北大生化课件Chapter14.ppt

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1、北大生化课件Chapter14 Still waters run deep.流静水深流静水深,人静心深人静心深 Where there is life,there is hope。有生命必有希望。有生命必有希望The chemistry of glucose catabolism was elucidated over about 100 yearslThe anaerobic breakdown(fermentation)of glucose to ethanol and CO2 by yeast has been exploited for many centuries in baking

2、 and winemaking.lScientific investigation of the chemistry of glucose breakdown began in the mid-19th century.lThe complete pathway was described around 1940.D-glucose D-glucose occupies a central occupies a central position in metabolismposition in metabolismNADPH+(fat)Other biomoleculesThe metabol

3、ic fates of glucose involves hundreds or thousands of chemical transformations.CO2+H2OD-glucoseD-glucose Go=2840 kJ/moleATPBiosynthesisUnderstanding sugar catabolism Understanding sugar catabolism occupied a central position in the occupied a central position in the development of biochemistrydevelo

4、pment of biochemistrylWhere cell-free fermentation with yeast extract was first observed by Buchner(1897):the vitalistic dogma was shaken and metabolim became chemistry.lWhere phosphorylated intermediates were first discovered(Harden and Young,1900s).lHeat-labile,nondialyzable enzymes were distingui

5、shed from the heat-stable,dialyzable coenzymes(e.g.,NAD+).lMuscle enzymes involved in the production of lactic acid were found to be astonishingly similar to the yeast enzymes involved in alcohol production.l ATP was revealed to be an energy currency.?The whole pathway ofThe whole pathway of glycoly

6、sis glycolysis(arbitrarily defined as(arbitrarily defined as from from glucose to pyruvateglucose to pyruvate)containing)containing ten stepsten steps of chemical reactions of chemical reactions with each catalyzed by a specific with each catalyzed by a specific enzyme,was elucidated by the enzyme,w

7、as elucidated by the 1940s.1940s.Glucose is first phosphorylated Glucose is first phosphorylated at C-6:consuming ATPat C-6:consuming ATP(Present in all cells of all organisms)Mg2+ATP2-is the actual substrateAn exergonic group An exergonic group transferring reactiontransferring reaction己糖激酶己糖激酶Gluc

8、ose is thus trappedGlucose is thus trappedand destabilized.and destabilized.Hexokinase Hexokinase exhibits exhibits induced induced fitfit property:The binding property:The binding of of glucoseglucose in the active site in the active sitecauses a major conformationalcauses a major conformationalcha

9、nge.change.Inactive conformationInactive conformationActive conformationActive conformationGlucoseGlucoseSubstrate-induced cleftclosing is a general featureof all kinases!Glucose 6-P is then converted to Glucose 6-P is then converted to fructose 6-P:via isomerizationfructose 6-P:via isomerization A

10、reversible Isomerization reaction A reversible Isomerization reaction(intramolecular redox reaction)An aldoseA ketose磷酸己糖异构酶磷酸己糖异构酶A necessary prelude for the next two steps ofreactions(phosphorylation and C-C cleavage).Fructose 6-P is then phosphorylated Fructose 6-P is then phosphorylated at C-1:A

11、TP consumingat C-1:ATP consumingAnother exergonic groupAnother exergonic grouptransferring reactiontransferring reaction(PFK-1)(PFK-1)磷酸果糖激酶磷酸果糖激酶-1-1The six-carbon fructose 1,6-bisphosphate is The six-carbon fructose 1,6-bisphosphate is then cleaved into two different triose then cleaved into two d

12、ifferent triose phosphatesphosphatesThe“lysis”stepThe“lysis”step123456醛缩酶醛缩酶Interconversion between dihydroxyacetone Interconversion between dihydroxyacetone phosphate and glyceraldehyde 3-P then phosphate and glyceraldehyde 3-P then occursoccurs(C3 or C4)(C5 or C2)(C1 or C6)Other hexoses(e.g.,Fru.,

13、Man.,Gal.)are alsoconverted to glyceraldehyde3-phosphate to enter glycolysis.A reversible Isomerization reactionA reversible Isomerization reaction (intramolecular redox reaction)磷酸丙糖异构酶磷酸丙糖异构酶An aldoseA ketoseGlyceraldehyde 3-P is then oxidized Glyceraldehyde 3-P is then oxidized to form 1,3-bispho

14、sphoglycerateto form 1,3-bisphosphoglycerateOxidation(dehydrogenation)Oxidation(dehydrogenation)followed by followed by phosphorolysis phosphorolysis An acyl phosphateAn acyl phosphate(a type of anhydride)(a type of anhydride)3-3-磷酸甘油醛脱氢酶磷酸甘油醛脱氢酶3 3HH3232P P3232P P3 3HHTetrameric A Cys residue is in

15、volved in the catalysis of A Cys residue is involved in the catalysis of glyceraldehyde 3-P dehydrogenaseglyceraldehyde 3-P dehydrogenaseOxidationOxidationstepstepPhosphorolysisPhosphorolysisstepstep(An acyl-enzyme intermediateAn acyl-enzyme intermediate)Inactivation of the enzymeInactivation of the

16、 enzymeNADH exchanged NADH exchanged for NADfor NAD+3-3-磷酸甘油醛脱氢酶磷酸甘油醛脱氢酶 HB:BHBHBThe phosphoryl group linked to the carboxyl The phosphoryl group linked to the carboxyl group of 1,3-bisphosphoglycerate is group of 1,3-bisphosphoglycerate is then transferred to ADP to form then transferred to ADP to

17、form ATPATP(This enzyme was named for the reverse reaction)substrate-level phosphorylationsubstrate-level phosphorylation(vs.respiration-linked phosphorylationvs.respiration-linked phosphorylation)磷酸甘油酸激酶磷酸甘油酸激酶ADPADPATPATP3-phosphoglycerate is then 3-phosphoglycerate is then converted to 2-phosphog

18、lycerateconverted to 2-phosphoglycerate磷酸甘油酸变位酶磷酸甘油酸变位酶Mutase:catalyzes group transferring from one position to another in a molecule.The phosphoglycerate mutase acts as a transient carrier of phosphoryl groups Phosphoglucomutase acts in a similar fashion!(Initially phosphorylated by using 2,3-BPG)2

19、-phosphoglycerate is then 2-phosphoglycerate is then dehydrated to produce dehydrated to produce phosphoenolpyruvatephosphoenolpyruvateA super high-energy A super high-energy phosphate compoundphosphate compoundDG0 for the hydrolysis of the phosphate group is changed from 17.6 to 61.9 kJ/mol.烯醇酶烯醇酶A

20、 Redistribution of energyA Redistribution of energy(PEP)The phosphoryl group of PEP is The phosphoryl group of PEP is transferred to ADP to form transferred to ADP to form ATP ATP at the endat the endThis enzyme was alsonamed for the reverse reactionNonenzymaticconversion丙酮酸激酶丙酮酸激酶ADPADPATPATPThe se

21、cond substrate-level ADP The second substrate-level ADP phosphorylation!phosphorylation!Major catabolic Major catabolic fates of pyruvatefates of pyruvatePyruvate also actsPyruvate also actsas precursors inas precursors inmany anabolic many anabolic reactionsreactions.The first stage of the glycolyt

22、ic pathway:The first stage of the glycolytic pathway:The The preparatory phasepreparatory phase Glucose is activated Glucose is activated (or primed),with (or primed),with 2 ATP molecules 2 ATP molecules invested.invested.primingprimingprimingprimingisomerizationisomerizationcleavagecleavageThe seco

23、nd stage of the glycolytic pathway:The second stage of the glycolytic pathway:The payoff phaseThe payoff phaseOnly 5%of thepotential energy of the glucose molecule is releasedduring glycolysis.All the enzymes catalyzing glycolysishave been found in the cytosol.The net production of ATP per glucose i

24、nglycolysis is 4-2=2.The pathways of carbons,phosphoryl groups,and electronsfor glycolysis:Glycolytic enzymes may form Glycolytic enzymes may form multienzyme complexesmultienzyme complexes within cells within cellslWhen proteins are purified from extracts of broken cells in diluted solutions,noncov

25、alent interactions between proteins could be destroyed(i.e.,higher level organization destroyed).lKinetic and physical evidences suggest that the enzymes act to catalyze the ten reactions of glycolysis pathway(as enzymes act in other metabolic pathways)may assemble into multienzyme complexes,where i

26、ntermediates are directly channeled from one enzyme to another,without entering the aqueous solutions,a phenomenon called“substrate channeling”.The glycolyticThe glycolyticenzymes very likelyenzymes very likelyform multienzymeform plexes.SubstrateSubstratechannelingchannelingDilutionDilutiondissocia

27、tiondissociationFermentationFermentation:pyruvatepyruvate is converted to is converted to lactic acidlactic acid or or ethanolethanol under under anaerobic conditionsanaerobic conditionsPyruvate is reducedto lactate,leading toNAD+regenerationwhen O2 lacks(thusallowing glycolysis to continue);occurri

28、ng in very active skeleton muscle,some bacteria like lactobacilliLactic acid Fermentation:乳酸脱氢酶乳酸脱氢酶Present only in those alcohol fermentativeOrganismsPresent in many organisms including humanPyruvate be decarboxylatedPyruvate be decarboxylatedand reduced to form and reduced to form ethanolethanolin

29、 some microorganismsin some microorganismsEthanol Fermentation:丙酮酸脱羧酶丙酮酸脱羧酶乙醇脱氢酶乙醇脱氢酶Thiamine pyrophosphate(TPP)is involved in the cleavage of C-C bondsadjacent to a carbonylgroup.Vitamin B1(thiamine)is used to synthesize TPP.A lack of Vitamin B1 in humans will result in a condition known as Beriber

30、iMany carbohydrates meet their catabolic fate in glycolysislPolysaccharides such as starch and glycogen are first degraded into glucoses via hydrolysis in extracellular spaces(catalyzed by a-amylases and other enzymes),but into glucose 1-phosphate via phosphorolysis inside cells(catalyzed by phospho

31、rylases).lOligosaccharides(e.g.,sucrose,lactose,trehalose etc)are degraded into monosaccharides before further transformed.lHexoses other than glucose can also be catabolized via the glycolytic pathway after being converted to a phosphorylated derivative.HOHOHOHa a-amylasePentose phosphate Pentose p

32、hosphate pathwaypathway converts glucose converts glucose to specialized products to specialized products needed by cell needed by cell biosynthesis:biosynthesis:NADPHNADPH and and Ribose 5-phosphateRibose 5-phosphateThis is primarily an anabolic pathway.To counter the damagingeffects of oxygen radi

33、calsFor Reductive biosynthesisSome biological roles served by PPPNucleotidebiosynthesisPPP in rapidlydividing cellsGlucoseGlycogenCyclic esterThe Non-oxidative phase of PPPIsomerization&epimerization of Ru5PCarbon-carbon bond cleavage and formationTPPTPP转羟乙醛酶转羟乙醛酶转二羟丙酮基酶转二羟丙酮基酶Ribulose 1,5-bisphopha

34、te is regenerated alsovia this pathway in the Calvin cycle.GlycolysisTwo other ways ofdisplaying the PPPTPP is involved in the C-C cleavage and formation reaction catalyzed by transketolase:transferringtwo carbons from a ketose to an aldose.Transaldolase catalyzes a reaction in which a three-carbonu

35、nit is transferred from a ketose to an aldose without being helped by any cofactor.G-6-P is partitioned between glycolysis and PPPSummary(1)lD-glucose is a commonly used fuel and versatile D-glucose is a commonly used fuel and versatile precursor in almost all organisms.precursor in almost all organ

36、isms.lThe study of glucose degradation has a rich The study of glucose degradation has a rich history in biochemistry(especially for history in biochemistry(especially for enzymology).enzymology).lGlucose is first converted into two three-carbon Glucose is first converted into two three-carbon pyruv

37、ates via the ten-step glycolysis pathway pyruvates via the ten-step glycolysis pathway without directly consuming Owithout directly consuming O2 2 and with a net and with a net production of two ATP molecules by substrate-production of two ATP molecules by substrate-level phosphorylation.level phosp

38、horylation.lLimited amount of energy can be released by Limited amount of energy can be released by oxidizing glucose under anaerobic conditions by oxidizing glucose under anaerobic conditions by fermentation.fermentation.lThe enzymes participating glycolysis may form The enzymes participating glyco

39、lysis may form multiple enzyme complexes,where substrate is multiple enzyme complexes,where substrate is channeled from one enzyme to another.channeled from one enzyme to another.lThe sugar units on glycogen is converted to The sugar units on glycogen is converted to glucose 1-phosphate via phosphor

40、olysis,which is glucose 1-phosphate via phosphorolysis,which is catalyzed by glycogen phosphorylase.catalyzed by glycogen phosphorylase.lGlucose 6-phosphate can also be oxidized to Glucose 6-phosphate can also be oxidized to form ribose 5-phosphate and NADPH via the form ribose 5-phosphate and NADPH via the pentose phosphate pathway.pentose phosphate pathway.Summary(2)