4 Обмен энергией (1160073)
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Лекция 4.Обмен энергиейP A R TBioenergetics and MetabolismMetabolism is a highly coordinated and directed cell activity, in whichmany multienzyme systems cooperate to accomplish four functions:(1) to obtain chemical energy by capturing solar energy or by degradingenergy-rich nutrients from the environment, (2) to convert nutrientmolecules into the cell's own characteristic molecules, including macromolecular precursors, (3) to polymerize monomeric precursors into proteins, nucleic acids, lipids, polysaccharides, and other cell components,and (4) to synthesize and degrade biomolecules required in specializedcellular functions.Although metabolism embraces hundreds of different enzymecatalyzed reactions, the central metabolic pathways—our major concern—are few in number and are remarkably similar in all forms oflife.
Living organisms can be divided into two large groups according tothe chemical form in which they obtain carbon from the environment.Autotrophs (such as photosynthetic bacteria and higher plants) canuse carbon dioxide from the atmosphere as their sole source of carbon,from which they construct all their carbon-containing biomolecules(see Fig. 2-4).
Some autotrophic organisms, such as cyanobacteria, canalso use atmospheric nitrogen to generate all their nitrogenous components. Heterotrophs cannot use atmospheric carbon dioxide andmust obtain carbon from their environment in the form of relativelycomplex organic molecules, such as glucose. The cells of higher animalsand most microorganisms are heterotrophic.
Autotrophic cells are relatively self-sufficient, whereas heterotrophic cells, with their requirements for carbon in more complex forms, must subsist on the productsof other cells.Many autotrophic organisms are photosynthetic and obtain theirenergy from sunlight, whereas heterotrophic cells obtain their energyfrom the degradation of organic nutrients made by autotrophs.
In ourbiosphere, autotrophs and heterotrophs live together in a vast, interdependent cycle in which autotrophic organisms use atmospheric CO2 tobuild their organic biomolecules, some of them generating oxygen fromH2O in the process. Heterotrophs in turn use the organic products ofautotrophs as nutrients and return CO2 to the atmosphere. The oxidation reactions that produce CO2 also consume O2, converting it to H2O.Thus carbon, oxygen, and water are constantly cycled between the heterotrophic and autotrophic worlds, solar energy ultimately providingthe driving force for this massive process (Fig.
1).Facing page: The active site of glyceraldehyde-3phosphate dehydrogenase, with the bound cofactornicotinamide adenine dinucleotide (NAD) shown inred. This enzyme catalyzes the oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, astep in glycolysis, a central pathway in glucosemetabolism. This is the earliest known example ofan enzymatic reaction in which the energy releasedby electron transfer (oxidation) drives the formationof a high-energy phosphate compound.PhotosyntheticautotrophsFigure 1 The cycling of carbon dioxide and oxygenbetween the autotrophic (photosynthetic) and theheterotrophic domains in the biosphere.
Theflowofmass through this cycle is enormous; about 4 x 1011metric tons of carbon are turned over in the biosphere annually.359360Part III Bioenergetics and MetabolismAll living organisms also require a source of nitrogen, which isnecessary for the synthesis of amino acids, nucleotides, and other compounds. Plants are generally able to use either ammonia or solublenitrates as their sole source of nitrogen, but vertebrate animals mustobtain some nitrogen in the form of amino acids or other organic compounds.
Only a few organisms—the cyanobacteria and a few species ofsoil bacteria that live symbiotically on the roots of certain plants (legumes)—are capable of converting ("fixing") atmospheric nitrogen (N2)into ammonia. Other microbial organisms (nitrifying bacteria) carryout the oxidation of ammonia to nitrites and nitrates. Thus, in additionto the global carbon and oxygen cycle (Fig. 1), a nitrogen cycle operatesin the biosphere in which huge amounts of nitrogen undergo cyclingand turnover (Fig.
2). The cycling of carbon, oxygen, and nitrogen,which involves many species of living organisms, depends on a properbalance between the activities of the producers (autotrophs) and consumers (heterotrophs) in our biosphere.Figure 2 The cycling of nitrogen in the biosphere.Gaseous nitrogen (N2) makes up 80% of our atmosphere.^Hj||^^^^^kNitrates, nitritesI NitrifyingbacteriarNitrogenAtmosphericN2m^lia, ^ Ammoniabacteria^fixing:• •H•t •i ^^'-~' ^' •^H^M• MjAnimals 1,-Hants I„_JAminoacidsThese great cycles of matter are driven by an enormous flow ofenergy through the biosphere, which begins with the capture of solarenergy by photosynthetic organisms and its use to generate energyrich carbohydrates and other organic nutrients; these nutrients arethen used as energy sources by heterotrophic organisms.
In the metabolic processes of each organism participating in these cycles, and inall energy-requiring activities, there is a loss of useful energy (freeenergy) and an inevitable increase in the amount of unavailable energyas heat and entropy.
In contrast to the cycling of matter, therefore,energy flows one-way through the biosphere; useful energy can neverbe regenerated in living organisms from energy dissipated as heat andentropy. Carbon, oxygen, and nitrogen recycle continuously, but energy is constantly transformed into unusable forms.Metabolism, the sum of all of the chemical transformations thatoccur in a cell or organism, occurs in a series of enzyme-catalyzed reactions that constitute metabolic pathways. Each of the consecutivesteps in such a pathway brings about a small, specific chemical change,usually the removal, transfer, or addition of a specific atom, functionalgroup, or molecule. In this sequence of steps (the pathway), a precursor is converted into a product through a series of metabolic intermediates (metabolites).
The term intermediary metabolism is often applied to the combined activities of all of the metabolic pathways thatinterconvert precursors, metabolites, and products of low molecularweight (not including macromolecules).Part III Bioenergetics and Metabolism361Catabolism is the degradative phase of metabolism, in which organic nutrient molecules (carbohydrates, fats, and proteins) are converted into smaller, simpler end products (e.g., lactic acid, CO2, NH3).Catabolic pathways release free energy, some of which is conserved inthe formation of ATP and reduced electron carriers (NADH andNADPH).
In anabolism, also called biosynthesis, small, simple precursors are built up into larger and more complex molecules, includinglipids, polysaccharides, proteins, and nucleic acids. Anabolic reactionsrequire the input of energy, generally in the forms of the free energy ofhydrolysis of ATP and the reducing power of NADH and NADPH(Fig. 3).Energy-yieldingnutrientsCarbohydratesPatsProteinsCellmacromolecules jProteinsjPolysaccharides iLipidsiNucleic acidsAnabolismEnergy-poorend productsPrecursormoleculesAmino acidsSugarsFatty acidsNitrogenousbasesMetabolic pathways are sometimes linear and sometimesbranched, yielding several useful end products from a single precursoror converting several starting materials into a single product. In general, catabolic pathways are convergent and anabolic pathways divergent (Fig.
4). Some pathways are even cyclic: one of the starting components of the pathway is regenerated in the series of reactions thatconverts another starting component into a product. We shall see examples of each type of pathway in the following chapters.Figure 3 Energy relationships between catabolicand anabolic pathways. Catabolic pathways deliverchemical energy in the form of ATP, NADH, andNADPH. These are used in anabolic pathways toconvert small precursor molecules into cell macromolecules.362Part III Bioenergetics and MetabolismRubberCarotenoidSteroidhormonespigmentsPhospholipidsTriacylglycerolsStarchGlycogenSucroseIsopentenylpyrophosphate* CholesterolBileacids^ ^ Fatty acidsAlaninePhenyl- "alanine^ ^ Glucose^ > PyruvateSerineLeucine(a)IsoleucineAcetate:(acetyl-CoA)Acetoacetyl-CoAEicosanoidsFatty acidsTTriacylglycerolsCDP-diglycerideCitrateOxaloacetateICholesterylestersVitamin KMevalonatePhospholipids(b)COoCOo(c)Figure 4 Three types of nonlinear metabolic pathways: (a) converging, catabolic; (b) diverging, anabolic; and (c) a cyclic pathway, in which one of thestarting materials (oxaloacetate) is regenerated andreenters the pathway.
Acetate, a key metabolic intermediate, can be produced by the breakdown of avariety of fuels (a), can serve as the precursor forthe biosynthesis of an array of products (b), or canbe consumed in the catabolic pathway known asthe citric acid cycle (c).Most organisms have the enzymatic equipment to carry out boththe degradation and the synthesis of certain compounds (fatty acids,for example). The simultaneous synthesis and degradation of fattyacids would be wasteful and is prevented by separately regulating anabolic and catabolic reaction sequences: when one occurs, the other issuppressed.
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