药学英语第五版第三单元.docx
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药学英语第五版第三单元.docx
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药学英语第五版第三单元
BiochemistrySeekstoExplainLifeinChemicalTerms
Themoleculesofwhichlivingorganismsarecomposedconformtoallthefamiliarlawsofchemistry,buttheyalso
interactwitheachotherinaccordancewithanothersetofprinciples,whichweshallrefertocollectivelyasthemolecular
logicoflife.Theseprinciplesdonotinvolveneworyetundiscoveredphysicallawsorforces.Instead,theyareasetof
relationshipscharacterizingthenature,function,andinteractionsofbiomolecules.
Iflivingorganismsarecomposedofmoleculesthatareintrinsicallyinanimate,howdothesemoleculesconferthe
remarkablecombinationofcharacteristicswecalllife?
Howisitthatalivingorganismappearstobemorethanthesumof
itsinanimateparts?
Philosophersonceansweredthatlivingorganismsareendowedwithamysteriousanddivinelifeforce,
butthisdoctrine(vitalism)hasbeenfirmlyrejectedbymodernscience.Thebasicgoalofthescienceofbiochemistryisto
determinehowthecollectionsofinanimatemoleculesthatconstitutelivingorganismsinteractwitheachothertomaintain
andperpetuatelife.Althoughbiochemistryyieldsimportantinsightsandpracticalapplicationsinmedicine,agriculture,
nutrition,andindustry,itisultimatelyconcernedwiththewonderoflifeitself.
AllMacromoleculesAreConstructedfromaFewSimpleCompounds
Mostofthemolecularconstituentsoflivingsystemsarecomposedofcarbonatomscovalentlyjoinedwithothercarbon
atomsandwithhydrogen,oxygen,ornitrogen.Thespecialbondingpropertiesofcarbonpermittheformationofagreat
varietyofmolecules.Organiccompoundsofmolecularweightlessthanabout500,suchasaminoacids,nucleotidase,and
monosaccharide,serveasmonomericsubunitsofproteins,nucleicacids,andpolysaccharides,respectively.Asingleprotein
moleculemayhave1,000ormoreaminoacids,anddeoxyribonucleicacidhasmillionsofnucleotides.
EachcellofthebacteriumEscherichiacoli(E.coli)containsmorethan6,000differentkindsoforganiccompounds,
includingabout3,000differentproteinsandasimilarnumberofdifferentnucleicacidmolecules.Inhumanstheremaybe
tensofthousandsofdifferentkindsofproteins,aswellasmanytypesofpolysaccharides(chainsofsimplesugars),a
varietyoflipids,andmanyothercompoundsoflowermolecularweight.
Topurifyandtocharacterizethoroughlyallofthesemoleculeswouldbeaninsuperabletask,itwerenotforthefact
thateachclassofmacromolecules(proteins,nucleicacids,polysaccharides)iscomposedofasmall,commonsetofmonomeric
subunits.Thesemonomericsubunitscanbecovalentlylinkedinavirtuallylimitlessvarietyofsequences,justasthe26
lettersoftheEnglishalphabetcanbearrangedintoalimitlessnumberofwords,sentiments,orbooks.
Deoxyribonucleicacids(DNA)areconstructedfromonlyfourdifferentkindsofsimplemonomericsubunits,the
deoxyribonucleotides,andribonucleicacids(RNA)arecomposedofjustfourtypesofribonucleotides.Proteinsarecomposed
of20differentkindsofaminoacids.Theeightkindsofnucleotidesfromwhichallnucleicacidsarebuiltandthe20
differentkindsofaminoacidsfromwhichallproteinsarebuiltareidenticalinalllivingorganisms.
Mostofthemonomericsubunitsfromwhichallmacromoleculesareconstructedservemorethanonefunctioninliving
cells.Thenucleotidesservenotonlyassubunitsofnucleicacids,butalsoasenergy-carryingmolecules.Theaminoacids
aresubunitsofproteinmolecules,andalsoprecursorsofhormones,neurotransmitters,pigments,andmanyotherkindsof
biomolecules.
Fromtheseconsiderationswecannowsetoutsomeoftheprinciplesinthemolecularlogicoflife:
Alllivingorganisms
havethesamekindsofmonomericsubunits.Thereareunderlyingpatternsinthestructureofbiologicalmacromolecules.The
identityofeachorganismispreservedbyitspossessionofdistinctivesetsofnucleicacidsandofproteins.
ATPIstheUniversalCarrierofMetabolicEnergy,LinkingCatabolismandAnabolism
Cellscapture,store,andtransportfreeenergyinachemicalform.Adenosinetriphosphate(ATP)functionsasthemajor
carrierofchemicalenergyinallcells.ATPcarriesenergyamongmetabolicpathwaysbyservingasthesharedintermediate
thatcouplesendergonicreactionstoexergonicones.TheterminalphosphategroupofATPistransferredtoavarietyof
acceptormolecules,whicharetherebyactivatedforfurtherchemicaltransformation.Theadenosinediphosphate(ADP)that
remainsafterthephosphatetransferisrecycledtobecomeATP,attheexpenseofeitherchemicalenergy(duringoxidative
phosphorylation)orsolarenergyinphotosyntheticcells(bytheprocessofphotophosphorylation).ATPisthemajor
connectinglink(thesharedintermediate)betweenthecatabolicandanabolicnetworksofenzyme-catalyzedreactionsinthe
cell.Theselinkednetworksofenzyme-catalyzedreactionsarevirtuallyidenticalinalllivingorganisms.
GeneticContinuityIsVestedinDNAMolecules
Perhapsthemostremarkableofallthepropertiesoflivingcellsandorganismsistheirabilitytoreproducethemselves
withnearlyperfectfidelityforcountlessgenerations.Thiscontinuityofinheritedtraitsimpliesconstancy,overthousands
ormillionsofyears,inthestructureofthemoleculesthatcontainthegeneticinformation.Veryfewhistoricalrecordsof
civilization,eventhoseetchedincopperorcarvedinstone,havesurvivedforathousandyears.Butthereisgoodevidence
thatthegeneticinstructionsinlivingorganismshaveremainednearlyunchangedovermuchlongerperiods;manybacteriahave
nearlythesamesize,shape,andinternalstructureandcontainthesamekindsofprecursormoleculesandenzymesasthose
thatlivedabillionyearsago.
HereditaryinformationispreservedinDNA,along,thinorganicpolymersofragilethatitwillfragmentfromtheshear
forcesarisinginasolutionthatisstirredorpipetted.Ahumanspermoregg,carryingtheaccumulatedhereditary
informationofmillionsofyearsofevolution,transmitstheseinstructionsintheformofDNAmolecules,inwhichthelinear
sequenceofcovalentlylinkednucleotidesubunitsencodesthegeneticmessage.Geneticinformationisencodedinthelinear
sequenceoffourkindsofsubunitsofDNA.Thedouble-helicalDNAmoleculehasaninternaltemplateforitsownreplication
andrepair.
TheStructureofDNAAllowsforItsRepairandReplicationwithNear-PerfectFidelity
Thecapacityoflivingcellstopreservetheirgeneticmaterialandtoduplicateitforthenextgenerationresultsfrom
thestructuralcomplementaritybetweenthetwohalvesoftheDNAmolecule.ThebasicunitofDNAisalinearpolymeroffour
differentmonomericsubunits,deoxyribonucleotides,arrangedinapreciselinearsequence.Itisthislinearsequencethat
encodesthegeneticinformation.TwoofthesepolymericstrandsaretwistedabouteachothertoformtheDNAdoublehelix,
inwhicheachmonomericsubunitinonestrandpairsspecificallywiththecomplementarysubunitintheoppositestrand.In
theenzymaticreplicationorrepairofDNA,oneofthetwostrandsservesasatemplatefortheassemblyofanother,
structurallycomplementaryDNAstrand.Beforeacelldivides,thetwoDNAstrandsseparateandeachservesasatemplatefor
thesynthesisofacomplementarystrand,generatingtwoidenticaldouble-helicalmolecules,oneforeachdaughtercell.If
onestrandisdamaged,continuityofinformationisassuredbytheinformationpresentontheotherstrand.
TheLinearSequenceinDNAEncodesProteinswithThree-DimensionalStructures
TheinformationinDNAisencodedasalinear(one-dimensional)sequenceofthenucleotideunitsofDNA,butthe
expressionofthisinformationresultsinathree-dimensionalcell.Thischangefromonetothreedimensionsoccursintwo
phases.AlinearsequenceofdeoxyribonucleotidesinDNAcodes(throughtheintermediary,RNA)fortheproductionofa
proteinwithacorrespondinglinearsequenceofaminoacids.Theproteinfoldsitselfintoaparticularthree-dimensional
shape,dictatedbyitsaminoacidsequence.Theprecisethree-dimensionalstructure(nativeconformation)iscrucialtothe
protein’sfunctionaseithercatalystorstructuralelement.Thisprincipleemerges:
Thelinearsequenceofaminoacidsinaproteinleadstotheacquisitionofauniquethree-dimensionalstructurebya
self-assemblyprocession.
Onceaproteinhasfoldedintoitsnativeconformation,itmayassociatenoncovalentlywithotherproteins,orwith
nucleicacidsorlipids,toformsupramolecularcomplexessuchaschromosomes,ribosomes,andmembranes.Thesecomplexesare
inmanycasesself-assembling.Theindividualmoleculesofthesecomplexeshavespecific,high-affinitybindingsitesfor
eachother,andwithinthecelltheyspontaneouslyformfunctionalcomplexes.
Individualmacromoleculeswithspecificaffinityforothermacromoleculesself-assembleintosupramolecularcomplexes.
NoncovalentInteractionsStabilizeThree-DimensionalStructures
Theforcesthatprovidestabilityandspecificitytothethree-dimensionalstructuresofmacromoleculesand
supramolecularcomplexesaremostlynoncovalentinteractions.Theseinteractions,individuallyweakbutcollectivelystrong,
includehydrogenbonds,ionicinteractionsamongchargedgroups,vanderWaalsinteractions,andhydrophobicinteractions
amongnonpolargroups.Theseweakinteractionsaretransient;individuallytheyformandbreakinsmallfractionsofasecond.
Thetransientnatureofnoncovalentinteractionsconfersaflexibilityonmacromole
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