPrizein1959).MarshallW.Nirenberg,HarGobindKhorana,interpretationofthegeneticcodesinproteinsynthesis(NoblePrizein1968).FrancoisJacob,AndreLwoff,andJacquesMonod,Mechanismstoswitchgenesoneandoffinprokaryotes(NobelPrizein1965).DavidBaltimore,RenatoDulbecco,andHowardMartinTemin,enzymaticRNA-dependentDNAsynthesisintumorviruses(NobelPrizein1975).BarbaraMcClintock,mobilegeneticelementsortransposons(NobelPrizein1983).SusumuTonegawa,geneticprincipleforthegenerationofantibodydiversity(NobelPrizein1987).SidneyAltmanandThomasR.Cech,RNAcatalyzedRNAprocessing(Ribozyme，核酶)(NobelPrizein1989).RichardJ.RobertsandPhilipA.Sharp,eukaryoticgenesaresplitandhavetobespliced(剪接）aftertranscription(NoblePrizein1993).GunterBlobel,intrinsicsignalsgovernproteinlocalization(NobelPrizein1999).HowtostudyBiochemistryIICompareandrelatethechemicalreactions(thesubstrates,theproductsandthetypeofconversion)enzymes,coenzymes,physiologicalroles,waysofregulationinvolved,etc.(Thismustbesimilar/relatedtothat!)Understandtheclassicalexperimentsandthoughtsthatledtotherevelationoftheknowledgedescribed(whywasoneawardedtheNobelPrize?).Beawarewiththedegreeofspeculativenessoncertainmodels(nothingis100%certaininscience).Understandtheaspectsthatneedfurtherstudies(howcouldIwinaNobelPrize?)Chapter20PrinciplesofBioenergeticsBioenergetics(生物能學(xué)）thequantitativestudyofenergytransductionsinlivingcellsandthechemicalnatureunderlyingtheseprocesses.一、BioenergeticsandThermodynamics1.Cellsneedenergytodoalltheirwork形成和保持細(xì)胞高度有序的結(jié)構(gòu)(biosynthesisofmacromolecules).推動(dòng)所有類型的運(yùn)動(dòng)(mechanicalworkandtransport).建立跨膜的離子濃度和電荷梯度.保持體溫.在某些生物中產(chǎn)生光.The“energyindustry”(production,storageanduse)iscentraltotheeconomyofthecellsociety!2.

ChemicalenergyisthefavorableenergyformforcellstodoalltheirworkAntoineLavoisier`sinsightonanimalrespirationinthe18thcentury:itisnothingbutaslowcombustionofcarbonandhydrogen(thesamenatureasalightingcandle).(拉瓦錫1743－1794)Livingcellsaregenerallyheldatconstanttemperatureandpressure:chemicalenergy(freeenergy)hastobeusedbylivingorganisms,nothermalenergy,neithermechanicalenergyisavailabletodoworkincells.Biologicalenergytransformationobeythetwobasiclawsofthermodynamicsrevealedbyphysicistsandchemistsinthe19thcentury:energycanneitherbecreatednorbedestroyed(butconserved);energyconversionisnever100%efficient(somewillalwaysbewastedinincreasingthedisorderor“entropy”oftheuniverse).Thefreeenergyconceptofthermodynamicismoreimportanttobiochemiststhantochemists(whocanalwaysincreasethetemperatureandpressuretomakeareactiontooccur!).3.Thefreeenergy(G)conceptofbiochemicalreactionsFreeenergy(G):

在恒定的溫度和壓力條件下，一個(gè)生物化學(xué)反應(yīng)或過程可以用來做功的能量的多少。注意：自由能強(qiáng)調(diào)的是反應(yīng)始態(tài)與終態(tài)之間的能量變化，任何體系的絕對(duì)自由能是無法測定的。Freeenergychange(

G):

即產(chǎn)物與反應(yīng)物之間的自由能差異。Gibbs發(fā)現(xiàn):在恒定的溫度和壓力條件下，任何自發(fā)過程的自由能都是減少的(productsshouldhavelessfreeenergythanreactantsforareactiontooccurspontaneously,i.e.,

Ghasanegativevalue).（注意：自發(fā)性與化學(xué)反應(yīng)的速率沒有任何關(guān)系

）（4）生物化學(xué)中的標(biāo)準(zhǔn)自由變化(

G'o):

valueofthechangeinfreeenergyunderconditionsof298K(25oC),1atmpressure,pH7.0(chemistsusepH0,i.e.,theconcentrationofH+theyuseis1M,not10-7Masbiochemistsusehere)andinitialconcentrationsof1Mforallreactantsandproducts(exceptH+).（5）Theactualfreeenergychang(G)dependson

G'o,temperature,ratioofproductandreactantconcentrations(Q):

G=

G'o+RTlnQ（6）Enzymesonlyspeedupthermodynamicallyfavorablereactions(havinganegative

G)!（7）

Go'與K'

eq

的關(guān)系(theprimeagainindicatesitsbiochemicaltransformation):atequilibrium,

G=0,Q=K`eq,thus

Go'

=-RTlnK'eq

（8）當(dāng)兩個(gè)反應(yīng)偶聯(lián)時(shí)，

Gand

G‘o

的值具有加和性（additive）(i.e.,sharingcommonintermediates),因此，一個(gè)熱力學(xué)允許的反應(yīng)可以驅(qū)動(dòng)一個(gè)熱力學(xué)不利的反應(yīng).這樣的現(xiàn)象在生物化學(xué)中經(jīng)常遇到。Glucose+Phosphate→Glucose-6-phosphate+H2O

Go‘=+13.8KJ/molATP+H2O→ADP+Pi

Go‘=-30.5KJ/mol二、PhosphateGroupTransfersandATPATP水解的標(biāo)準(zhǔn)自由能（

Go‘

）：[ATP]=[ADP]=[Pi]=1mol/L,[H+]=10-7mol/L

Go‘=-30.5KJ/mol因此我們稱ATP為高能磷酸化合物，而發(fā)生斷裂的焦磷酸鍵稱為高能鍵。（注意：我們所說的高能與化學(xué)中的高能鍵是完全不同的兩個(gè)概念）2.在細(xì)胞中，ATP水解的實(shí)際自由能變化（

G）與

Go‘差別很大：(

G'o

is-30.5kJ/mol;Gincellsis-50to-65kJ/mol)Forexample,inhumanerythrocytestheconcentrationsofATP,ADP,andPiare2.25,0.25,and1.65mM,respectively,thepHis7.0andthetemperatureis25°C.ΔG=ΔG°'+RTln[ADP][Pi][ATP]=-30,500J/mol+(8.315J/mol?K)(298K)ln[(2.50?10-4)(1.65?10-3)/(2.25?10-3)]=-30,500+2,480ln(1.83?10-4)

=-51.8(kJ/mol)3.TheATPmoleculeiskineticallystableatpH7(i.e.,ithasahighactivationenergy,G?forhydrolysis)andenzymecatalysisisneededforitshydrolysis.4.ATPisnotalong-termstorageformoffreeenergyinlivingcells,beingconsumedwithinaminutefollowingitsformation.Arestinghumanconsumesabout40kgofATPin24hours!5.其他高能磷酸化物和硫酯類化合物：6.ATPhasanintermediatephosphorylgrouptransferpotential,thusADPcanacceptandATPcandonatephosphorylgroups.ATPprovidesenergybygrouptransfer(donatingaPi,PPiorAMPtoformcovalentintermediates),notbysimplehydrolysis.ATPhasanintermediatephosphorylgrouptransferpotentialATPcantransferaPi,PPiorAMPtoareactant7.Inthelab,aslittleasafewpicomoles(10-12mol)ofATPcanbemeasuredusingfireflyluciferinandluciferase(熒光素酶),usingspectroscopicmethods.三、生物主要是通過氧化還原反應(yīng)的電子轉(zhuǎn)移獲得生物能1.Whenelectronsflowfromalowaffinitycarrier(reductant,e.g.,glucose)toahighaffinitycarrier(oxidant,e.g.,O2),eitherinanelectricbatteryorinalivingcell,energyisreleasedandworkcanbedone.Oxidationofenergy-richbiologicalfuelsoftenmeansdehydrogenation(catalyzedbydehydrogenases,脫氫酶)fromcarbonshavingvariousoxidationstates.Inthelivingcells,electronsaretransferreddirectlyaselectrons(betweenmetalions),ashydrogenatoms(H++e-),orasahydrideion(:H-orH++2e-).生物細(xì)胞中氧化還原反應(yīng)電子轉(zhuǎn)移方式：以電子的形式傳遞：呼吸鏈中細(xì)胞色素之間的氧化還原反應(yīng)。2.以氫原子的形式傳遞：以FMN或FAD為氫受體的脫氫反應(yīng)。3.以氫負(fù)離子（或H++2e-）形式傳遞：以NADN+或NADP+為受體的脫氫反應(yīng)。2.Reductionpotentials(E)measureaffinityforelectrons:Theaffinityforelectronsofacompound(initsoxidizedform)isindicatedbyitsreductionpotential(E).Standardreductionpotential(Eo')ofeachoxidant(aconstant)ismeasuredbyconnectingahalf-cellhavingtheoxidizedandreducedspeciesoftheredoxpaireachat1M,or1atmforgases,pH7toareferencehalf-cellhaving1MH+and1atmH2,whoseEo'isarbitrarilyassignedas0.00V.ApositivevalueofEo'indicatesatendencytoacquireelectronfromthereferencehalfcell(with1MH+/1atmH2).Thestandardreductionpotential(Eo')ofaconjugateredoxpairismeasuredbyconnectingthesamplehalf-celltotheH+/H2referencehalf-cell.pH7pH0WaltherNernstderivedanequationthatrelatesstandardreductionpotential(E0)toreductionpotential(E)atanyconcentrationofoxidizedandreducedspeciesinthecell:在生物化學(xué)中用Eo'取代EoEo'3.生物化學(xué)反應(yīng)的自由能變化與電池電動(dòng)勢：Theenergymadeavailabletodoworkbythisspontaneouselectronflow(thefree-energychangefortheoxidation-reductionreaction)isproportionaltoΔE:ΔG=-nFΔEorΔGo'=-nFΔEo'Acetaldehyde+NADH+H+

→ethanol+NAD+TherelevanthalfreactionsandtheirEovaluesare:(1)Acetaldehyde+2H++2e-

→ethanol

Eo‘=-0.197V(2)NAD++2H++2e-

→NADH+H+

Eo'=-0.320VFortheoverallreaction,ΔEo'=-0.197V-(-0.320V)=0.123V,andnis2.Therefore,ΔGo'=-nFΔEo'=-2(96.5kJ/V?mol)(0.123V)=-23.7kJ/mol.5.Afewuniversalcarrierscollectelectronsfromthestepwiseoxidationofvarioussubstrates.Cellularoxidationofanutrientoccursviastepwisereactions(pathways)forefficientenergytransduction.NAD+,NADP+,FAD,andFMNareuniversalreversibleelectroncarriers(ascoenzymesofvariousenzymes).NADandNADParedinucleotidesabletoaccept/donateahydrideion(with2e-)foreachroundofreduction/oxidation.NAD(asNAD+)usuallyactsinoxidationsandNADP(asNADPH)inreductions.NADandNADPcaneasilydiffuseoutoftheenzymes,butFMNandFADaretightlyboundtotheenzymes(thusbeingcalledprostheticgroups,andthecomplexproteinsbeingcalledflavoproteins).NADHandFADH2willbefurtheroxidizedviatherespiratorychainforATPproduction.:H-QuinonoidNAD(NicotinamideAdenineDinucleotide)andNADP(NicotinamideAdenineDinucleotidePhosphate)Nicotinamide(derivedfromniacin)Benzenoid(insolution)IsoalloxazineFMN(Flavinmononucleotide)andFAD(FlavinAdenineDinucleotide)Eo'ofFAD/FMNoftendiffersindifferentflavoproteins,whichareoftencomplexandcontainotherinorganicionstohelpelectrontransfer異咯嗪環(huán)(derivedfromriboflavin)

ATP的水解和鎂離子的作用Carbonshavevariousoxidationstates,withhydrocarbonbeingthemostreducedandCO2beingthemostoxidizaed.

首先，在你們這個(gè)年齡應(yīng)該充滿理想和抱負(fù)，對(duì)知識(shí)、科學(xué)和人生有著無限追求。所以，我覺得考研的主要目的是為了進(jìn)一步提高自己的知識(shí)層次，培養(yǎng)從事科學(xué)研究的能力，為實(shí)現(xiàn)更大和更遠(yuǎn)的目標(biāo)奠定基礎(chǔ)。如果這樣，你就應(yīng)該選一個(gè)自己喜歡的學(xué)科專業(yè)，立志為其不懈努力，奮斗不止。

其次，可能你對(duì)學(xué)科專業(yè)的認(rèn)識(shí)還很模糊，難以確定自己喜歡什么，并且近期目標(biāo)就是考上研究生，那么，就根據(jù)自己的專業(yè)知識(shí)能力和英語水平，選擇比較有把握考上的單位。因?yàn)槿暌院笕绾巫兓桶l(fā)展是很難預(yù)料的。也許，在這三年中，你對(duì)科學(xué)的認(rèn)識(shí)更加深刻，業(yè)務(wù)知識(shí)和科研能力都取得了飛躍進(jìn)展，到時(shí)考博和出國深造都有可能。往后的發(fā)展機(jī)遇可能更多更好。最后，如果你的目的就是為了研究生畢業(yè)后容易就業(yè)或找一份好的工作，那么，我認(rèn)為生物化學(xué)和分子生物學(xué)專業(yè)最好。一是目前以及在未來一段時(shí)間內(nèi)，這方面的專業(yè)人員還比較短缺，例如，近年來，我們學(xué)院來了許多研究生和博士生，但沒有一個(gè)生物化學(xué)和分子生物學(xué)專業(yè)的，而我們學(xué)院急需這方面的師資和學(xué)術(shù)骨干。并且據(jù)我了解，全國普通高校都存在類似問題。二是隨著生物技術(shù)的發(fā)展和應(yīng)用，我國生物技術(shù)產(chǎn)業(yè)將迅速發(fā)展，而生物化學(xué)和分子生物學(xué)是生物技術(shù)的理論基礎(chǔ)和技術(shù)支撐，所以未來的就業(yè)機(jī)會(huì)很多。三是生物化學(xué)和分子生物學(xué)專業(yè)比較容易轉(zhuǎn)行，如你所說的健康和營養(yǎng)問題都離不開生物化學(xué)的理論和技術(shù)。既是到一些公司搞營銷，也屬生物化學(xué)專業(yè)選擇面最廣。

以上所述，僅是我個(gè)人的看法，還要根據(jù)你自己的情況而定。

糖酵解GlycolysisandtheCatabolismofHexosesAnoverviewonD-glucosemetabolism大多數(shù)機(jī)體的能源物質(zhì)，徹底氧化：

G'o=–2840kJ/mole通過磷酸戊糖途徑可產(chǎn)生NADPH和ribose-5-P?？梢砸远嗵?glycogenorstarch)或轉(zhuǎn)化為脂肪進(jìn)行長期儲(chǔ)存。幾乎所有的生物分子都可由葡萄糖合成（aminoacids,nucleotides,fattyacids,coenzymesandothermetabolicintermediates.）MetabolismofglucoseunderAerobicandAnaerobicConditions:AnoverviewonD-glucosemetabolismTheDevelopmentofBiochemistryandtheDelineationofGlycolysisWentHandbyHand1897,EduardBuchner(Germany),accidentalobservation:sucrose(asapreservative)wasrapidlyfermentedintoalcoholbycell-freeyeastextract.Theacceptedviewthatfermentationisinextricablytiedtolivingcells(i.e.,thevitalisticdogma,活力論)wasshakenandBiochemistrywasborn:Metabolismbecamechemistry!1900s,ArthurHardenandWilliamYoungPiisneededforyeastjuicetofermentglucose,ahexosediphosphate(fructose1,6-bisphosphate)wasisolated.1900s,ArthurHardenandWilliamYoung(GreatBritain)separatedtheyeastjuiceintotwofractions:oneheat-labile,non－dialyzablezymase(enzymes)andtheotherheat-stable,dialyzablecozymase(metalions,ATP,ADP,NAD+).1910s-1930s,GustavEmbdenandOttoMeyerhof(Germany),studiedmuscleanditsextracts:Reconstructedallthetransformationstepsfromglycogentolacticacidinvitro;revealedthatmanyreactionsoflacticacid(muscle)andalcohol(yeast)fermentationswerethesame!DiscoveredthatlacticacidisreconvertedtocarbohydrateinthepresenceofO2(gluconeogenesis);observedthatsomephosphorylatedcompoundsareenergy-rich.GlycolysiswasalsoknownasEmbden-Meyerhofpathway.Thewholepathwayofglycolysis(Glucosetopyruvate)waselucidatedbythe1940s.一、糖酵解途徑（Embden-Meyerhofpathway

）葡萄糖經(jīng)10步酶促反應(yīng)，分解為2分子丙酮酸，產(chǎn)生2分子ATP和2分子NADH。1.糖酵解的十步反應(yīng)AnetgainoftwoATP,twoNADH,twopyruvatesareresultedwhenaglucosemoleculeisoxidizedviatheglycolysispathway:

Glucose+2ADP+2Pi+2NAD+

2pyruvate+2ATP+2H2O+2NADH+2H+2.Tenenzymescatalyzethetenreactionsofglycolysis（1）Hexokinase(alsoglucokinaseinliver)催化糖酵解的第一步反應(yīng)?！颩g2+ATP2-,notATP4-

是該酶的另一個(gè)底物;★己糖激酶是一個(gè)調(diào)節(jié)酶，6-磷酸葡萄糖和ATP是該酶的變構(gòu)抑制劑?！镌摲磻?yīng)熱力學(xué)有利，在細(xì)胞中為不可逆反應(yīng)。（2）Phosphohexoseisomerase(alsocalledphosphoglucoseisomerase)catalyzestheisomerizationfromglucose6-Ptofructose6-P,convertinganaldosetoaketose.（3）Phosphofructokinase-1(PFK-1,磷酸果糖激酶-1)催化第二步磷酸化反應(yīng)?！?/p>

PFK-1是一個(gè)變構(gòu)酶，催化效率很低，糖酵解途徑進(jìn)行的速度依賴于該酶的活性水平?！颬FK-1是一個(gè)調(diào)節(jié)酶，ATP和H+是該酶的變構(gòu)抑制劑，AMP可解除ATP對(duì)該酶的抑制作用，因此細(xì)胞中ATP/AMP值對(duì)此酶具有明顯的調(diào)節(jié)作用。★H+

濃度升高抑制該酶的活性。★在動(dòng)物細(xì)胞中，發(fā)現(xiàn)三種同工酶：PFKA,B,C?！颰heplantPFK-1makesuseofPPi,insteadofATPatthisstep.（4）Aldolase(醛縮酶),namedforthereversereactioncatalyzesthecleavage(“l(fā)ysis”)offructose1,6-bisphosphatefromthemiddleC-Cbondtoformtwo3-carbonsugars,dihydroxyacetonephosphateandglyceraldehyde3-phosphate;thisisareversalaldolcondensationreaction;thermodynamicallyveryunfavorableunderstandardconditions.（5）Triosephosphateisomerase(anextremelyefficientenzyme)convertsdihydroacetonephosphatetoglyceraldehyde3-phosphate;anintramolecularredoxreaction(ahydrogenatomistransferredfromC-1toC-3).（6）Glyceraldehyde3-phosphatedehydrogenase

催化糖酵解的唯一的氧化還原反應(yīng)?！颪AD+

為該酶的輔酶，從底物接受2e-?！锩摎浞磻?yīng)的同時(shí)，底物發(fā)生磷酸化，無機(jī)磷酸直接參與反應(yīng)，砷酸可取代磷酸參與該反應(yīng)，但產(chǎn)物為3-磷酸甘油酸?！飵€基為該酶的催化基團(tuán)，形成硫酯中間產(chǎn)物，碘乙酸為是該酶的抑制劑。（7）Thephosphoglyceratekinasecatalyzesthedirecttransferoftheanhydridephosphatein1,3-BPGtoanADPtogenerateanATP;thisiscalledthesubstrate-levelphosphorylation;1,3-BPGisahighenergyintermediatethatleadstoATPformation.（8）Thephosphoglyceratemutasecatalyzestheshiftofphosphorylgroupon3-phosphoglyceratefromC-3toC-2;2,3-bisphosphoglycerateisbothacoenzymeforthemutaseandanintermediateforthereaction.（9）Enolase(烯醇酶)catalyzestheeliminationofaH2Ofrom2-phosphoglyceratetogeneratephosphoenolglycerate(PEP)withthetransferpotentialofthephosphorylgroupdramaticallyincreased(

G0`

changedfrom–17.6to–61.9kJ/mol).（10）Thepyruvatekinase(namedforthereversereaction)催化糖酵解的第二分子ATP的合成。★該酶為變構(gòu)調(diào)節(jié)酶，ATP、長鏈脂肪酸、乙酰輔酶A、和丙氨酸為抑制劑，果糖-1，6-二磷酸為激活劑?！镌摲磻?yīng)在細(xì)胞中為不可逆反應(yīng)?！镌撁钢辽儆腥N不同類型的同工酶：L型（肝臟）、M型（肌肉和腦）、A型（其他組織）。二

其他單糖進(jìn)入糖酵解途徑三、糖酵解途徑的調(diào)節(jié)Therateofglycolysisinmammalsismainlycontrolledatthestepactedbyphosphofructokinase-1(PFK-1)(Why?)(1)

PFK-1catalyzesanirreversibleexergonicreaction,whichcommitsglucosetotheglycolysispathway(awayfromthepentosephosphatepathway).(2)PFK-1isacomplextetramericenzymeregulatedbymultipleintracellularsignals(allostericeffectors):ATP,citratebeingnegativeones;AMP,ADPandfructose2,6-bisphosphateaspositiveones.(3)Aregulatedbifunctionalenzyme(PFK-2andFBPase-2)synthesizes(fromFru-6-P)anddegradesfructose2,6-bisphosphate.Afeedforwardstimulation:Fru-6-PstimulatethesynthesisandinhibitsthehydrolysisofFru-2,6-bisphosphate,whichinturnstimulatesPFK-1.2.Hexokinaseandpyruvatekinasealsosetthepaceofglycolysis(1)Thesetwoenzymesalsocatalyzedirreversibleexergonicreactions.(2)MusclehexokinaseisallostericallyinhibitedbyitsreactionproductGlc-6-P,whichaccumulateswhenPFK-1isinhibited.(3)Theliverhexokinase(alsocalledhexokinaseDorglucokinase)hasabout100XlessaffinityforglucosethanthatinmuscleandisnotinhibitedbyGlc-6-P:itsmainroleistoconvertexcessglucosetoGlc-6-Pforglycogensynthesis.(4)PyruvatekinaseisallostericallyinhibitedbyATP,

alanine,acetyl-CoA,andlong-chainfattyacids.(5)Thecatalyticactivityoftheliverpyruvatekinaseisozyme(theLtype)isalsocontrolledbyreversiblephosphorylation.四、無氧條件下，丙酮酸的去路

－－發(fā)酵ThisoccurstoregenerateNAD+fortheglycolysispathwaytocontinuewhenO2lacks.乳酸發(fā)酵：

PyruvateisreducedtolactatewhenO2lacksinareactioncatalyzedbylactatedehydrogenase(occurringinveryactiveskeletonmuscle,somebacterialikelactobacilli)2.乙醇發(fā)酵：pyruvateisfirstdecarboxylatedandthenreducedbyNADH,catalyzedbypyruvatedecarboxylaseandalcoholdehydrogenaserespectively.PresentonlyinthosealcoholfermentativeorganismsPresentinmanyorganismsincludinghumanHexokinaseGlucoseInducedfitIrreversibleincells

Onesubunitofthetetramericphosphofructokinase-1(PFK-1)RegulatoryADPThecommittingstepPhosphorolysis（磷酸解作用）C-1nolongercarriesalargepositivecharge:hydrideionleavesreadilyEnergy-richintermediate(thioester)InactiveenzymeProposedactionmechanismOfglyceraldehyde3-PdehydrogenaseAproposedactionmechanismforphosphoglyceratemutase第二十三章

三羧酸循環(huán)TricarboxylicAcidCycle一、葡萄糖的有氧分解代謝在有氧條件下，葡萄糖的分解代謝可分為三個(gè)階段進(jìn)行討論：（1）葡萄糖到丙酮酸階段同糖酵解途徑；（2）三羧酸循環(huán)；（3）呼吸鏈電子傳遞和ATP的合成。1.Pyruvateisoxidizedtoacetyl-CoAbythecatalysisofpyruvatedehydrogenasecomplexStructureofthepyruvatedehydrogenasecomplex

ThreedimensionalimageofPDHcomplex,showingthesubunitstructure:E1,pyruvatedehydrogenase;E2,dihydrolipoyltransacetylase;andE3,dihydrolipoyldehydrogenase.Thecore(green)consists

of60moleculesofE2,arrangedin20trimerstoformapentagonal

dodecahedron.ThelipoyldomainofE2(blue)reachesoutwardto

touchtheactivesitesofE1molecules(yellow)arrangedontheE2core.

AnumberofE3subunits(red)arealsoboundtothecore,wherethe

swingingarmonE2canreachtheiractivesites.Anasteriskmarksthe

sitewherealipoylgroupisattachedtothelipoyldomainofE2.Numberoflipoyl

domainsvariesbyspecies.

2.Thecompleteoxidationofpyruvateinanimaltissueswasproposedtoundergoviaacyclicpathway(1)O2consumptionandpyruvateoxidationinmincedmuscletissueswerefoundtobestimulatedbysomefour-carbondicarboxylicacids(Fumarate,succinate,malateandoxaloacetate,five-carbondicarboxylicacid(a-ketoglutarate),orsix-carbontricarboxylicacids(citrate,isocitrate,cis-aconitate).(2)Asmallamountofanyoftheseorganicacidsstimulatesmanyfoldsofpyruvateoxidation!(3)Malonateinhibitspyruvateoxidationregardlessofwhichactiveorganicacidisadded!(4)HansKrebsproposedthe“citricacidcycle”forthecompleteoxidationofpyruvateinanimaltissuesin1937(hewronglyhypothesizedthatpyruvatecondenseswithoxaloacetateinhisoriginalproposal).(5)Thecitricacidcyclewasconfirmedtobeuniversalincellsbyinvitrostudieswithpurifiedenzymesandinvivostudieswithradioisotopes(“radioisotopetracerexperiments”).(6)KrebswasawardedtheNobelprizeinmedicinein1953forrevealingthecitricacidcycle(thusalsocalledtheKrebscycle).3.Theacetylgroup(carriedbyCoA)iscompletelyoxidizedtoCO2viathecitricacidcycleThe4-carbonoxaloacetate(草酰乙酸)actsasthe“carrier”fortheoxidation.Thetwocarbonsreleasedas2CO2inthefirstcycleofoxidationarenotfromtheacetyl-CoAjustjoined.The8electronsreleasedarecollectedbythreeNAD+andoneFAD.OnemoleculeofATP(orGTP)isproducedpercyclebysubstrate-levelphosphorylation.4.Thecitricacidcycle5.Thecompleteoxidationofoneglucosemayyieldasmanyas32ATPAlltheNADHandFADH2willeventuallypasstheirelectronstoO2afterbeingtransferredthroughaseriesofelectroncarriers.ThecompleteoxidationofeachNADHmoleculeleadstothegenerationofabout2.5ATP,andFADH2ofabout1.5ATP.Overallefficiencyofenergyconservationisabout34%usingthefreeenergychangesunderstandardconditionsandabout65%usingactualfreeenergychangesincells.6.TheregulationofTCAcycle(1)ThepyruvatedehydrogenasecomplexinvertebratesisregulatedallosetericallyandcovalentlyTheformationofacetyl-CoAfrompyruvateisakeyirreversiblestepinanimalsbecausetheyareunabletoconvertacetyl-CoAintoglucose.Thecomplex(inallorganisms)isallostericallyinhibitedbysignalingmoleculesindicatingarichsourceofenergy,e.g.,ATP,acetyl-CoA,NADH,fattyacids;activatedbymoleculesindicatingalack(ordemand)ofenergy,e.g.,AMP,CoA,NAD+,Ca2+.Theactivityofthecomplex(invertebrates,probablyalsoinplants,butnotinE.coli)isalsoregulatedbyreversiblephosphorylationofoneoftheenzymes,E1,inthecomplex:phosphorylationofaspecificSerresidueinhibitsanddephosphorylationactivatesthecomplex.Thekinaseandphosphataseisalsopartoftheenzymecomplex.ThekinaseisactivatedbyahighconcentrationofATP.(2)TherateofthecitricacidcycleiscontrolledatthreeexergonicirreversiblestepsCitratesynthase,isocitratedehydrogenaseanda-ketoglutaratedehydrogenase;Inhibitedbyproductfeedback(citrate,succinyl-CoA)andhighenergycharge(ATP,NADH);Activatedbyalowenergycharge(ADP)orasignalforenergyrequirement(Ca2+).二、三羧酸循環(huán)的回補(bǔ)反應(yīng)Themostcommonanapleroticreactionscoverteitherpyruvateorphosphoenolpyruvatetooxaloacetateormalate.Someanaerobicbacteria,lackingthea-ketoglutaratedehydrogenaseenzyme,makebiosyntheticprecursorsviatheincompletecitricacidcycle;couldbeanearlyevolutionstageofthecitricacidcycle.

三、三羧酸循環(huán)的中間產(chǎn)物是重要的生物合成的前體分子Thecitricacidcycleisthehubofintermediarymetabolismservingboththecatabolicandanaboli