B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 33
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The high-energy intermediate formed—anucleoside triphosphate—exists free in solution until it reacts with the growing end of an RNA or aDNA chain with release of pyrophosphate. Hydrolysis of the latter to inorganic phosphate is highlyfavorable and helps to drive the overall reaction in the direction of polynucleotide synthesis.
Fordetails, see Chapter 5.MBoC6 m2.67/2.43Figure 2–42 An alternative pathway ofATP hydrolysis, in which pyrophosphateis first formed and then hydrolyzed. Thisroute releases about twice as much freeenergy (approximately –100 kJ/mole) asthe reaction shown earlier in Figure 2–33,and it forms AMP instead of ADP. (A) Inthe two successive hydrolysis reactions,oxygen atoms from the participating watermolecules are retained in the products, asindicated, whereas the hydrogen atomsdissociate to form free hydrogen ions(H+, not shown).
(B) Summary of overallreaction.HOW CELLS OBTAIN ENERGY FROM FOODHEAD POLYMERIZATION(e.g., PROTEINS, FATTY ACIDS)6673+TAIL POLYMERIZATION7+1each monomer carries a high-energybond that will be used for theaddition of the next monomer7(e.g., DNA, RNA, POLYSACCHARIDES)77each monomer carriesa high-energy bondfor its own addition1growing polymer, and it must therefore be regenerated each time that a monomeris added. In this case, each monomer brings with it the reactive bond that will beused in adding the next monomer in the series. In tail polymerization, the reactivebond carried by each monomer is instead used immediately for its own addition(Figure 2–44).We shall see in later chapters that both of these types of polymerization areused.
The synthesis of polynucleotides and some simple polysaccharides occursby tail polymerization, for example, whereas the synthesisof proteins occurs by aMBoC6 m2.68/2.44head polymerization process.SummaryLiving cells need to create and maintain order within themselves to survive andgrow. This is thermodynamically possible only because of a continual input ofenergy, part of which must be released from the cells to their environment as heatthat disorders the surroundings. The only chemical reactions possible are those thatincrease the total amount of disorder in the universe.
The free-energy change for areaction, ∆G, measures this disorder, and it must be less than zero for a reactionto proceed spontaneously. This ∆G depends both on the intrinsic properties of thereacting molecules and their concentrations, and it can be calculated from theseconcentrations if either the equilibrium constant (K) for the reaction or its standardfree-energy change, ∆G°, is known.The energy needed for life comes ultimately from the electromagnetic radiationof the sun, which drives the formation of organic molecules in photosynthetic organisms such as green plants. Animals obtain their energy by eating organic moleculesand oxidizing them in a series of enzyme-catalyzed reactions that are coupled to theformation of ATP—a common currency of energy in all cells.To make possible the continual generation of order in cells, energetically favorable reactions, such as the hydrolysis of ATP, are coupled to energetically unfavorable reactions.
In the biosynthesis of macromolecules, ATP is used to form reactivephosphorylated intermediates. Because the energetically unfavorable reaction ofbiosynthesis now becomes energetically favorable, ATP hydrolysis is said to drivethe reaction. Polymeric molecules such as proteins, nucleic acids, and polysaccharides are assembled from small activated precursor molecules by repetitive condensation reactions that are driven in this way. Other reactive molecules, called eitheractivated carriers or coenzymes, transfer other chemical groups in the course ofbiosynthesis: NADPH transfers hydrogen as a proton plus two electrons (a hydrideion), for example, whereas acetyl CoA transfers an acetyl group.HOW CELLS OBTAIN ENERGY FROM FOODThe constant supply of energy that cells need to generate and maintain the biological order that keeps them alive comes from the chemical-bond energy in foodmolecules.The proteins, lipids, and polysaccharides that make up most of the food we eatmust be broken down into smaller molecules before our cells can use them—eitherFigure 2–44 The orientation of theactive intermediates in the repetitivecondensation reactions that formbiological polymers.
The head growth ofpolymers is compared with its alternative,tail growth. As indicated, these twomechanisms are used to produce differenttypes of biological macromolecules.74Chapter 2: Cell Chemistry and Bioenergeticsas a source of energy or as building blocks for other molecules. Enzymatic digestion breaks down the large polymeric molecules in food into their monomer subunits—proteins into amino acids, polysaccharides into sugars, and fats into fattyacids and glycerol. After digestion, the small organic molecules derived from foodenter the cytosol of cells, where their gradual oxidation begins.Sugars are particularly important fuel molecules, and they are oxidized insmall controlled steps to carbon dioxide (CO2) and water (Figure 2–45).
In thissection, we trace the major steps in the breakdown, or catabolism, of sugars andshow how they produce ATP, NADH, and other activated carrier molecules in animal cells. A very similar pathway also operates in plants, fungi, and many bacteria.As we shall see, the oxidation of fatty acids is equally important for cells. Othermolecules, such as proteins, can also serve as energy sources when they are funneled through appropriate enzymatic pathways.Glycolysis Is a Central ATP-Producing PathwayThe major process for oxidizing sugars is the sequence of reactions known asglycolysis—from the Greek glukus, “sweet,” and lusis, “rupture.” Glycolysis produces ATP without the involvement of molecular oxygen (O2 gas). It occurs in thecytosol of most cells, including many anaerobic microorganisms.
Glycolysis probably evolved early in the history of life, before photosynthetic organisms introduced oxygen into the atmosphere. During glycolysis, a glucose molecule with sixcarbon atoms is converted into two molecules of pyruvate, each of which containsthree carbon atoms. For each glucose molecule, two molecules of ATP are hydrolyzed to provide energy to drive the early steps, but four molecules of ATP are produced in the later steps. At the end of glycolysis, there is consequently a net gainof two molecules of ATP for each glucose molecule broken down. Two moleculesof the activated carrier NADH are also produced.The glycolytic pathway is outlined in Figure 2–46 and shown in more detail inPanel 2–8 (pp.
104–105) and Movie 2.5. Glycolysis involves a sequence of 10 separate reactions, each producing a different sugar intermediate and each catalyzedby a different enzyme. Like most enzymes, these have names ending in ase—suchas isomerase and dehydrogenase—to indicate the type of reaction they catalyze.Although no molecular oxygen is used in glycolysis, oxidation occurs, in thatelectrons are removed by NAD+ (producing NADH) from some of the carbonsderived from the glucose molecule. The stepwise nature of the process releasesthe energy of oxidation in small packets, so that much of it can be stored in activated carrier molecules rather than all of it being released as heat (see Figure2–45).
Thus, some of the energy released by oxidation drives the direct synthesisof ATP molecules from ADP and Pi, and some remains with the electrons in theelectron carrier NADH.(A) DIRECT BURNING OF SUGARIN NONLIVING SYSTEM(B) STEPWISE OXIDATION OF SUGAR IN CELLSlarge activationenergy overcomeby the heat froma fireSUGAR + O2free energyFigure 2–45 Schematic representationof the controlled stepwise oxidation ofsugar in a cell, compared with ordinaryburning. (A) If the sugar were oxidized toCO2 and H2O in a single step, it wouldrelease an amount of energy much largerthan could be captured for useful purposes.(B) In the cell, enzymes catalyze oxidationvia a series of small steps in which freeenergy is transferred in conveniently sizedpackets to carrier molecules—most oftenATP and NADH.
At each step, an enzymecontrols the reaction by reducing theactivation-energy barrier that has to besurmounted before the specific reactioncan occur. The total free energy released isexactly the same in (A) and (B).small activation energiesovercome by enzymes thatwork at body temperatureSUGAR + O2all free energy isreleased as heat;none is storedCO2 + H2Osome freeenergy stored inactivated carriermoleculesCO2 + H2OHOW CELLS OBTAIN ENERGY FROM FOOD75Two molecules of NADH are formed per molecule of glucose in the course ofglycolysis. In aerobic organisms, these NADH molecules donate their electrons tothe electron-transport chain described in Chapter 14, and the NAD+ formed fromthe NADH is used again for glycolysis (see step 6 in Panel 2–8, pp.
104–105).Fermentations Produce ATP in the Absence of OxygenFor most animal and plant cells, glycolysis is only a prelude to the final stage ofthe breakdown of food molecules. In these cells, the pyruvate formed by glycolysisis rapidly transported into the mitochondria, where it is converted into CO2 plusacetyl CoA, whose acetyl group is then completely oxidized to CO2 and H2O.In contrast, for many anaerobic organisms—which do not utilize molecularoxygen and can grow and divide without it—glycolysis is the principal source ofthe cell’s ATP. Certain animal tissues, such as skeletal muscle, can also continueto function when molecular oxygen is limited. In these anaerobic conditions, thepyruvate and the NADH electrons stay in the cytosol.
The pyruvate is convertedinto products excreted from the cell—for example, into ethanol and CO2 in theyeasts used in brewing and breadmaking, or into lactate in muscle. In this process,the NADH gives up its electrons and is converted back into NAD+. This regeneration of NAD+ is required to maintain the reactions of glycolysis (Figure 2–47).Energy-yielding pathways like these, in which organic molecules both donateand accept electrons (and which are often, as in these cases, anaerobic), are calledCH2OHOone moleculeof glucoseOHHOOHenergyinvestmentto berecoupedlaterOHATPSTEP 1STEP 2ATPSTEP 3P OH2CCH2O POfructose 1,6bisphosphateHOOHOHSTEP 4STEP 5two molecules ofglyceraldehyde3-phosphateCHOCHOCHOHCHOHCH2O Pcleavage ofsix-carbonsugar to twothree-carbonsugarsCH2O PNADHSTEP 6NADHATPSTEP 7ATPSTEP 8STEP 9STEP 10ATPCOO–two moleculesof pyruvateenergygenerationCCH3OATPCOO–CCH3OFigure 2–46 An outline of glycolysis.Each of the 10 steps shown is catalyzedby a different enzyme.
Note that step 4cleaves a six-carbon sugar into two threecarbon sugars, so that the number ofmolecules at every stage after this doubles.As indicated, step 6 begins the energygeneration phase of glycolysis. Becausetwo molecules of ATP are hydrolyzed in theearly, energy-investment phase, glycolysisresults in the net synthesis of 2 ATP and 2NADH molecules per molecule of glucose(see also Panel 2–8).76Chapter 2: Cell Chemistry and BioenergeticsFigure 2–47 Two pathways for theanaerobic breakdown of pyruvate.(A) When there is inadequate oxygen,for example, in a muscle cell undergoingvigorous contraction, the pyruvateproduced by glycolysis is converted tolactate as shown. This reaction regeneratesthe NAD+ consumed in step 6 of glycolysis,but the whole pathway yields much lessenergy overall than complete oxidation.(B) In some organisms that can growanaerobically, such as yeasts, pyruvate isconverted via acetaldehyde into carbondioxide and ethanol.
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