北大生化课件Chapter14.ppt

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1、Chapter 14 Glycolysis and the Pentose Phosphate PathwayGlycolytic pathway;Fermentation;substrate channeling;Pentose phosphate pathway.,To be lectured by Professor Zengyi Chang on September 19,2007,The chemistry of glucose catabolism was elucidated over about 100 years,The anaerobic breakdown(ferment

2、ation)of glucose to ethanol and CO2 by yeast has been exploited for many centuries in baking and winemaking.Scientific investigation of the chemistry of glucose breakdown began in the mid-19th century.The complete pathway was described around 1940.,D-glucose occupies a central position in metabolism

3、,NADPH+,(fat),Other biomolecules,The metabolic fates of glucose involves hundreds or thousands of chemical transformations.,CO2+H2O,D-glucose,Go=2840 kJ/mole,ATP,Biosynthesis,Understanding sugar catabolism occupied a central position in the development of biochemistry,Where cell-free fermentation wi

4、th yeast extract was first observed by Buchner(1897):the vitalistic dogma was shaken and metabolim became chemistry.Where phosphorylated intermediates were first discovered(Harden and Young,1900s).Heat-labile,nondialyzable enzymes were distinguished from the heat-stable,dialyzable coenzymes(e.g.,NAD

5、+).Muscle enzymes involved in the production of lactic acid were found to be astonishingly similar to the yeast enzymes involved in alcohol production.ATP was revealed to be an energy currency.,?,The whole pathway of glycolysis(arbitrarily defined as from glucose to pyruvate)containing ten steps of

6、chemical reactions with each catalyzed by a specific enzyme,was elucidated by the 1940s.,Glucose is first phosphorylated at C-6:consuming ATP,(Present in all cells of all organisms),Mg2+ATP2-is the actual substrate,An exergonic group transferring reaction,己糖激酶,Glucose is thus trappedand destabilized

7、.,Hexokinase exhibits induced fit property:The binding of glucose in the active sitecauses a major conformationalchange.,Inactive conformation,Active conformation,Glucose,Substrate-induced cleftclosing is a general featureof all kinases!,Glucose 6-P is then converted to fructose 6-P:via isomerizatio

8、n,A reversible Isomerization reaction(intramolecular redox reaction),An aldose,A ketose,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 at C-1:ATP consuming,Another exergonic grouptransferring reaction,(PFK-1),磷酸果糖

9、激酶-1,The six-carbon fructose 1,6-bisphosphate is then cleaved into two different triose phosphates,The“lysis”step,1,2,3,4,5,6,醛缩酶,Interconversion between dihydroxyacetone phosphate and glyceraldehyde 3-P then occurs,(C3 or C4),(C5 or C2),(C1 or C6),Other hexoses(e.g.,Fru.,Man.,Gal.)are alsoconverted

10、 to glyceraldehyde3-phosphate to enter glycolysis.,A reversible Isomerization reaction(intramolecular redox reaction),磷酸丙糖异构酶,An aldose,A ketose,Glyceraldehyde 3-P is then oxidized to form 1,3-bisphosphoglycerate,Oxidation(dehydrogenation)followed by phosphorolysis,An acyl phosphate(a type of anhydr

11、ide),3-磷酸甘油醛脱氢酶,3H,32P,32P,3H,Tetrameric,A Cys residue is involved in the catalysis of glyceraldehyde 3-P dehydrogenase,Oxidationstep,Phosphorolysisstep,(An acyl-enzyme intermediate),Inactivation of the enzyme,NADH exchanged for NAD+,3-磷酸甘油醛脱氢酶,HB,:B,HB,HB,The phosphoryl group linked to the carboxyl

12、 group of 1,3-bisphosphoglycerate is then transferred to ADP to form ATP,(This enzyme was named for the reverse reaction),substrate-level phosphorylation(vs.respiration-linked phosphorylation),磷酸甘油酸激酶,ADP,ATP,3-phosphoglycerate is then converted to 2-phosphoglycerate,磷酸甘油酸变位酶,Mutase:catalyzes group

13、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-phosphoglycerate is then dehydrated to produce phosphoenolpyruvate,A super

14、 high-energy phosphate compound,G0 for the hydrolysis of the phosphate group is changed from 17.6 to 61.9 kJ/mol.,烯醇酶,A Redistribution of energy,(PEP),The phosphoryl group of PEP is transferred to ADP to form ATP at the end,This enzyme was alsonamed for the reverse reaction,Nonenzymaticconversion,丙酮

15、酸激酶,ADP,ATP,The second substrate-level ADP phosphorylation!,Major catabolic fates of pyruvate,Pyruvate also actsas precursors inmany anabolic reactions.,The first stage of the glycolytic pathway:The preparatory phase,Glucose is activated(or primed),with 2 ATP molecules invested.,priming,priming,isom

16、erization,cleavage,The second stage of the glycolytic pathway:The payoff phase,Only 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 inglycolysis is 4-2=2.,The pathw

17、ays of carbons,phosphoryl groups,and electronsfor glycolysis:,Glycolytic enzymes may form multienzyme complexes within cells,When proteins are purified from extracts of broken cells in diluted solutions,noncovalent interactions between proteins could be destroyed(i.e.,higher level organization destr

18、oyed).Kinetic 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 intermediates are directly channeled from one enzyme to another,without entering the aqueous

19、solutions,a phenomenon called“substrate channeling”.,The glycolyticenzymes very likelyform multienzymecomplexes.,Substratechanneling,Dilutiondissociation,Fermentation:pyruvate is converted to lactic acid or ethanol under anaerobic conditions,Pyruvate is reducedto lactate,leading toNAD+regenerationwh

20、en O2 lacks(thusallowing glycolysis to continue);occurring in very active skeleton muscle,some bacteria like lactobacilli,Lactic acid Fermentation:,乳酸脱氢酶,Present only in those alcohol fermentativeOrganisms,Present in many organisms including human,Pyruvate be decarboxylatedand reduced to form ethano

21、lin some microorganisms,Ethanol Fermentation:,丙酮酸脱羧酶,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 Beriberi,Many carbohydrates meet

22、 their catabolic fate in glycolysis,Polysaccharides 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 phosphorylases).Oligosaccharide

23、s(e.g.,sucrose,lactose,trehalose etc)are degraded into monosaccharides before further transformed.Hexoses other than glucose can also be catabolized via the glycolytic pathway after being converted to a phosphorylated derivative.,HOH,OH,a-amylase,Pentose phosphate pathway converts glucose to special

24、ized products needed by cell biosynthesis:NADPH and Ribose 5-phosphate,This is primarily an anabolic pathway.,To counter the damagingeffects of oxygen radicals,For Reductive biosynthesis,Some biological roles served by PPP,Nucleotidebiosynthesis,PPP in rapidlydividing cells,GlucoseGlycogen,Cyclic es

25、ter,The Non-oxidative phase of PPP,Isomerization&epimerization of Ru5P,Carbon-carbon bond cleavage and formation,TPP,TPP,转羟乙醛酶,转二羟丙酮基酶,Ribulose 1,5-bisphophate is regenerated alsovia this pathway in the Calvin cycle.,Glycolysis,Two other ways ofdisplaying the PPP,TPP is involved in the C-C cleavage

26、and formation reaction catalyzed by transketolase:transferringtwo carbons from a ketose to an aldose.,Transaldolase catalyzes a reaction in which a three-carbonunit is transferred from a ketose to an aldose without being helped by any cofactor.,G-6-P is partitioned between glycolysis and PPP,Summary

27、(1),D-glucose is a commonly used fuel and versatile precursor in almost all organisms.The study of glucose degradation has a rich history in biochemistry(especially for enzymology).Glucose is first converted into two three-carbon pyruvates via the ten-step glycolysis pathway without directly consumi

28、ng O2 and with a net production of two ATP molecules by substrate-level phosphorylation.Limited amount of energy can be released by oxidizing glucose under anaerobic conditions by fermentation.,The enzymes participating glycolysis may form multiple enzyme complexes,where substrate is channeled from one enzyme to another.The sugar units on glycogen is converted to glucose 1-phosphate via phosphorolysis,which is catalyzed by glycogen phosphorylase.Glucose 6-phosphate can also be oxidized to form ribose 5-phosphate and NADPH via the pentose phosphate pathway.,Summary(2),