B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 25
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Although manycompounds present in cells do not fit into these categories, these four familiesof small organic molecules, together with the macromolecules made by linkingthem into long chains, account for a large fraction of the cell mass.Amino acids and the proteins that they form will be the subject of Chapter3. A summary of the structures and properties of the remaining three families—sugars, fatty acids, and nucleotides—is presented in Panels 2–4, 2–5, and 2–6,respectively (see pages 96–101).The Chemistry of Cells Is Dominated by Macromolecules withRemarkable PropertiesBy weight, macromolecules are the most abundant carbon-containing moleculesin a living cell (Figure 2–7). They are the principal building blocks from whicha cell is constructed and also the components that confer the most distinctiveproperties of living things. The macromolecules in cells are polymers that areconstructed by covalently linking small organic molecules (called monomers) into47Chapter 2: Cell Chemistry and Bioenergetics48CH2OHHCHOCHOOHHCCHOH+H3NCHHCCOOCH3OHA SUGARAN AMINO ACIDH H H H H H H H H H H H H HH C C C C C C C C C C C C C CNPO–OPO–POCH2O–NOOHFATTY ACIDSFATS, LIPIDS, MEMBRANESAMINO ACIDSPROTEINSNUCLEOTIDESNUCLEIC ACIDSFigure 2–6 The four main families of small organic molecules incells.
These small molecules form the monomeric building blocks, orsubunits, for most of the macromolecules and other assemblies of thecell. Some, such as the sugars and the fatty acids, are also energysources. Their structures are outlined here and shown in more detail inthe Panels at the end of this chapter and in Chapter 3.NOOPOLYSACCHARIDES_NH2–OSUGARSOA FATTY ACIDOlarger unitsof the cellOCH H H H H H H H H H H H H HObuilding blocksof the cellNOHA NUCLEOTIDElong chains (Figure 2–8). They have remarkable properties that could not havebeen predicted from their simple constituents.Proteins are abundant and spectacularly versatile, performing thousands ofdistinct functions in cells.
Many proteins serve as enzymes, the catalysts that facilitate the many covalent bond-making and bond-breaking reactions that the cellneeds. Enzymes catalyze all of the reactions whereby cells extract energy fromfood molecules, for example, and an enzyme called ribulose bisphosphate carboxylase helps to convert CO2 to sugars in photosynthetic organisms, producingmost of the organic matter needed for life on Earth. Otherproteins are used toMBoC6 m2.17/2.06build structural components, such as tubulin, a protein that self-assembles tomake the cell’s long microtubules, or histones, proteins that compact the DNA inchromosomes.
Yet other proteins act as molecular motors to produce force andbacterialcell30%chemicalsCELLVOLUME OF2 × 10–12 cm3inorganic ions (1%)small molecules (3%)phospholipid (2%)DNA (1%)RNA (6%)MACROMOLECULES70%H2Oprotein (15%)polysaccharide (2%)Figure 2–7 The distribution of molecules in cells. The approximate composition of a bacterial cellis shown by weight. The composition of an animal cell is similar, even though its volume is roughly1000 times greater. Note that macromolecules dominate.
The major inorganic ions include Na+, K+,Mg2+, Ca2+, and Cl–.THE CHEMICAL COMPONENTS OF A CELL49movement, as for myosin in muscle. Proteins perform many other functions, andwe shall examine the molecular basis for many of them later in this book.Although the chemical reactions for adding subunits to each polymer are different in detail for proteins, nucleic acids, and polysaccharides, they share important features. Each polymer grows by the addition of a monomer onto the end of agrowing chain in a condensation reaction, in which one molecule of water is lostwith each subunit added (Figure 2–9).
The stepwise polymerization of monomersinto a long chain is a simple way to manufacture a large, complex molecule, sincethe subunits are added by the same reaction performed over and over again by thesame set of enzymes. Apart from some of the polysaccharides, most macromolecules are made from a limited set of monomers that are slightly different fromone another—for example, the 20 different amino acids from which proteins aremade. It is critical to life that the polymer chain is not assembled at random fromthese subunits; instead the subunits are added in a precise order, or sequence.
Theelaborate mechanisms that allow enzymes to accomplish this task are describedin detail in Chapters 5 and 6.SUBUNITMACROMOLECULEsugarpolysaccharideaminoacidproteinnucleotidenucleic acidFigure 2–8 Three families ofmacromolecules. Each is a polymerformed from small molecules (calledmonomers) linked together by covalentbonds.Noncovalent Bonds Specify Both the Precise Shape of aMacromolecule and Its Binding to Other MoleculesMost of the covalent bonds in a macromolecule allow rotation of the atoms theyjoin, giving the polymer chain great flexibility. In principle, this allows a macromolecule to adopt an almost unlimited number of shapes, or conformations, asrandom thermal energy causes the polymer chain to writhe and rotate.
However,the shapes of most biological macromolecules are highly constrained because ofthe many weak noncovalent bonds that form between different parts of the samemolecule. If these noncovalent bonds are formed in sufficient numbers, the polymer chain can strongly prefer one particular conformation, determined by thelinear sequence of monomers in its chain. Most protein molecules and many ofthe small RNA molecules found in cells fold tightly into a highly preferred conformation in this way (Figure 2–10).The four types of noncovalent interactions important in biological moleculeswere presented earlier, and they are discussed further in Panel 2–3 (pp. 94–95).
Inaddition to folding biological macromolecules into unique shapes, they can alsoadd up to create a strong attraction between two different molecules (see Figure2–3). This form of molecular interaction provides for great specificity, inasmuchas the close multipoint contacts required for strong binding make it possible for amacromolecule to select out—through binding—just one of the many thousandsof other types of molecules present inside a cell.
Moreover, because the strength ofthe binding depends on the number of noncovalent bonds that are formed, interactions of almost any affinity are possible—allowing rapid dissociation whereappropriate.As we discuss next, binding of this type underlies all biological catalysis, making it possible for proteins to function as enzymes. In addition, noncovalent interactions allow macromolecules to be used as building blocks for the formation ofH2OAH + HOBCONDENSATIONenergeticallyunfavorableH2OABHYDROLYSISAH + HOBenergeticallyfavorableFigure 2–9 Condensation and hydrolysis as opposite reactions.
The macromolecules of the cellare polymers that are formed from subunits (or monomers) by a condensation reaction, and theyare broken down by hydrolysis. The condensation reactions are all energetically unfavorable; thuspolymer formation requires an energy input, as will be described in the text.MBoC6 m2.30/2.0850Chapter 2: Cell Chemistry and BioenergeticsFigure 2–10 The folding of proteinsand RNA molecules into a particularlystable three-dimensional shape, orconformation. If the noncovalent bondsmaintaining the stable conformation aredisrupted, the molecule becomes a flexiblechain that loses its biological activity.many unstableconformationsone stable foldedconformationlarger structures, thereby forming intricate machines with multiple moving partsthat perform such complex tasks as DNA replication and protein synthesis (Figure 2–11).MBoC6 m2.31/2.10SummaryLiving organisms are autonomous, self-propagating chemical systems.
They areformed from a distinctive and restricted set of small carbon-based molecules thatare essentially the same for every living species. Each of these small molecules iscomposed of a small set of atoms linked to each other in a precise configurationthrough covalent bonds. The main categories are sugars, fatty acids, amino acids,and nucleotides. Sugars are a primary source of chemical energy for cells and can beincorporated into polysaccharides for energy storage. Fatty acids are also important for energy storage, but their most critical function is in the formation of cellmembranes.
Long chains of amino acids form the remarkably diverse and versatilemacromolecules known as proteins. Nucleotides play a central part in energy transfer, while also serving as the subunits for the informational macromolecules, RNAand DNA.Most of the dry mass of a cell consists of macromolecules that have been produced as linear polymers of amino acids (proteins) or nucleotides (DNA and RNA),covalently linked to each other in an exact order. Most of the protein moleculesand many of the RNAs fold into a unique conformation that is determined by theirsequence of subunits. This folding process creates unique surfaces, and it dependson a large set of weak attractions produced by noncovalent forces between atoms.SUBUNITScovalent bondsMACROMOLECULESnoncovalent bondsMACROMOLECULARASSEMBLIESe.g., sugars, amino acids,and nucleotidese.g., globular proteinsand RNA30 nme.g., ribosomeFigure 2–11 Small molecules become covalently linked to form macromolecules, which in turn assemble through noncovalent interactionsto form large complexes.
Small molecules, proteins, and a ribosome are drawn approximately to scale. Ribosomes are a central part of themachinery that the cell uses to make proteins: each ribosome is formed as a complex of about 90 macromolecules (protein and RNA molecules).MBoC6 m2.32/2.11CATALYSIS AND THE USE OF ENERGY BY CELLS51These forces are of four types: electrostatic attractions, hydrogen bonds, van derWaals attractions, and an interaction between nonpolar groups caused by theirhydrophobic expulsion from water.
The same set of weak forces governs the specificbinding of other molecules to macromolecules, making possible the myriad associations between biological molecules that produce the structure and the chemistry ofa cell.CATALYSIS AND THE USE OF ENERGY BY CELLSOne property of living things above all makes them seem almost miraculously different from nonliving matter: they create and maintain order, in a universe that istending always to greater disorder (Figure 2–12). To create this order, the cells ina living organism must perform a never-ending stream of chemical reactions.
Insome of these reactions, small organic molecules—amino acids, sugars, nucleotides, and lipids—are being taken apart or modified to supply the many othersmall molecules that the cell requires. In other reactions, small molecules arebeing used to construct an enormously diverse range of proteins, nucleic acids,and other macromolecules that endow living systems with all of their most distinctive properties. Each cell can be viewed as a tiny chemical factory, performingmany millions of reactions every second.Cell Metabolism Is Organized by EnzymesThe chemical reactions that a cell carries out would normally occur only at muchhigher temperatures than those existing inside cells. For this reason, each reaction requires a specific boost in chemical reactivity. This requirement is crucial,because it allows the cell to control its chemistry.
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