清華大學(xué)生物化學(xué)課件糖酵解

1、Chapter 14 Glycolysis, the pentose phosphate pathway and the catabolism of glycogenGlycolysis (糖酵解糖酵解)The process in which a molecule of glucose is degraded in a series of enzyme-catalyzed reactions to yield two molecules of pyruvate.Overview on glucose metabolism The major fuel of most organisms an

2、d occupies a central position in metabolism ( Go= -2840 kJ/mol when completely oxidized). Can be stored in polymer form (glycogen or starch) or be converted to fat for long term storage. Can also be oxidized to make NADPH and ribose 5-phosphate via the pentose phosphate pathway. Is also a versatile

3、precursor for carbon skeletons of almost all kinds of biomolecules.Major pathways of glucose utilization Glycolysis The first stage in the complete oxidation of glucose An universal central pathway of glucose metabolism The chemistry of the reaction sequence completely conserved during evolution The

4、 first metabolic pathway to be elucidated and probably the best understood1. The Development of Biochemistry and the Delineation of Glycolysis Went Hand by Hand In 1897, accidental observation by Eduard Buchner: sucrose (as a preservative) was rapidly fermented into alcohol by cell-free yeast extrac

5、t. The accepted view that fermentation is inextricably tied to living cells (i.e., the vitalistic dogma) was shaken and Biochemistry was born: Metabolism became chemistry! In 1900s, Arthur Harden and William Young (Great Britain) found that Pi is needed for yeast juice to ferment glucose, a hexose d

6、iphosphate (fructose 1,6-bisphosphate) was isolated. They also separated the yeast juice into two fractions: one heat-labile, nondialyzable zymase (enzymes) and the other heat-stable, dialyzable cozymase (metal ions, ATP, ADP, NAD+). 1910s-1930s, Gustav Embden and Otto Meyerhof (Germany), studied mu

7、scle and its extracts: Reconstructed all the transformation steps from glycogen to lactic acid in vitro; revealed that many reactions of lactic acid (muscle) and alcohol (yeast) fermentations were the same! Discovered that lactic acid is reconverted to carbohydrate in the presence of O2 (gluconeogen

8、esis); observed that some phosphorylated compounds are energy-rich. The whole pathway of glycolysis (from glucose to pyruvate) was elucidated by the 1940s.Basic facts about glycolysis Ten steps of reactions are involved in the pathway. Six types of reactions occur: group transfer, isomerization, ald

9、ol cleavage, dehydrogenation, group shift, and dehydration. All the enzymes are found in the cytosol. All intermediates are phosphorylated. Only a small fraction (5.2%) of the potential energy of the glucose molecule is released and much still remains in the final product of glycolysis, pyruvate.2.

10、The overall glycolytic pathway can be divided into two phases Preparatory phase: Glucose is converted into glyceraldehyde 3-phosphate, consuming ATP. Payoff phase: Glyceraldehyde 3-phosphate is oxidized to generate pyruvate, generating ATP and NADH.Group transferIsomerizationGroup transferAldol clea

11、vageIsomerizationIsomerizationDehydrogenationGroup transferGroup shiftDehydrationGroup transferIrreversible in cellsMgATP2-, not ATP4-, is the actual substratePreparatory Phase Step 1 IrreversibleexergonicAldoseKetoseReversiblePreparatory Phase Step 2 ReversiblePreparatory Phase Step 3 The commitmen

12、t step (PFK1)IrreversibleexergonicPreparatory Phase Step 4 The “l(fā)ysis” stepKetoseAldoseKetoseAldoseStep 5 Preparatory Phase ReversibleKetoseGroup transferIsomerizationGroup transferAldol cleavageIsomerizationTwo molecules of ATP are consumedStep 6 Oxidation and phosphorylation reactionPayoff Phase A

13、cyl phosphatePayoff Phase Step 7 Substrate-level phosphorylationfor ATP generationPayoff Phase Step 82,3-bisphosphoglycerate is both a coenzyme and an intermediate of the reactionPayoff Phase Step 9ReversiblePayoff Phase Step 10Substrate-level phosphorylationfor ATP generationSpontaneousIsomerizatio

14、nDehydrogenationGroup transferGroup shiftDehydrationGroup transferFour molecules of ATP and two molecules of NADH are generatedThe chemical logic of the glycolytic pathway A net gain of two ATP, two NADH, two molecules of pyruvate are resulted when a glucose molecule is oxidized via the glycolysis p

15、athway: Glucose + 2 ADP + 2Pi + 2NAD+ 2 pyruvate + 2ATP + 2H2O + 2NADH + 2H+ Go = -85 kJ/molGlucoseGlucose 6-phosphateFructose 1,6-bisphosphate + Glyceraldehyde 3-phosphateDihydroxyacetone phosphateGlyceraldehyde 3-phosphate (2)1,3-Bisphosphoglycerate (2)3-Phosphoglycerate (2)2-Phosphoglycerate (2)P

16、hosphoenolpyruvate (2)Pyruvate (2)hexokinasephosphofructokinase-1phosphohexose isomerasealdolasetriose phosphate isomeraseglyceraldehyde 3-phosphate dehydrogenaseenolasephosphoglycerate mutasepyruvate kinaseATP2NADHADPATPADP2ATP2ADP2Pi2NAD+2ATP+ H+2ADPFructose 6-phosphatephosphoglycerate kinaseImpor

17、tance of phosphorylated intermediates Negatively charged, cant diffuse out of the cell, therefore, no energy is needed to retain them in the cell Energy conserved in the phosphorylated compounds Lower the activation energy and increase specificity of the enzymatic reactionsFates of pyruvateServe as

18、a precursor in anabolic reactions3. Fermentation: pyruvate is converted to lactic acid or ethanol under anaerobic conditions This occurs to regenerate NAD+ for the glycolysis pathway to continue when O2 lacks. Lactic acid fermentation: pyruvate is reduced to lactate by NADH, catalyzed by lactate deh

19、ydrogenase. The lactate produced in muscle can be converted back to glucose by gluconeogenesis in the liver of vertebrates (via the Cori cycle).Pyruvate is reduced to lactate when O2 lacks in a reaction catalyzed by lactate dehydrogenaseNamed for the reverse reactionThe Cori cycle Ethanol fermentati

20、on (occurring in yeast and other microorganisms): pyruvate is first decarboxylated and then reduced by NADH, catalyzed by pyruvate decarboxylase and alcohol dehydrogenase respectively. Thiamine pyrophosphate (TPP, 硫胺焦磷酸硫胺焦磷酸, derived from vitamin B1) act as the coenzyme of the decarboxylase.Pyruvate

21、 can be reduced to ethanol in some microorganismsPresent only inthose alcohol fermentative organismsPresent in many organisms, including humanIndustrial-scale fermentation4. Other hexoses are also oxidized via the glycolysis pathway They are also first primed by phosphorylation (at C-1 or C-6). Fruc

22、tose is primed and cleaved to form dihydroxyacetone phosphate and glyceraldehyde, which are further converted to glyceraldehyde 3-phosphate. Galactose is first converted to Glucose-1-phosphate via a UDP-galactose intermediate and UDP-glucose intermediate, then to Glucose-6-phosphate.in liverin muscl

23、e and kidneyGalactose is convertedto glucose-1-phosphatevia a UDP-galactose intermediate Several human genetic diseases result in disordered galactose metabolism (galactosemia)5. Dietary poly- and disaccharides are hydrolyzed to monosaccharides in the digestive system Salivary a a-amylase (a a-淀粉酶淀粉

24、酶) in the mouth hydrolyzes starch (glycogen) into short polysaccharides or oligosaccharides. Pancreatic a a-amylase (active at low pH) continue to convert the saccharides to mainly maltose and dextrin (from amylopectin, 枝鏈淀粉枝鏈淀粉). Specific enzymes on the microvilli of the intestinal epithelial cells

25、 finally hydrolyze all disaccharides into monosaccharides. The monosaccharides are then absorbed at the intestinal microvilli and transported to various tissues for oxidative degradation via the glycolytic pathway. Adults lacking lactase will have lactose intolerance syndrome: the lactose is convert

26、ed to toxic compounds in the large intestine by the bacteria there, causing abdominal cramps and diarrhea. Lactose + H2O D-galactose + D-glucoselactase6. Glycogen and starch are degraded by phosphorolysis The glucose unit at the nonreducing terminal of glycogen is removed as glucose 1-phosphate via

27、phosphorolysis catalyzed by glycogen phosphorylase. Glucose 1-phosphate is then converted to glucose 6-phosphate by phosphoglucomutase. No ATP needed!Glycogen breakdown by glycogen phosphorylaseA bifunctional debranching enzyme aids the phosphorylase in degrading glycogen(Figure 15-26)7. Pentose pho

28、sphate pathway converts glucose to specialized products needed by the cells Glucose 6-phosphate + 2NADP+ + H2O ribose 5-phosphate +2NADPH + CO2 + 2H+Oxidative reactionsof the pentosephosphate pathway Prominent in tissues actively synthesizing fatty acids and steroids, such as mammary gland, adrenal

29、cortex, liver and adipose tissues. Pentose generated is necessary for biosynthesis of nucleic acids. If not needed, six five-carbon sugar phosphates are converted to five six-carbon sugar phosphates. The reverse of this rearrangement, regeneration of six five-carbon sugar phosphate from five six-car

30、bon sugar phosphate occurs in the Calvin cycle for photosynthetic fixation of CO2 in plants (To be discussed in Chapter 20).General scheme of the pentose phosphate pathwayThe regeneration of six-carbon glucose-6-phosphate from five-carbon ribose-5-phosphate in the pentose phosphate pathway (nonoxida

31、tive)Role of NADPH in regulation8. Regulation of glycolysis Regulatory enzymes of glycolysis:HexokinasePhosphofructokinase-1 (PFK-1)Pyruvate kinaseThey all catalyze exergonic and irreversible reactions, and are all regulated by allosteric effectors.Factors that determine the activity of an enzymeFac

32、tors that determine the activity of an enzymeAllosteric regulation Protein phosphorylation and dephosphorylationThe rate of glycolysis in mammals is mainly controlled at the step acted by phosphofructokinase-1 (PFK-1) PFK-1 catalyzes an irreversible and exergonic reaction, which commits glucose to t

33、he glycolysis pathway (away from the pentose phosphate pathway). PFK-1 is a complex tetrameric enzyme regulated by multiple intracellular signals (allosteric effectors): ATP, citrate being negative ones; AMP, ADP and fructose 2,6-bisphosphate as positive ones.E. coli PFK-1Allosteric regulation of mu

34、scle PFK-1 by ATPRegulators that affect PFK-1 activity Hexokinase and pyruvate kinase also set the pace of glycolysis These two enzymes also catalyze irreversible and exergonic reactions. Pyruvate kinase is allosterically inhibited by ATP, acetyl-CoA, and long-chain fatty acids. The catalytic activi

35、ty of the liver pyruvate kinase isozyme (the L type) is also controlled by reversible phosphorylation. Hexokinase isozymes are regulated differently.Regulation of pyruvate kinase Different roles of muscle and liver in glucose metabolism Muscle consumes glucose, using it for energy production. Liver

36、produces and distributes glucose for other tissues, maintaining a constant blood glucose level. Liver and muscle hexokinase isozymes are regulated differently Muscle hexokinase is allosterically inhibited by its reaction product glucose 6-phosphate, which accumulates when PFK-1 is inhibited. Liver h

37、exokinase (also called hexokinase IV or glucokinase) has about 100 X less affinity for glucose than that in muscle, therefore, is regulated by the level of blood glucose. Liver hexokinase is not inhibited by glucose 6-phosphate: its main role is to convert excess glucose to glucose-6-phosphate for g

38、lycogen synthesis.Comparison of kinetic properties of liver Hexokinase IV and muscle Hexokinase I(Liver)(Muscle)Glycogen phosphorylase isozymes Two isozymes exist : one in liver and one in muscle Both are in two interconvertible forms: the a form is phosphorylated and more active; the b form is deph

39、osphorylated and less active. Both phosphorylation and dephosphorylation occur and catalyzed by specific phosphorylase b kinase and phosphorylase a phosphatase respectively.Covalent modification of glycogen phosphorylaseGlycogen phosphorylase kinase is regulated by different hormones in muscle and i

40、n liver In muscle, by epinephrine (腎上腺素腎上腺素) -secreted by the medulla of the adrenal gland (腎上腺髓質(zhì)腎上腺髓質(zhì)) In liver, by glucagon (胰增血糖素胰增血糖素) -secreted by the pancreasCascade mechanism ofepinephrine and glucagon actionGlycogen phosphorylase is also regulated allosterically, but differently in muscle an

41、d liverIn muscle: Ca2+ binds and activates phosphorylase b kinase. AMP binds and activates phosphorylase b. ATP inactivates phosphorylase.In liver: glucose binds to the a form of enzyme, exposing the phosphorylated Ser residues to the action of phosphorylase a phosphatase and converting it to the le

42、ss active b form.The liver glycogen phosphorylase a is negatively regulated by glucose, therefore,can function as a glucose sensorGlycolysis and gluconeogenesis are coordinately regulated to avoid the wasteful “futile cycling” Gluconeogenesis: The pathway converting simple precursors (e.g., pyruvate

43、 and lactate) to glucose, mainly occurring in the liver of mammals. Gluconeogenesis uses most of the same enzymes of glycolysis, but the three exergonic irreversible reactions (catalyzed by the three regulatory enzymes) are detoured (bypassed).Bypass reaction The unique enzymes catalyzing the two re

44、versing reactions at one detouring step are reciprocally regulated by common allosteric effectors: fructose 2,6-bisphosphate activates PFK-1 (thus activate glycolysis) and at the same time inhibits fructose 1,6-bisphosphatase 1 or FBPase-1 (thus inhibit gluconeogenesis). Enzymes catalyzing the non-common steps of paired catabolic and anabolic pathways are often reciprocally regulated to avoid futile cycling.Keywords Glycolysis-3 irreversible reactions Pentose phosphate pathway-products PFK-1-commitment step, regulation Glycogen phosphorylase-regulation Words of the week aerobic (有氧的有氧的)