《北大生化》ppt课件

1、To be lectured by Professor Zengyi Chang on September 19, 2007To be lectured by Professor Zengyi Chang on September 19, 2007lThe anaerobic breakdown (fermentation) of glucose to ethanol and CO2 by yeast has been exploited for many centuries in baking and winemaking.lScientific investigation of the c

2、hemistry of glucose breakdown began in the mid-19th century.lThe complete pathway was described around 1940.NADPH+(fat)Other biomoleculesThe metabolic fates of glucose involves hundreds or thousands of chemical transformations.CO2 + H2O Go = 2840 kJ/moleATPBiosynthesislWhere cell-free fermentation w

3、ith 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 distinguished from the heat-stable, dialyzable coenzymes

4、 (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. (Present in all cells of all organisms)Mg2+ATP2- is the actual substrateAn exergonic group A

5、n exergonic group transferring reactiontransferring reaction己糖激酶己糖激酶Hexokinase exhibits induced Hexokinase exhibits induced fit property: The binding fit property: The binding of glucose in the active siteof glucose in the active sitecauses a major conformationalcauses a major conformationalchange.c

6、hange.Inactive conformationInactive conformationActive conformationActive conformationGlucoseGlucoseSubstrate-induced cleftclosing is a general featureof all kinases!A reversible Isomerization reaction A reversible Isomerization reaction (intramolecular redox reaction)(intramolecular redox reaction)

7、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).Another exergonic groupAnother exergonic grouptransferring reactiontransferring reaction(PFK-1)(PFK-1)磷酸果糖激酶磷酸果糖激酶-1 -1The The “lysislysis step step123456醛缩酶醛缩酶(C3 or C4)(C5 or C2

8、)(C1 or C6)Other hexoses (e.g., Fru.,Man., Gal.) are alsoconverted to glyceraldehyde3-phosphate to enter glycolysis.A reversible Isomerization reaction A reversible Isomerization reaction (intramolecular redox reaction)(intramolecular redox reaction)磷酸丙糖异构酶磷酸丙糖异构酶An aldoseA ketoseOxidation (dehydrog

9、enation) followed by Oxidation (dehydrogenation) 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Tetrameric OxidationOxidationstepstepPhosphorolysisPhosphorolysisstepstep(An acyl-enzyme intermediate)(An acyl-en

10、zyme intermediate)Inactivation of the enzymeInactivation of the enzymeNADH exchanged NADH exchanged for NAD+for NAD+3-3-磷酸甘油醛脱氢酶磷酸甘油醛脱氢酶 HB:BHBHB(This enzyme was named for the reverse reaction)substrate-level phosphorylationsubstrate-level phosphorylation(vs. respiration-linked phosphorylation)(vs.

11、respiration-linked phosphorylation)磷酸甘油酸激酶磷酸甘油酸激酶ADPADPATPATP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.Phosphoglucomutase acts in a similar fashion!(Initially phosphorylated by using 2,3-BPG)A super high-energy A super high-energy phosphate compo

12、undphosphate compoundG0 for the hydrolysis of the phosphate group is changed from 17.6 to 61.9 kJ/mol.烯醇酶烯醇酶A Redistribution of energyA Redistribution of energy(PEP)This enzyme was alsonamed for the reverse reactionNonenzymaticconversion丙酮酸激酶丙酮酸激酶ADPADPATPATPThe second substrate-level ADP The second

13、 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 reactions.reactions. Glucose is activated Glucose is activated (or primed), with (or

14、primed), with 2 ATP molecules 2 ATP molecules invested. invested.primingprimingprimingprimingisomerizationisomerizationcleavagecleavageOnly 5% of thepotential energy of the glucose molecule is releasedduring glycolysis.All the enzymes catalyzing glycolysishave been found in the cytosol. The net prod

15、uction of ATP per glucose inglycolysis is 4-2=2.lWhen proteins are purified from extracts of broken cells in diluted solutions, noncovalent interactions between proteins could be destroyed (i.e., higher level organization destroyed).lKinetic and physical evidences suggest that the enzymes act to cat

16、alyze the ten reactions of glycolysis pathway (as enzymes act in other metabolic pathways) may assemble into multienzyme complexes, where intermediates are directly channeled from one enzyme to another, without entering the aqueous solutions, a phenomenon called “substrate channeling.The glycolyticT

17、he glycolyticenzymes very likelyenzymes very likelyform multienzymeform plexes.SubstrateSubstratechannelingchannelingDilutionDilutiondissociationdissociationPyruvate is reducedto lactate, leading toNAD+ regenerationwhen O2 lacks (thusallowing glycolysis to continue);occurring in very active skeleton

18、 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 ethanoland reduced to form ethanolin some microorganismsin so

19、me 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 BeriberilPolysaccharides suc

20、h 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 phosphorylases).lOligosaccharides (e.g., sucrose, lactose, trehalose etc) are degra

21、ded into monosaccharides before further transformed.lHexoses other than glucose can also be catabolized via the glycolytic pathway after being converted to a phosphorylated derivative.To counter the damagingeffects of oxygen radicalsFor Reductive biosynthesisNucleotidebiosynthesisPPP in rapidlyPPP i

22、n rapidlydividing cellsdividing 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-bisphophate is regenerated alsovia this pathway in the Calvin cycle.GlycolysisTwo othe

23、r ways ofdisplaying the PPPTPP is involved in the C-C cleavage and formation TPP is involved in the C-C cleavage and formation reaction catalyzed by transketolase: transferringreaction catalyzed by transketolase: transferringtwo carbons from a ketose to an aldose. two carbons from a ketose to an ald

24、ose. Transaldolase catalyzes a reaction in which a three-carbonunit is transferred from a ketose to an aldose without being helped by any cofactor.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 al

25、l organisms.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-car

26、bon pyruvates via the ten-step glycolysis pathway pyruvates via the ten-step glycolysis pathway without directly consuming O2 and with a net without directly consuming O2 and with a net production of two ATP molecules by substrate-production of two ATP molecules by substrate-level phosphorylation.level phosphorylation.lLimited amount of energy can be released by Limited amount of energy can be