B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 36
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The CO2is released as a waste product, while the high-energy electrons from NADH arepassed to a membrane-bound electron-transport chain (discussed in Chapter14), eventually combining with O2 to produce H2O. The citric acid cycle itself doesnot use gaseous O2 (it uses oxygen atoms from H2O).
But the cycle does require O2in subsequent reactions to keep it going. This is because there is no other efficientway for the NADH to get rid of its electrons and thus regenerate the NAD+ that isneeded.The citric acid cycle takes place inside mitochondria in eukaryotic cells. Itresults in the complete oxidation of the carbon atoms of the acetyl groups in acetyl CoA, converting them into CO2. But the acetyl group is not oxidized directly.Instead, this group is transferred from acetyl CoA to a larger, four-carbon molecule, oxaloacetate, to form the six-carbon tricarboxylic acid, citric acid, for whichthe subsequent cycle of reactions is named. The citric acid molecule is then gradually oxidized, allowing the energy of this oxidation to be harnessed to produceenergy-rich activated carrier molecules.
The chain of eight reactions forms a cyclebecause at the end the oxaloacetate is regenerated and enters a new turn of thecycle, as shown in outline in Figure 2–57.We have thus far discussed only one of the three types of activated carriermolecules that are produced by the citric acid cycle; NADH, the reduced form ofthe NAD+/NADH electron carrier system (see Figure 2–36). In addition to threemolecules of NADH, each turn of the cycle also produces one molecule of FADH2(reduced flavin adenine dinucleotide) from FAD (see Figure 2–39), and one molecule of the ribonucleoside triphosphate GTP from GDP.
The structure of GTP isillustrated in Figure 2–58. GTP is a close relative of ATP, and the transfer of itsterminal phosphate group to ADP produces one ATP molecule in each cycle. Aswe discuss shortly, the energy that is stored in the readily transferred electrons ofNADH and FADH2 will be utilized subsequently for ATP production through the(A)Figure 2–56 The oxidation of fatty acidsto acetyl CoA. (A) Electron micrographof a lipid droplet in the cytoplasm. (B) Thestructure of fats. Fats are triacylglycerols.The glycerol portion, to which three fattyacids are linked through ester bonds,is shown in blue. Fats are insoluble inwater and form large lipid droplets in thespecialized fat cells (adipocytes) in whichthey are stored.
(C) The fatty acid oxidationcycle. The cycle is catalyzed by a series offour enzymes in mitochondria. Each turn ofthe cycle shortens the fatty acid chain bytwo carbons (shown in red) and generatesone molecule of acetyl CoA and onemolecule each of NADH and FADH2.(A, courtesy of Daniel S. Friend.)(C)RCH2CCH2CH2S–CoArest ofhydrocarbon tailfat dropletfatty acyl CoAshortened bytwo carbonscycle repeatsuntil fatty acidis completelydegradedORCH2activated fatty acidenters cycleOfatty acyl CoACS–CoAOCH31 µmS–CoAOCH2OCFADCFADH2acetyl CoAORhydrocarbon tailCH2CHCHHS–CoAOCHOCOhydrocarbon tailRCH2CRS–CoAOCH2OChydrocarbon tailester bond(B)triacylglycerolNADH+ H+S–CoAH2OOH HOCH2 CCNAD+CH2CCHHOCS–CoAChapter 2: Cell Chemistry and Bioenergetics84Figure 2–57 Simple overview of the citricacid cycle.
The reaction of acetyl CoA withoxaloacetate starts the cycle by producingcitrate (citric acid). In each turn of thecycle, two molecules of CO2 are producedas waste products, plus three moleculesof NADH, one molecule of GTP, and onemolecule of FADH2. The number of carbonatoms in each intermediate is shown in ayellow box. For details, see Panel 2–9(pp.
106–107).OH3CCS–CoAacetyl CoA2Coxaloacetate6CSTEP 14CSTEP 2NADH++Hcitrate6CSTEP 8+NADH + HSTEP 34CSTEP 7C O25CSTEP 44CSTEP 6STEP 54CFADH2+NADH + H4CC O2GTPNET RESULT: ONE TURN OF THE CYCLE PRODUCES THREE NADH, ONE GTP, ANDONE FADH2 MOLECULE, AND RELEASES TWO MOLECULES OF CO2process of oxidative phosphorylation, the only step in the oxidative catabolism offoodstuffs that directly requires gaseous oxygen (O2) from the atmosphere.Panel 2–9 (pp. 106–107) and Movie 2.6 present the complete citric acid cycle.Water, rather than molecular oxygen, supplies the extra oxygen atoms requiredto make CO2 from the acetyl groups entering the citric acid cycle. As illustrated inthe panel, three molecules of MBoC6water m2.82/2.57are split in each cycle, and the oxygen atomsof some of them are ultimately used to make CO2.In addition to pyruvate and fatty acids, some amino acids pass from the cytosolinto mitochondria, where they are also converted into acetyl CoA or one of theother intermediates of the citric acid cycle.
Thus, in the eukaryotic cell, the mitochondrion is the center toward which all energy-yielding processes lead, whetherthey begin with sugars, fats, or proteins.Both the citric acid cycle and glycolysis also function as starting points forimportant biosynthetic reactions by producing vital carbon-containing intermediates, such as oxaloacetate and α-ketoglutarate. Some of these substances produced by catabolism are transferred back from the mitochondria to the cytosol,where they serve in anabolic reactions as precursors for the synthesis of manyessential molecules, such as amino acids (Figure 2–59).Electron Transport Drives the Synthesis of the Majority of the ATPin Most CellsMost chemical energy is released in the last stage in the degradation of a foodmolecule. In this final process, NADH and FADH2 transfer the electrons that theygained when oxidizing food-derived organic molecules to the electron-transportchain, which is embedded in the inner membrane of the mitochondrion (seeFigure 14–10).
As the electrons pass along this long chain of specialized electronacceptor and donor molecules, they fall to successively lower energy states. Theenergy that the electrons release in this process pumps H+ ions (protons) acrossthe membrane—from the innermost mitochondrial compartment (the matrix)to the intermembrane space (and then to the cytosol)—generating a gradient ofH+ ions (Figure 2–60). This gradient serves as a major source of energy for cells,being tapped like a battery to drive a variety of energy-requiring reactions.
Themost prominent of these reactions is the generation of ATP by the phosphorylation of ADP.HOW CELLS OBTAIN ENERGY FROM FOOD85guanineNO–OPOOOP–OHCOO–CPCH2 OOCNFigure 2–58 The structure of GTP. GTPand GDP are close relatives of ATP andADP, respectively.OCNNHCNH2O–riboseOHOHGDPGTPAt the end of this series of electron transfers, the electrons are passed to molecules of oxygen gas (O2) that have diffused into the mitochondrion, which simultaneously combine with protons (H+) from the surrounding solution to producewater.
The electrons have now reached a low energy level, and all the availableenergy has been extracted from the oxidized food molecule. This process, termedoxidative phosphorylation (Figure 2–61), also occurs in the plasma membraneof bacteria. As one of the most remarkable achievements of cell evolution, it is acentral topic of Chapter 14.In total, the complete oxidation of a molecule of glucose to H2O and CO2 isused by the cell to produce about 30 molecules of ATP.
In contrast, only 2 molecules of ATP are produced per molecule of glucose by glycolysis alone.Amino Acids and Nucleotides Are Part of the Nitrogen CycleMBoC6 m2.83/2.58So far we have concentrated mainly on carbohydrate metabolismand have not yetconsidered the metabolism of nitrogen or sulfur.
These two elements are important constituents of biological macromolecules. Nitrogen and sulfur atoms passGLUCOSEnucleotidesglucose 6-phosphateamino sugarsglycolipidsglycoproteinsfructose 6-phosphateGLYCOLYSISserinedihydroxyacetonephosphate3-phosphoglyceratelipidsamino acidspyrimidinesphosphoenolpyruvatealaninepyruvatecholesterolfatty acidsaspartateother amino acidspurinespyrimidinescitrateoxaloacetateCITRICACIDCYCLEα-ketoglutaratehemechlorophyllsuccinyl CoAglutamateother amino acidspurinesFigure 2–59 Glycolysis and the citricacid cycle provide the precursorsneeded to synthesize many importantbiological molecules. The amino acids,nucleotides, lipids, sugars, and othermolecules—shown here as products—inturn serve as the precursors for the manymacromolecules of the cell.
Each blackarrow in this diagram denotes a singleenzyme-catalyzed reaction; the red arrowsgenerally represent pathways with manysteps that are required to produce theindicated products.86Chapter 2: Cell Chemistry and Bioenergeticsfrom compound to compound and between organisms and their environment ina series of reversible cycles.Although molecular nitrogen is abundant in the Earth’s atmosphere, nitrogenis chemically unreactive as a gas.
Only a few living species are able to incorporate it into organic molecules, a process called nitrogen fixation. Nitrogen fixation occurs in certain microorganisms and by some geophysical processes, suchas lightning discharge. It is essential to the biosphere as a whole, for without it lifecould not exist on this planet. Only a small fraction of the nitrogenous compoundsin today’s organisms, however, is due to fresh products of nitrogen fixation fromthe atmosphere.
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