浙大食化考研.docx

浙大食化考研.docx

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浙大食化考研

Chapter3Proteins

5.1Introduction

Proteins

Proteinsaremoleculesofgreatsize,complexity,anddiversity.Theyarethesourceofdietaryaminoacids,bothessentialandnonessential.Attheelementallevel,proteinscontain50-55%carbon,6-7%hydrogen,20-23%oxygen,12-19%nitrogen,and0.2-3.0%sulfur.

Proteinsynthesisoccursinribosomes.Afterthesynthesis,someaminoacidconstituentsaremodifiedbycytoplasmicenzymes.Thischangeselementalcompositionofsomeproteins.Proteinsthatarenotenzymaticallymodifiedincellsarecalledhomoproteins,andthosethataremodifiedorcomplexedwithnonproteincomponentsarecalledconjugatedproteinsorheteroproteins.Thenonproteincomponentsareoftenreferredtoasprostheticgroups.Examplesofconjugatedproteinsincludenucleoproteins（ribosomes）,glycoproteins（ovalbumin,k-casein）,phosphoproteins（a-andb-casalns,kinases,phosphorylases）,lipoproteins（proteinsofeggyolk,severalplasmaproteins）,andmetalloproteins（hemoglobin,myoglobin,andseveralenzymes）.Glyco-andphosphoproteinscontaincovalentlylinkedcarbohydrateandphosphategroups,respectively,whereastheotherconjugatedproteinnoncovalentcomplexescontainingnucleicacids,lipids,ormetalions.Thesecomplexescanbedissociatedunderappropriateconditions.

Proteinsalsocanbeclassifiedaccordingtotheirgrossstructuralorganization.Thus,globularproteinsatethosethatexistinsphericalorellipsoidalshapes,resultingfromfoldingofthepolypeptidechain（s）onitself.Ontheotherhand,fibrousproteinsarerodshapedmoleculescontainingtwistedlinearpolypeptidechains（e.g.,tropomyosin,collagen,keratin,andelastin）.Fibrousproteinsalsocanheformedasaresultoflinearaggregationofsmallglobularproteins,suchasactinandfibrin.Amajorityofenzymesareglobularproteins,andfibrousproteinsinvariablyfunctionasstructuralproteins.

Thevariousbiologicalfunctionsofproteinscanbecategorizedasenzymecatalysts,structuralproteins,contractileproteins（myosin,actin,tubulin）,hormones（insulin,growthhormone）,transferproteins（serumalbumin,transferrin,hemoglobin）,antibodies（immunoglobulins）,storageproteins（eggalbumen,seedproteins）,andprotectiveproteins（toxinsandallergens）.Storageproteinsarefoundmainlyineggsandplantseeds.Theseproteinsactassourcesofnitrogenandaminoacidsforgerminatingseedsandembryos.Theprotectiveproteinsareapartofthedefensemechanismforthesurvivalofcertainmicroorganismsandanimals.

Allproteinsareessentiallymadeupofthesameprimary20aminoacids;however,someproteinsmaynotcontainoneorafewofthe20aminoacids.Thedifferencesinstructureandfunctionofthesethousandsofproteinsarisefromthesequenceinwhichtheaminoacidsarelinkedtogetherviaamidebonds.Literally,billionsofproteinswithuniquepropertiescanbesynthesizedbychangingtheaminoacidsequence,thetypeandratioofaminoacids,andthechainlengthofpolypeptides.

Allbiologicallyproducedproteinscanbeusedasfoodproteins.However,forpracticalpurposes,feedproteinsmaybedefinedasthosethatareeasilydigestible,nontoxic,nutritionallyadequate,functionallyuseableinfoodproducts,andavailableinabundance.Traditionally,milk,meats（includingfishandpoultry）,eggs,cereals,legumes,andoilseedshavebeenthemajorsourcesoffoodproteins.However,becauseoftheburgeoningworldpopulation,nontraditionalsourcesofproteinsforhumannutritionneedtobedevelopedtomeetthefuturedemand.Thesuitabilityofsuchnewproteinsourcesforuseinfoods,however,dependsontheircostandtheirabilitytofulfillthenormalroleofproteiningredientsinprocessedandhome-cookedfoods.Thefunctionalpropertiesofproteinsinfoodsarerelatedtotheirstructuralandotherphysico-chemicalcharacteristics.Afundamentalunderstandingofthephysical,chemicalnutritional,andfunctionalpropertiesofproteinsandthechangesthesepropertiesundergoduringprocessingisessentialiftheperformanceofproteinsinfoodsistobeimproved,andifneworlesscostlysourcesofproteinsaretocompetewithtraditionalfoodproteins.

Aminoacids

Aminoacidsarethe"buildingblocks"ofproteinswhichcontainbothbasicaminogroupsandacidiccarboxylgroups.α-aminoacidsarethebasicstructuralunitsofproteins.Theseaminoacidsconsistofanα-carbonatomcovalentlyattachedtoahydrogenatom,anaminogroup,acarboxylgroup,andasidechainRgroup.

Naturalproteinscontainupto20differentprimaryaminoacidslinkedtogetherviaamidebonds.TheseaminoacidsdifferonlyinthechemicalnatureofthesidechainRgroup.Thephysicochemicalproperties,suchasnetcharge,solubility,chemicalreactivity,andhydrogenbondingpotential,oftheaminoacidsaredependentonthechemicalnatureoftheRgroup.

AtneutralpHvaluesinaqueoussolutionsboththeaminoandthecarboxylgroupsareionized.Thecarboxylgrouplosesaprotonandobtainsanegativecharge,whiletheaminogroupgainsaprotonandhenceacquiresapositivecharge.Asaconsequence,aminoacidspossessdipolarcharacteristics.

Whentheaminogroupofoneaminoacidreactswiththecarboxylgroupofanotheraminoacid,apeptidebondisformedandamoleculeofwaterisreleased.ThisC-Nbondjoinsaminoacidstogethertoformproteins。

Proteinstructure

Proteinsaremacromoleculeswithdifferentlevelsofstructuralorganization.Theprimarystructureofproteinsrelatestothepeptidebondsbetweencomponentaminoacidsandalsototheaminoacidsequenceinthemolecule.Researchershaveelucidatedtheaminoacidsequenceinmanyproteins.

Thesecondarystructureofproteinsinvolvesfoldingtheprimarystructure.Hydrogenbondsbetweenamidenitrogenandcarbonyloxygenarethemajorstabilizingforce.Thesebondsmaybeformedbetweendifferentareasofthesamepolypeptidechainorbetweenadjacentchains.

Thetertiarystructureofproteinsinvolvesapatternoffoldingofthechainsintoacompactunitthatisstabilizedbyhydrogenbonds,vanderWaalsforces,disulfidebridges,andhydrophobicinteractions.Thetertiarystructureresultsintheformationofatightlypackedunitwithmostofthepolaraminoacidresidueslocatedontheoutsideandhydrated.

Largemoleculesofmolecularweightsaboveabout50,000mayformquaternarystructuresbyassociationofsubunits.Thesestructuresmaybestabilizedbyhydrogenbonds,disulfidebridges,andhydrophobicinteractions.

5.2DenaturationofProteins

Denaturationisaprocessthatchangesthemolecularstructurewithoutbreakinganyofthepeptidebandsofaprotein.Theprocessispeculiartoproteinsandaffectsdifferentproteinstodifferentdegrees,dependingonthestructureofaprotein.Denaturationcanbebroughtaboutbyavarietyofagents,ofwhichthemostimportantareheat,pH,saltsandsurfaceeffects.Consideringthecomplexityofmanyfoodsystems,itisnotsurprisingthatdenaturationisacomplexprocessthatcannoteasilybedescribedinsimpleterms.Denaturationusuallyinvolveslossofbiologicalactivityandsignificantchangesinsomephysicalorfunctionalpropertiessuchassolubility.Thedestructionofenzymeactivitybyheatisanimportantoperationinfoodprocessing.Inmostcasesdenaturationisnonreversible;however,therearesomeexceptions,suchastherecoveryofsometypesofenzymeactivityafterheating.Heatdenaturationissometimesdesirable-forexample,thedenaturationofwheyproteinsfortheproductionofmilkpowderusedinbaking.

Denaturationmaysometimesresultinflocculationofglobularproteinsbutmayalsoleadtotheformationofgels.Foodsmaybedenatured,andtheirproteinsdestabilized,duringfreezingandfrozenstorage.Fishproteinsareparticularlysusceptibletodestabilization.

Proteindenaturationandcoagulationareaspectsofheatstabilitythatcanberelatedtotheaminoacidcompositionandsequenceoftheprotein.Denaturationcanbedefinedasamajorchangeinthenativestructurethatdoesnotinvolvealterationoftheaminoacidsequence.Theeffectofheatusuallyinvolvesachangeinthetertiarystructure,leadingtoalessorderedarrangementofthepolypeptidechains.Thetemperaturerangeinwhichdenaturationandcoagulationofmostproteinstakeplaceisabout55to75℃.Therearesomenotableexceptionstothisgeneralpattern.Caseinandgelatinareexamplesofproteinsthatcanbeboiledwithoutapparentchangeinstability.Theexceptionalstabilityofcaseinmakesitpossibletoboil,sterilize,andconcentratemilk,withoutcoagulation.

DenaturingAgents

PhysicalAgents

TemperatureandDenaturation

Heatisthemostcommonlyusedagentinfoodprocessingandpreservation.Proteinsundergovaryingdegreesofdenaturationduringprocessing.Thiscanaffecttheirfunctionalpropertiesinfoods,anditisthereforeimportanttounderstandthefactorsaffectingproteindenaturation.

Whenaproteinsolutionisgraduallyheatedaboveacriticaltemperature,itundergoesasharptransitionfromthenativestatetothedenaturedstate.Thetemperatureatthetransitionmidpoint,wheretheconcentrationratioofnativeanddenaturedstatesis1，isknowneitherasthemeltingtemperatureTm,orthedenaturationtemperatureTd.Themechanismoftemperature-induceddenaturationishighlycomplexandinvolvesprimarilydestabilizadonofthemajornoncovalentinteractions.Hydrogenbonding,electrostatic,andvanderWaalsinteractionsareexothermic（enthalpydriven）innature.Therefore，theyaredestabilizedathightemperaturesandstabilizedatlowtemperatures.However，sincepeptidehydrogenbondsinproteinsaremostlyburiedintheinterio