B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 34
Текст из файла (страница 34)
Again, this pathwayregenerates NAD+ from NADH, as requiredto enable glycolysis to continue. Both (A)and (B) are examples of fermentations.(A) FERMENTATION LEADING TO EXCRETION OF LACTATEglucoseNAD+glycolysisADPATPNADH + HO–ONAD+O–O+NADregenerationCC+CHOCH3COHCH3lactatepyruvate(B) FERMENTATION LEADING TO EXCRETION OF ETHANOL AND CO2glucoseNAD+glycolysisADPATPNADH + HO–O+H+HCOCH3pyruvateNAD+NADregenerationCC+OCH3acetaldehydeH2COHCH3ethanolCO2fermentations. Studies of the commercially important fermentations carried outby yeasts inspired much of early biochemistry. Work in the nineteenth centuryled in 1896 to the then startling recognition that these processes could be studiedoutside living organisms, in cell extracts.
This revolutionary discovery eventuallymade it possible to dissect out and study each of the individual reactions in thefermentation process. The piecing together of the complete glycolytic pathway inthe 1930s was a major triumph of biochemistry, and it was quickly followed by therecognition of the central role of ATP in cell processes.Glycolysis Illustrates How Enzymes Couple Oxidation to EnergyStorageThe formation of ATP during glycolysis provides a particularly clear demonstration of how enzymes couple energetically unfavorable reactions with favorableones, thereby driving the many chemical reactions that make life possible. Twocentral reactions in glycolysis (steps 6 and 7) convert the three-carbon sugar intermediate glyceraldehyde 3-phosphate (an aldehyde) into 3-phosphoglycerate (acarboxylic acid; see Panel 2–8, pp. 104–105), thus oxidizing an aldehyde group to acarboxylic acid group.
The overall reactionreleases enough free energy to convertMBoC6 m2.71/2.47a molecule of ADP to ATP and to transfer two electrons (and a proton) from thealdehyde to NAD+ to form NADH, while still liberating enough heat to the environment to make the overall reaction energetically favorable (∆G° for the overallreaction is –12.5 kJ/mole).Figure 2–48 outlines this remarkable feat of energy harvesting. The chemicalreactions are precisely guided by two enzymes to which the sugar intermediatesHOW CELLS OBTAIN ENERGY FROM FOOD(A)STEPS 6 AND 7 OF GYCOLYSISHOCHCglyceraldehyde3-phosphateOHCH2OHSPA short-lived covalent bond isformed between glyceraldehyde3-phosphate and the –SH group ofa cysteine side chain of the enzymeglyceraldehyde 3-phosphatedehydrogenase. The enzyme alsobinds noncovalently to NAD+.ENZYMEENZYMESHCOHHCOHCH2OSTEP 6glyceraldehyde 3-phosphate dehydrogenaseNAD++high-energythioester bondSENZYMEGlyceraldehyde 3-phosphate isoxidized as the enzyme removes ahydrogen atom (yellow) andtransfers it, along with an electron,to NAD+, forming NADH (seeFigure 2–37).
Part of the energyreleased by the oxidation of thealdehyde is thus stored in NADH,and part is stored in the highenergy thioester bond that linksglyceraldehyde 3-phosphate to theenzyme.PNADH + HHCOCOHCH2Ohigh-energyphosphatebondPA molecule of inorganic phosphatedisplaces the high-energy thioesterbond to create 1,3-bisphosphoglycerate, which contains ahigh-energy phosphate bond.inorganicphosphatePiOPHCOCOH1,3-bisphosphoglycerateCH2OPP APSTEP 7phosphoglycerate kinase77PHOADPP APThe high-energy phosphate groupis transferred to ADP to form ATP.ATPOCHCOHCH2O(B)3-phosphoglyceratePSUMMARY OF STEPS 6 AND 7HOCHONADHaldehydeOCcarboxylicacidATPThe oxidation of an aldehyde to acarboxylic acid releases energy,much of which is captured in theactivated carriers ATP and NADH.Figure 2–48 Energy storage in steps6 and 7 of glycolysis.
(A) In step 6, theenzyme glyceraldehyde 3-phosphatedehydrogenase couples the energeticallyfavorable oxidation of an aldehyde tothe energetically unfavorable formationof a high-energy phosphate bond. Atthe same time, it enables energy to bestored in NADH. The formation of thehigh-energy phosphate bond is driven bythe oxidation reaction, and the enzymethereby acts like the “paddle wheel”coupler in Figure 2–32B. In step 7, thenewly formed high-energy phosphate bondin 1,3-bisphosphoglycerate is transferredto ADP, forming a molecule of ATP andleaving a free carboxylic acid group on theoxidized sugar. The part of the moleculethat undergoes a change is shaded inblue; the rest of the molecule remainsunchanged throughout all these reactions.(B) Summary of the overall chemicalchange produced by reactions 6 and 7.78Chapter 2: Cell Chemistry and Bioenergeticsare tightly bound.
As detailed in Figure 2–48, the first enzyme (glyceraldehyde3-phosphate dehydrogenase) forms a short-lived covalent bond to the aldehydethrough a reactive –SH group on the enzyme, and catalyzes its oxidation by NAD+in this attached state. The reactive enzyme–substrate bond is then displaced byan inorganic phosphate ion to produce a high-energy phosphate intermediate,which is released from the enzyme. This intermediate binds to the second enzyme(phosphoglycerate kinase), which catalyzes the energetically favorable transfer ofthe high-energy phosphate just created to ADP, forming ATP and completing theprocess of oxidizing an aldehyde to a carboxylic acid.
Note that the C–H bond oxidation energy in step 6 drives the formation of both NADH and a high-energyphosphate bond. The breakage of the high-energy bond then drives ATP formation.We have shown this particular oxidation process in some detail because it provides a clear example of enzyme-mediated energy storage through coupled reactions (Figure 2–49).
Steps 6 and 7 are the only reactions in glycolysis that create ahigh-energy phosphate linkage directly from inorganic phosphate. As such, theyaccount for the net yield of two ATP molecules and two NADH molecules per molecule of glucose (see Panel 2–8, pp. 104–105).As we have just seen, ATP can be formed readily from ADP when a reactionintermediate is formed with a phosphate bond of higher energy than the terminalphosphate bond in ATP.
Phosphate bonds can be ordered in energy by comparingthe standard free-energy change (∆G°) for the breakage of each bond by hydrolysis. Figure 2–50 compares the high-energy phosphoanhydride bonds in ATP withthe energy of some other phosphate bonds, several of which are generated duringglycolysis.Organisms Store Food Molecules in Special ReservoirsAll organisms need to maintain a high ATP/ADP ratio to maintain biological orderin their cells. Yet animals have only periodic access to food, and plants need tosurvive overnight without sunlight, when they are unable to produce sugar fromphotosynthesis. For this reason, both plants and animals convert sugars and fatsto special forms for storage (Figure 2–51).To compensate for long periods of fasting, animals store fatty acids as fatdroplets composed of water-insoluble triacylglycerols (also called triglycerides).The triacylglycerols in animals are mostly stored in the cytoplasm of specializedfat cells called adipocytes.
For shorter-term storage, sugar is stored as glucosePOOCP1,3-bisphosphoglycerateOOCATPNADHfree energyformation ofhigh-energy bondhydrolysis ofhigh-energy bondADPNAD+HOCglyceraldehyde3-phosphateHO3-phosphoglycerateOCC–H bondoxidationSTEP 6STEP 7TOTAL ENERGY CHANGE for step 6 followed by step 7 is a favorable –12.5 kJ/moleFigure 2–49 Schematic view of thecoupled reactions that form NADH andATP in steps 6 and 7 of glycolysis. TheC–H bond oxidation energy drives theformation of both NADH and a high-energyphosphate bond. The breakage of the highenergy bond then drives ATP formation.HOW CELLS OBTAIN ENERGY FROM FOOD–OOOCH 2CCOOCCOC–OCHNCCH3PO–O–+NH2HOphosphoenolpyruvate(see Panel 2–8, pp. 104–105)–61.9 kJfor example,1,3-bisphosphoglycerate(see Panel 2–8)–49.0 kJONPHO–O–creatine phosphate(activated carrier thatstores energy in muscle)–43.0 kJfor example,ATP when hydrolyzedto ADP–30.6 kJH2Oanhydridebond tophosphate(phosphoanhydridebond)OCOPOOO–POOO–PO––20OHCCH–40O–H2Ophosphoesterbond–60OH2Ophosphatebond increatinephosphateO–O–H2Oanhydridebond to carbonPΔGo FOR HYDROLYSISenol phosphatebond79OPO–O–for example,glucose 6-phosphate(see Panel 2–8)–17.5 kJH2Otype of phosphate bondspecific examples showing thestandard free-energy change (ΔG ˚)for hydrolysis of phosphate bond0Figure 2–50 Phosphate bonds have different energies.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
