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Manahan,StanleyE.ENVIRONMENTALBIOCHEMISTRYEnvironmentalChemistryBocaRaton:CRCPressLLC,200021ENVIRONMENTALBIOCHEMISTRY____________________________________________________21.1.BIOCHEMISTRYTheeffectsofpollutantsandpotentiallyharmfulchemicalsonlivingorganismsareofparticularimportanceinenvironmentalchemistry.Theseeffectsareaddressedunderthetopicof“ToxicologicalChemistry”inChapter22,andforspecificsub-stancesinChapter23.Thecurrentchapter,“EnvironmentalBiochemistry,”isdesignedtoprovidethefundamentalbackgroundinbiochemistryrequiredtounder-standtoxicologicalchemistry.Mostpeoplehavehadtheexperienceoflookingthroughamicroscopeatasinglecell.Itmayhavebeenanameba,aliveandoozingaboutlikeablobofjellyonthemicroscopeslide,oracellofbacteria,stainedwithadyetomakeitshowupmoreplainly.Or,itmayhavebeenabeautifulcellofalgaewithitsbrightgreenchloro-phyll.Eventhesimplestofthesecellsiscapableofcarryingoutathousandormorechemicalreactions.Theselifeprocessesfallundertheheadingofbiochemistry,thatbranchofchemistrythatdealswiththechemicalproperties,composition,andbiologically-mediatedprocessesofcomplexsubstancesinlivingsystems.Biochemicalphenomenathatoccurinlivingorganismsareextremelysophisti-cated.Inthehumanbodycomplexmetabolicprocessesbreakdownavarietyoffoodmaterialstosimplerchemicals,yieldingenergyandtherawmaterialstobuildbodyconstituentssuchasmuscle,blood,andbraintissue.Impressiveasthismaybe,considerahumblemicroscopiccellofphotosyntheticcyanobacteriaonlyaboutamicrometerinsize,whichrequiresonlyafewsimpleinorganicchemicalsandsunlightforitsexistence.ThiscellusessunlightenergytoconvertcarbonfromCO2,hydrogenandoxygenfromH2O,nitrogenfromNO3-,sulfurfromSO42-,andphosphorusfrominorganicphosphateintoalltheproteins,nucleicacids,carbohydrates,andothermaterialsthatitrequirestoexistandreproduce.Suchasimplecellaccomplisheswhatcouldnotbedonebyhumansinevenavastchemicalfactorycostingbillionsofdollars.Ultimately,mostenvironmentalpollutantsandhazardoussubstancesareofconcernbecauseoftheireffectsuponlivingorganisms.Thestudyoftheadverse©2000CRCPressLLCeffectsofsubstancesonlifeprocessesrequiressomebasicknowledgeofbiochem-istry.Biochemistryisdiscussedinthischapter,withemphasisuponaspectsthatareespeciallypertinenttoenvironmentallyhazardousandtoxicsubstances,includingcellmembranes,DNA,andenzymes.Biochemicalprocessesnotonlyareprofoundlyinfluencedbychemicalspeciesintheenvironment,theylargelydeterminethenatureofthesespecies,theirdegrada-tion,andeventheirsyntheses,particularlyintheaquaticandsoilenvironments.Thestudyofsuchphenomenaformsthebasisofenvironmentalbiochemistry.1BiomoleculesThebiomoleculesthatconstitutematterinlivingorganismsareoftenpolymerswithmolecularmassesoftheorderofamillionorevenlarger.Asdiscussedlaterinthischapter,thesebiomoleculesmaybedividedintothecategoriesofcarbohydrates,proteins,lipids,andnucleicacids.Proteinsandnucleicacidsconsistofmacromole-cules,lipidsareusuallyrelativelysmallmolecules,carbohydratesrangefromrela-tivelysmallsugarmoleculestohighmolarmassmacromoleculessuchasthoseincellulose.Thebehaviorofasubstanceinabiologicalsystemdependstoalargeextentuponwhetherthesubstanceishydrophilic(“water-loving”)orhydrophobic(“water-hating”).Someimportanttoxicsubstancesarehydrophobic,acharacteristicthatenablesthemtotraversecellmembranesreadily.Partofthedetoxificationprocesscarriedonbylivingorganismsistorendersuchmoleculeshydrophilic,thereforewater-solubleandreadilyeliminatedfromthebody.21.2.BIOCHEMISTRYANDTHECELLThefocalpointofbiochemistryandbiochemicalaspectsoftoxicantsisthecell,thebasicbuildingblockoflivingsystemswheremostlifeprocessesarecarriedout.Bacteria,yeasts,andsomealgaeconsistofsinglecells.However,mostlivingthingsaremadeupofmanycells.Inamorecomplicatedorganismthecellshavedifferentfunctions.Livercells,musclecells,braincells,andskincellsinthehumanbodyarequitedifferentfromeachotheranddodifferentthings.Cellsaredividedintotwomajorcategoriesdependinguponwhetherornottheyhaveanucleus:eukaryoticcellshaveanucleusandprokaryoticcellsdonot.Prokaryoticcellsarefoundpredominatelyinsingle-celledorganismssuchasbacteria.2Eukaryoticcellsoccurinmulticellularplantsandanimals—higherlifeforms.MajorCellFeaturesFigure21.1showsthemajorfeaturesoftheeukaryoticcell,whichisthebasicstructureinwhichbiochemicalprocessesoccurinmulticellularorganisms.Thesefeaturesarethefollowing:•Cellmembrane,whichenclosesthecellandregulatesthepassageofions,nutrients,lipid-soluble(“fat-soluble”)substances,metabolicproducts,toxi-cants,andtoxicantmetabolitesintoandoutofthecellinteriorbecauseofitsvaryingpermeabilityfordifferentsubstances.Thecellmembrane©2000CRCPressLLCprotectsthecontentsofthecellfromundesirableoutsideinfluences.Cellmembranesarecomposedinpartofphospholipidsthatarearrangedwiththeirhydrophilic(“water-seeking”)headsonthecellmembranesurfacesandtheirhydrophobic(“water-repelling”)tailsinsidethemembrane.Cellmembranescontainbodiesofproteinsthatareinvolvedinthetransportofsomesubstancesthroughthemembrane.Onereasonthecellmembraneisveryimportantintoxicologyandenvironmentalbiochemistryisbecauseitregu