第十章 糖類合成代謝

第一節(jié)光合作用HowisCO2converted(fixed)toglucoseinplants?WhatarethemajorstagesforCO2assimilationviatheCalvincycle?Fixationstage:CO2iscondensedtoafive-carbonacceptor(ribulose-1,5-bisphosphate)toform3-phosphoglycerate;Reductionstage:3-phosphoglycerateisreducedtoglyceraldehyde-3-phosphate;Regenerationstage:ribulose-1,5-bisphosphateisregeneratedusingglyceraldehyde-3-phosphate.Howwasthe“pathofcarbon”inphotosynthesisrevealedbyCalvin?Mainlytwomethodswereused:isotopictracingandchromatographicseparationsoflabeledintermediates;Inject14CO2intoaalgaesuspension,thenilluminateforashortperiodoftime;Theproductswereanalyzedbymethodslikeradiochromatograms.Wheredoes14CO2goattheverybeginningoftheassimilation?3-phosphoglycerateisthefirstmetabolitelabeledwhen14CO2isinjected;Allthe14Cwasfoundtobeinthecarboxylgroupof3-phosphoglycerate;Ribulose-1,5-bisphosphatewasrevealedtobetheCO2acceptorbycomparingthesteady-stateconcentrationsofvariouscompoundsbysuddenlyraisingorloweringtheCO2levels.WhatistheproposedreactionsCO2fixation(fixationstage)?Ribulose-1,5-bisphosphateisconvertedtoanenediolintermediatebeforecondensedtoCO2;CO2(notbicarbonate)isaddedtothesecondcarbonoftheenediolintermediatetoformasix-carbonb-ketoacidintermediate,whichisthenhydratedtoformanothersix-carbonintermediate;Two3-phosphoglyceratemoleculesareformedfromthesix-carbonintermediateviaacarbanion;Thewholeconversioniscatalyzedbyribulose-1,5-bisphosphatecarboxylase/oxygenase(rubiscoinshort).Whatisknownaboutrubisco?Theplantenzymeconsistsof8large(withbothcatalyticandregulatorysites)and8smallsubunits(functionunknown);Thebacterialenzymeisadimer(similartothelargesununitsoftheplantenzyme);Itisbothacarboxylaseandaoxygenasesharingthesameactivesite,locatedattheinterfaceofthelargesubunits;O2competeswithCO2attheactivesite;Theenzymehasalowefficiency,thuswithlargeamountsofenzymeneeded(beingthemostabundantenzymeinthebiosphere);Howis3-phosphoglyceratereducedtoglyceraldehyde-3-P(reductionstage)?Essentiallythereversalofthetwostepsofglycolysispathway;3-phosphoglyceratekinaseconvertsitto1,3-bisphophoglycerate(consumingoneATP);Thenreducedtoglyceraldehyde-3-phosphatebyGlyceraldehyde-3-phosphatedehydrogenase,BUTusingNADPH,notNADH.Whatarethealternativefatesofglyceraldehyde-3-phosphate?Canbeconvertedviathegluconeogenesispathwaytoglucose-1-P,whichisthenusedtosynthesizestarchinthestroma;Canbetransportedoutofthechloroplast(usingthePi-triosephosphateantiporter)andthenusedforglycolysisinthecytosoltogenerateATP;Orbeusedforgluconeogenesisinthecytosoltoeventuallygeneratesucrose(fortransportorstorage).Howisribulose-1,5-bisphosphateregeneratedfromtriosephosphates?Foreach6triosephosphates,5areusedforregenerating3moleculesofribulose-1,5-bisphosphate(leavingoneforthealternativefates);Itoccursbyrearrangementofcarbonskeletons,involvingthree-,four-,five-,six-,andseven-carbonsugars;Enzymesinvolvedincludetransketolase,aldolase,bisphosphatase,isomerase,epimerase,andkinase;Thepathwayisessentiallythereversalofthepentosephosphatepathway.WhatistheenergycostforconvertingthreemoleculesofCO2toonemoleculeofglyceraldehyde-3-phosphate?

SixNADPHandnineATP(i.e.,twoNADPHandthreeATPforfixingoneCO2).HowareATPandreducingequivalents(NADPH/NADH)movedoutofthechloroplasts?

Viathedihydroxyacetonephosphate/3-phosphoglyceratecycle.第二節(jié)

蔗糖的合成蔗糖是光合作用的產(chǎn)物，是糖積累貯存的形式，在植物體內(nèi)也以蔗糖形式進行運輸?shù)摹Ｒ?、單糖供體的活化--糖核苷酸的合成:現(xiàn)在發(fā)現(xiàn)的核苷二磷酸葡萄糖主要有UDPG、ADPG、GDPG。

Whatistheprecursorformofglucoseforitspolymerization?

Leloirdiscoveredin1949thatonesugar is transformedtoanothersugarviasugarnucleotide;

Hediscoveredin1959thatglycogenissynthesizedfromUDP-glucose!

Sugarnucleotidesareusedfor transformationandsynthesis!核苷二磷酸葡萄糖在生物體內(nèi)不同聚糖合成時，由不同的核苷二磷酸提供糖基和能量，在植物細胞中用于蔗糖合成的主要是由UDPG；用于淀粉合成為UDPG、ADPG；而合成纖維素的為GDPG和UDPG等。

HowisUDP-glucosesynthesized?

Glucose-6-phosphate(fromglucosephosphorylationorgluconeogenesis)isconvertedtoglucose-1-phosphatebyphosphoglucomutase;UDP-glucoseissynthesizedfromglucose-1-phosphateandUTPbytheactionofUDP-pyrophosphorylase,releasingapyrophosphate,whichisfurtherhydrolyzed.在高等植物中，蔗糖的生物合成問題經(jīng)過長期的爭論，自Leloir等人發(fā)現(xiàn)UDPG以來，已基本清楚，