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Nucleic acid chains are sgrthesizedfrom energy-rich nucleoside triphosphates by a condensation reaction thatreleasesinorganic plrophosphate during phosphodiesterbond formation.There are two main types of nucleic acids, differing in the type of sugar intheir sugar-phosphate backbone. Those based on the sugar ribose are knor,tmasribonucleic acids, or RNA, and normally contain the basesA, G, C, and U.
Thosebased on deoxyribose(in which the hydroxyl at the 2' position of the ribose carbon ring is replaced by a hydrogen are knor,rm as deoxyribonucleic acids, orDNA, and contain the bases A, G, C, and T (T is chemically similar to the U inRNA, merely adding the methyl group on the pyrimidine ring; see Panel2-6).RNA usually occurs in cells as a single polynucleotide chain, but DNA is virtuallyalways a double-stranded molecule-a DNA double helix composed of twopolynucleotide chains running antiparallel to each other and held together byhydrogen-bonding between the basesof the two chains.The linear sequence of nucleotides in a DNA or an RNA encodes the geneticinformation of the cell. The ability of the bases in different nucleic acidmolecules to recognize and pair with each other by hydrogen-bonding (calledbase-pairing)-G with C, and A with either T or U-underlies all of heredity andevolution, as explained in Chapter 4.5'endIGoIo-ToIoTheChemistryof Cellsls Dominatedby Macromoleculeswith RemarkablePropertiesI3'endBy weight, macromolecules are the most abundant carbon-containingmolecules in a living cell (Figure 2-29 and Table 2-3).
They are the principalbuilding blocks from which a cell is constructed and also the components thatconfer the most distinctive properties of living things. The macromolecules incells are polymers that are constructed by covalently linking small organicmolecules (called monomers) into long chains (Figure 2-3O). Yet they haveremarkable properties that could not have been predicted from their simpleconstituents.Proteins are especially abundant and versatile. They perform thousands ofdistinct functions in cells. Many proteins serve as enzymes,the catalysts thati o n s ,s m a l lm o l e c u l e s( 4 % )p h o s p h o l i p i d(s2 % )D N A( 1 % )R N A( 5 % )^77Oo/oHzop r o t e i n s( 1 5 % )o-mr)C-mpolysaccharide( 2s% )Figure2-29 Macromoleculesareabundantin cells.Theapproximatecompositionof a bacterialcellis shownby weight.The compositionof an animalcellis similar(seeTable2-3).T H EC H E M I C ACL O M P O N E N TOSF A C E L L63Table2-3 ApproximateChemicalCompositionsof a TypicalBacteriumand aTypicalMammalianCellHzolnorganicions(Na+,K*,Mg2*,ca2+,cl-, etc.)MiscellaneoussmallmetabolitesProteinsRNADNAPhospholipidsOtherlipidsPolysaccharidesTotalcellvolumeRelativecellvolume70170131561')3181.10.25?2.\2 y 1 g - 1.2r 31z4 x 1 0 - ec m 32000P r o t e i n s p, o i y s a c c h a r i d e sD,N A ,a n d R N Aa r e m a c r o m o l e c u l e sL i p i d sa r e n o t g e n e r a l l yc a s s e da sb o t h m a m m a la n a n d b a c L e r r ace tsbles to make the cell'slong microtubules, or histones, proteins that compact theDNA in chromosomes.
Yet other proteins act as molecular motors to produceforce and movement, as in the caseof myosin in muscle. proteins perform manyother functions, and we shall examine the molecular basis for many of themlater in this book. Here we identifu some general principles of macromolecularchemistry that make such functions possible.Although the chemical reactions for adding subunits to each polyrner aredifferent in detail for proteins, nucleic acids, and polysaccharides, they shareimportant features.
Each polymer grows by the addition of a monomer ontothe end of a growing polymer chain in a condensation reaction, in which amade from a set of monomers that are slightly different from one another-forexample, the 20 different amino acids from which proteins are made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a particular order, or sequence.T]neelaborate mechanisms that allow this to be accomplished by enzymes aredescribedin detail in Chapters5 and 6.NoncovalentBondsSpecifyBoth the preciseShapeof aMacromoleculeand its Bindingto 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, asSUBUNITs u g aroMACROMOLECULEp o l y s a c c hr ai d eamrnou.,0"proternnu c l e o t i d en u c l e i ca c i dFigure2-30 Threefamiliesofmacromolecules.Eachis a polymerformedfrom smallmolecules(calledmonomers)linkedtogetherbycovalentbonds.64Chapter2: CellChemistryand BiosynthesisFigure2-31 Most proteinsand manyRNAmoleculesfold into only one stablebondsconformation.lf the noncovalentaremaintainingthis stableconformationdisrupted,the moleculebecomesaflexiblechainthat usuallyhasnobiologicalvalue.many unstableconformationsone stable foldedconformationrandom thermal energy causesthe polymer chain to writhe and rotate.
However,the shapesof most biological macromolecules are highly constrained becauseofthe many we ak noncoualent bonds that form between different parts of the samemolecule. If these noncovalent bonds are formed in sufficient numbers, thepolyrner chain can strongly prefer one particular conformation, determined bythe linear sequenceof monomers in its chain. Most protein molecules and manyof the small RNA molecules found in cells fold tightly into one highly preferredconformation in this way (Figure 2-31).The four types of noncovalent interactions important in biologicalmolecules were described earlier, and they are reviewed in Panel 2-3 (pp.110-lll).
Although individuallyveryweak, these interactions cooperate to foldbiological macromolecules into unique shapes.In addition, theycan also add upto create a strong attraction between two different molecules when thesemolecules fit together very closely,like a hand in a glove.This form of molecularinteraction provides for great specificity, inasmuch as the multipoint contactsrequired for strong binding make it possible for a macromolecule to select outthrough binding-just one of the many thousands of other types of moleculespresent inside a cell.
Moreover, because the strength of the binding depends onthe number of noncovalent bonds that are formed, interactions of almost anyaffinity are possible-allowing rapid dissociation when necessary.Binding of this type underlies all biological catalysis,making it possible forproteins to function as enzymes. Noncovalent interactions also allow macromolecules to be used as building blocks for the formation of larger structures. Incells, macromolecules often bind together into large complexes, thereby forming intricate machines with multiple moving parts that perform such complextasks as DNA replication and protein synthesis (Figure 2-32).SUBUNITSMACROMOLECULESc o v a l e n tb o n d sn o n c o v a l e nbt o n d sMACROMOLECULARASSEMBLIESe g , s u g a r sa, m i n o a c i d s ,and nucleotides30 nmeg,globularproteinsand RNAe 9., ribosomeFigure2-32 Smallmolecules,proteins,and a ribosomedrawn approximatelyto scale.Ribosomesarea centralpart of the machinerythat the(proteinand RNAmolecules).cellusesto makeproteins:eachribosomeisformedasa complexof about90 macromoleculesCATALYSISANDTHEUsEOFENERGYBYCELLS65SummaryLiuing organismsare autonomous, self-propagatingchemical systems.Theyare madefrom a distinctiue and restrictedset of small carbon-basedmoleculesthat are essentially the samefor eueryliuing species.Each of thesemoleculesis composedof a smallset of atoms linked to each other in a preciseconftguration through coualent bonds.The main categoriesare sugars,fatty acids,amino acids,and nucleotides.Sugarsare aprimary sourceof chemical energyfor cellsand can be incorporated into polysaccharides for energy storage.Fatty acids are also important for energy storage,but theirmost critical function is in the formation of cell membranes.Polymers consisting ofamino acids constitute the remarkably diuerseand uersatilemacromoleculesknownas proteins.
Nucleotidesplay a central part in energy transfer.They are also the subunits for the informational macromolecules,RNAand DNA.Most of the dry massof a cell consistsof macromoleculesthat hauebeenproducedas linear polymersof amino acids (proteinsl or nucleotides(DNA and RNA),coualentlylinked to each other in an exact ordex Most of the protein moleculesand many of theRNAsfold into a unique conformation that depends on their sequenceof subunits.This folding processcreatesunique surfaces,and it depends on a large set of weakattractions produced by noncoualentforces between atoms. Theseforces are of fourtypes:electrostaticattrqctions, hydrogen bonds, uan der Waals attractions, and aninteraction between nonpolar groups caused by their hydrophobic expulsion f'romwater. The same set of weak forcesgouernsthe specific binding of other moleculestomacromolecules,making possible the myriad associations between biologicalmoleculesthat produce the structure and the chemistrv of a cell.CATALYSISANDTHEUSEOFENERGYBYCELLSOne property of living things above all makes them seem almost miraculouslydifferent from nonliving matter: they create and maintain order, in a universethat is tending always to greater disorder (Figure 2-33).
To create this order, thecells in a living organism must perform a never-ending stream of chemical reactions. In some of these reactions, small organic molecules-amino acids, sugars,nucleotides, and lipids-are being taken apart or modified to supply the manyother small molecules that the cell requires. In other reactions, these smallmolecules are being used to construct an enormously diverse range of proteins,nucleic acids, and other macromolecules that endow living systems with all oftheir most distinctive properties.
Each cell can be viewed as a tiny chemical factory performing many millions of reactions every second.Figure2-33 Order in biologicalstructures.Well-defined,ornate,and beautifulspatialpatternscan besize:(A)proteinmoleculesinIn orderof increasingfoundat everylevelof organizationin livingorganisms.the coat of a virus;(B)the regulararrayof microtubulesseenin a crosssectionof a spermtail; (C)surfacecontoursof a pollengrain(a singlecell);(D)close-upof the wing of a butterflyshowingthe patterncreatedby scales,eachscalebeingthe productof a singlecell;(E)spiralarrayof seeds,madeof millionsof cells,inthe headof a sunflower.(A,courtesyof R.A.Grantand J.M.Hogle;B,courtesyof LewisTilney;C,courtesyofColinMacFarlaneand ChrisJeffree;D and E,courtesyof KjellB.Sandved.)66Chapter2:CellChemistryand Biosynthesisotecuelmoleculemoleculemoleculemorecuremotecutec a t a l y s ibs ye n z y m e15A B B R E V I A T EADo-o-o-a-a-oe n z y m e2e n z y m e3e n z y m e4e n z y m e5Figure2-34 How a set ofenzyme-catalyzedreactionsgeneratesa metabolic pathway.EachenzymeIn this example,a setof enzymescatalyzesa particularchemicalreaction,leavingthe enzymeunchanged.actingin seriesconvertsmoleculeA to moleculeF,forminga metabolicpathway.CellMetabolismls Organizedby EnzymesThe chemical reactions that a cell carries out would normally occur only atmuch higher temperatures than those existing inside cells.
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