B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 37
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Most organic nitrogen has been in circulation for some time,passing from one living organism to another. Thus, present-day nitrogen-fixingreactions can be said to perform a “topping-up” function for the total nitrogensupply.Vertebrates receive virtually all of their nitrogen from their dietary intake ofproteins and nucleic acids. In the body, these macromolecules are broken downto amino acids and the components of nucleotides, and the nitrogen they containis used to produce new proteins and nucleic acids—or other molecules. About halfof the 20 amino acids found in proteins are essential amino acids for vertebrates(Figure 2–62), which means that they cannot be synthesized from other ingredients of the diet.
The other amino acids can be so synthesized, using a variety ofraw materials, including intermediates of the citric acid cycle. The essential aminoacids are made by plants and other organisms, usually by long and energeticallyexpensive pathways that have been lost in the course of vertebrate evolution.The nucleotides needed to make RNA and DNA can be synthesized using specialized biosynthetic pathways. All of the nitrogens in the purine and pyrimidinebases (as well as some of the carbons) are derived from the plentiful amino acidsglutamine, aspartic acid, and glycine, whereas the ribose and deoxyribose sugarsare derived from glucose. There are no “essential nucleotides” that must be provided in the diet.Amino acids not used in biosynthesis can be oxidized to generate metabolicenergy.
Most of their carbon and hydrogen atoms eventually form CO2 or H2O,whereas their nitrogen atoms are shuttled through various forms and eventuallyappear as urea, which is excreted. Each amino acid is processed differently, and awhole constellation of enzymatic reactions exists for their catabolism.Sulfur is abundant on Earth in its most oxidized form, sulfate (SO42–). To beuseful for life, sulfate must be reduced to sulfide (S2–), the oxidation state of sulfurrequired for the synthesis of essential biological molecules, including the aminoacids methionine and cysteine, coenzyme A (see Figure 2–39), and the iron-sulfurcenters essential for electron transport (see Figure 14–16).
The sulfur-reductionprocess begins in bacteria, fungi, and plants, where a special group of enzymesuse ATP and reducing power to create a sulfate assimilation pathway. Humansand other animals cannot reduce sulfate and must therefore acquire the sulfurthey need for their metabolism in the food that they eat.pyruvate fromglycolysisCO2NADH fromglycolysisCoAADP + PiCITRICACIDCYCLENADHNAD+2 e–OXIDATIVEPHOSPHORYLATIONATPMITOCHONDRIONeAmembraneproteinCBmembraneeAB HCABCH+e-electron inlow-energystateFigure 2–60 The generation of anH+ gradient across a membrane byelectron-transport reactions. An electronheld in a high-energy state (derived, forexample, from the oxidation of a metabolite)is passed sequentially by carriers A, B, andC to a lower energy state.
In this diagram,carrier B is arranged in the membranein such a way that it takes up H+ fromone side and releases it to the other asthe electronMBoC6passes.m2.85/2.60The result is an H+gradient. As discussed in Chapter 14, thisgradient is an important form of energy thatis harnessed by other membrane proteinsto drive the formation of ATP (for an actualexample, see Figure 14–21).O2pyruvateacetyl CoAH+electron inhigh-energystateH 2OFigure 2–61 The final stages of oxidationof food molecules. Molecules of NADHand FADH2 (FADH2 is not shown) areproduced by the citric acid cycle.
Theseactivated carriers donate high-energyelectrons that are eventually used to reduceoxygen gas to water. A major portion ofthe energy released during the transfer ofthese electrons along an electron-transferchain in the mitochondrial inner membrane(or in the plasma membrane of bacteria) isharnessed to drive the synthesis of ATP—hence the name oxidative phosphorylation(discussed in Chapter 14).HOW CELLS OBTAIN ENERGY FROM FOOD87Metabolism Is Highly Organized and RegulatedTHE ESSENTIAL AMINO ACIDSOne gets a sense of the intricacy of a cell as a chemical machine from the relationof glycolysis and the citric acid cycle to the other metabolic pathways sketchedout in Figure 2–63.
This chart represents only some of the enzymatic pathways ina human cell. It is obvious that our discussion of cell metabolism has dealt withonly a tiny fraction of the broad field of cell chemistry.All these reactions occur in a cell that is less than 0.1 mm in diameter, and eachrequires a different enzyme. As is clear from Figure 2–63, the same molecule canoften be part of many different pathways. Pyruvate, for example, is a substrate forhalf a dozen or more different enzymes, each of which modifies it chemically ina different way.
One enzyme converts pyruvate to acetyl CoA, another to oxaloacetate; a third enzyme changes pyruvate to the amino acid alanine, a fourth tolactate, and so on. All of these different pathways compete for the same pyruvatemolecule, and similar competitions for thousands of other small molecules go onat the same time.The situation is further complicated in a multicellular organism. Different celltypes will in general require somewhat different sets of enzymes. And differenttissues make distinct contributions to the chemistry of the organism as a whole.In addition to differences in specialized products such as hormones or antibodies, there are significant differences in the “common” metabolic pathways amongvarious types of cells in the same organism.Although virtually all cells contain the enzymes of glycolysis, the citric acidcycle, lipid synthesis and breakdown, and amino acid metabolism, the levelsof these processes required in different tissues are not the same.
For example,nerve cells, which are probably the most fastidious cells in the body, maintainalmost no reserves of glycogen or fatty acids and rely almost entirely on a constantTHREONINEMETHIONINELYSINEVALINELEUCINEISOLEUCINEHISTIDINEPHENYLALANINETRYPTOPHANFigure 2–62 The nine essential aminoacids. These cannot be synthesized byhuman cells and so must be supplied inthe diet.MBoC6 m2.87/2.62glucose 6-phosphatepyruvateacetyl CoAFigure 2–63 Glycolysis and the citric acid cycle are at the center of an elaborate set of metabolic pathways in human cells.
Some 2000metabolic reactions are shown schematically with the reactions of glycolysis and the citric acid cycle in red. Many other reactions either lead intothese two central pathways—delivering small molecules to be catabolized with production of energy—or they lead outward and thereby supplycarbon compounds for the purpose of biosynthesis. (Adapted with permission from Kanehisa Laboratories.)MBoC6 n2.300/2.6388Chapter 2: Cell Chemistry and Bioenergeticssupply of glucose from the bloodstream. In contrast, liver cells supply glucose toactively contracting muscle cells and recycle the lactic acid produced by musclecells back into glucose. All types of cells have their distinctive metabolic traits, andthey cooperate extensively in the normal state, as well as in response to stress andstarvation.
One might think that the whole system would need to be so finely balanced that any minor upset, such as a temporary change in dietary intake, wouldbe disastrous.In fact, the metabolic balance of a cell is amazingly stable. Whenever the balance is perturbed, the cell reacts so as to restore the initial state.
The cell can adaptand continue to function during starvation or disease. Mutations of many kindscan damage or even eliminate particular reaction pathways, and yet—providedthat certain minimum requirements are met—the cell survives. It does so becausean elaborate network of control mechanisms regulates and coordinates the ratesof all of its reactions. These controls rest, ultimately, on the remarkable abilitiesof proteins to change their shape and their chemistry in response to changes intheir immediate environment. The principles that underlie how large moleculessuch as proteins are built and the chemistry behind their regulation will be ournext concern.SummaryGlucose and other food molecules are broken down by controlled stepwise oxidationto provide chemical energy in the form of ATP and NADH.
There are three main setsof reactions that act in series, the products of each being the starting material for thenext: glycolysis (which occurs in the cytosol), the citric acid cycle (in the mitochondrial matrix), and oxidative phosphorylation (on the inner mitochondrial membrane). The intermediate products of glycolysis and the citric acid cycle are usedboth as sources of metabolic energy and to produce many of the small moleculesused as the raw materials for biosynthesis.
Cells store sugar molecules as glycogenin animals and starch in plants; both plants and animals also use fats extensivelyas a food store. These storage materials in turn serve as a major source of food forhumans, along with the proteins that comprise the majority of the dry mass of mostof the cells in the foods we eat.WHAT WE DON’T KNOW• Did chemiosmosis precedefermentation as the source ofbiological energy, or did some form offermentation come first, as had beenassumed for many years?• What is the minimum number ofcomponents required to make a livingcell from scratch? How might we findout?• Are other life chemistries possiblebesides the single one known on Earth(and described in this chapter)? Whenscreening for life on other planets,what type of chemical signaturesshould we search for?• Is the shared chemistry inside allliving cells a clue for deciphering theenvironment on Earth where the firstcells originated? For example, whatmight we conclude from the universallyshared high K+/Na+ ratio, neutral pH,and central role of phosphates?PROBLEMSWhich statements are true? Explain why or why not.2–1A 10–8 M solution of HCl has a pH of 8.2–2Most of the interactions between macromoleculescould be mediated just as well by covalent bonds as bynoncovalent bonds.2–3Animals and plants use oxidation to extract energyfrom food molecules.2–4If an oxidation occurs in a reaction, it must beaccompanied by a reduction.2–5Linking the energetically unfavorable reaction A→ B to a second, favorable reaction B → C will shift theequilibrium constant for the first reaction.2–6The criterion for whether a reaction proceedsspontaneously is ΔG not ΔG°, because ΔG takes intoaccount the concentrations of the substrates and products.2–7The oxygen consumed during the oxidation of glucose in animal cells is returned as CO2 to the atmosphere.Discuss the following problems.2–8The organic chemistry of living cells is said to bespecial for two reasons: it occurs in an aqueous environment and it accomplishes some very complex reactions.But do you suppose it is really all that much different fromthe organic chemistry carried out in the top laboratories inthe world? Why or why not?2–9The molecular weight of ethanol (CH3CH2OH) is46 and its density is 0.789 g/cm3.A.What is the molarity of ethanol in beer that is 5%ethanol by volume? [Alcohol content of beer varies fromabout 4% (lite beer) to 8% (stout beer).]B.The legal limit for a driver’s blood alcohol contentvaries, but 80 mg of ethanol per 100 mL of blood (usuallyreferred to as a blood alcohol level of 0.08) is typical.
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