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Cofactors
Chemicalspeciesthatparticipateincatalysisofenzymaticreactions.Cofactorsarerequiredbyinactiveapoenzymestoconvertthemtoactiveholoenzymes.Typeofcofactors:
Someenzymescontainingorrequiringinorganicelementsascofactors
Somecoenzymesservingastransientcarriersofspecificatomsorfunctionalgroups
Majorcoenzymes
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Phosphatehydrolysis
Hydrolysisreactionsforsomebiochemicallyimportantphosphatecompounds.
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Thelabilephosphategroupofeachcompoundisshowninyellow.Themorestablereactionproduct,Piisingray.Ascaleofphosphatetransferpotentialisshowntotheright.
ATPproducts
HydrolysisofATP
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FormationofSadenosylmethionine
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NADandNADP
Formulas:
OxidizedandreducedfromofNAD(andNADP)
ThepyridineringofNAD+isreducedbyadditionofahydrideiontoC4whenNAD+isconvertedtoNADH(andwhenNADP+isconvertedtoNADPH).InNADP,the2'hydroxylgroupofthesugarringofadenosineisphosphorylated.Thereactivecenterofthesecoenzymesisshowninred.SomereactionscatalyzedbyNAD+
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TheUVabsorptionspectraofNAD+andNADH.
Reductionofthenicotinamideringproducesanew,broadabsorptionbandwithamaximumat340nm.TheproductionofNADHduringanenzymecatalyzedoxidationcanbeconvenientlyfollowedbyobservingtheappearenceoftheabsorbaanceat340nm.LactatedehydrogenaseaNADdependentenzyme
LactatedehydrogenazecatalysesthereversibleoxidationoflactateMechanismoflactatedehydrogenase.
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His195,abasecatalystintheactivesite,abstractsaprotonfromtheC2hydroxylgroupoflactate,facilitatingtransferofthehydrideion(H)fromC2ofthesubstratetoC4oftheboundNAD+.Becauselactatedehydrogenasehasaspecificbindingsiteforthecarboxamidegroup(C(O)NH2)ofNAD+,onlyonefaceofthepyridineringispositionedtowardlactateandthereforethehydrideionisalwaysaddedtothesameface.Arg171formsanionpairwiththecarboxylategroupofthesubstrate.Inthereversereaction,Histransferredstereospecificallyfromthereducedcoenzyme,NADH,toC2oftheoxidizedsubstrate,pyruvate.Orderedkineticmechanismforlactatedehydrogenase
Intheforwarddirection,thecoenzymeNAD+isboundfirstanditsreducedform,NADH,isreleasedlast.
FADandFMN
FAD:flavinadenincdinucleotideFMN:flavinmononucleotidebotharederivedfromribofiavin,orvitaminB2Riboflavinconsistofthefivecarbonalcoholribitol(areducedformofribose)linkedtoanitrogen
atomofisoalloxazine,aheterocyclicringssystem.LikeNAP+andNADP+,FADcontainsAMPandapyrophosphatclinkage.
Riboflavinanditscoenzymes
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Riboflaivin.Ribitolislinkedtotheisoalloxazineringsystem.
Flavinmononucleotide(FMNblack)andflavinadeninedinucleotide(FADblackandblue).Thereactivecenterisshowninred.
ReductionandreoxidationofFMNorFAD
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TheconjugateddoublebondsbetweenN1andN5arereducedbyadditionofahydrideionandaprotontoformFMNH2orFADH2,respectively,thehydroquinoneformofeachcoenzyme.Oxidationoccursintwosteps.Asingleelectronisremovedbyaoneelectronoxidizingagent,withlossofaproton,toformarelativelystablefreeradicalintermediate.ThissemiquinoneisthenoxidizedbyremovalofaprotonandanelectrontoformfullyoxidizedFMNorFAD.
Reactionscalatyzedbyflavoproteins
Mechanismoftheflavindependentglutathionereductasereaction.
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Thefirststeps,notshown,involvethereductionofFADtoFADH2byNADPHandthebindingofglutathione(glutathioneisasulfhydrylcompound).ThemechanismbywhichoxidizedglutathioneisreducedbytheE.FADH2isshown.
Vitaminsandvitaminlikenutrients
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Majorvitaminsrequiredinhumannutrition
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Niacin
PellagraAclinicaldeficiencysyndromemanifestedinskin,nervoussystem,anddigestivetractduetodeficiencyofniacin.LesionofskinexposedtolightSpinalpain,digestvedisturbance
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VitaminB1(Thiamine)
Structureofthiamine
Mechanismofthiaminepyrophosphateaction
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Mechanismofyeastpyruvatedecarboxylase
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ThepositivechargeofthethiazoliumringofTPPattractselectrons,weakeningthebondbetweenC2andhydrogen.Thisprotonispresumablyremovedbyabasicresidue.Ionizationgeneratesaresonancestabilizeddipolarcarbanionknownasanylid(amoleculewithoppositechargesonadjacentatoms).ThenegativelychargedC2attackstheelectrondeficientcarbonylcarbonofthesubstratepyruvate,andthefirstproduct(CO2)isreleased.Twocarbonsofpyruvatearenowattachedtothethiazoleringaspartofaresonancestabilizedcarbanion.Inthefollowingstep,protonationofthecarbanionproduceshydroxyethylthiaminepyrophosphate(HETPP).HETPPiscleaved,releasingacetaldehyde(thesecondproduct)andregeneratingtheylidformoftheenzymeTPPcomplex.TPPreformswhentheylidisprotonatedbytheenzyme.
ViatminB2(Riboflavin)
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VitaminB6(Pyridoxine)
B6vitaminsandpyridoxalphosphate
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Pyridoxalphosphate
BindingofPLP
Thefundamentalbiochemicalfunctionofpyridoxal5phosphateistheformationofaldimineswithaminoacidsthatstabilizethedevelopmentofcarbanioniccharacterattheandcarbonsofaminoacidsinintermediates.
Structuresofcatalyticintermediatesinpyridoxalphosphatedependentreactions.
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TheinitialaldimineintermediateresultingfromSchiff'sbaseformationbetweenthecoenzymeandtheaminogroupofanaminoacid(a).Thisaldimineisconvertedtotheresonancestabilizedintermediate(b)bylossofaprotonatthecarbon.Furtherenzymecatalyzedprotontransferstointermediates(c)and(d)mayoccur,dependingonthespecificityofagivenenzyme.Theenzymesusetheirgeneralacidsandbasestocatalyzetheseprotontransfers.
Mechanismoftransaminases
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Pyridoxalphosphate(continued)Someimportantmetabolicrolesofpyridoxalphosphate
Mechanismofactionofpyridoxalphosphateinglutamateoxalacetatetransaminase
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Mechanismofactionofpyridoxalphosphateinaspartatedecarboxylase
Tetrahydrofolate
Formationoftetrahydrofolate
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Pterinispartoffolate,amoleculecontainingpaminobenzoate(red)andglutamate(blue).Thepolyglutamateformsoftetrahydrofolateusuallycontainfiveorsixglutamateresidues.Thereactivecentersofthecoenzyme,N5andN10areshowninred.
Onecarbonderivativesoftetrahydrofolate
Thederivativescanbeinterconvertedenzymaticallybytheroutesshown.Rrepresentsthebenzoylpolyglutamateportionontetrahydrofolate
VitaminB12(Cobalamin)
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MetabolismofvitaminB12
ThemetabolicinterconversionsofB12areindicatedwithlightarrows,andB12requiringreactionsareindicatedwithheavyarrows.Otherrelatedpathwaysareindicatedwithdashedarrows.
Biotin
BiotinisacofactorforenzymesthatcatalyzecarboxylgrouptransferreactionsandATPdependentcarboxylationreactions.
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Thecarboxylategroupofbiotiniscovalentlyboundviaamidelinkagetotheaminogroupofalysineresidue(blue).ThereactivecenterofthebiotinmoietyisN1(red).
Reactioncatalyzedbypyruvatecarboxylase
Biotin,bicarbonateandATPreacttoformcarboxybiotinCarboxybiotinylenzymecomplexprovidesastableactivatedformofCO2thatcanbetransfererd
topyruvateTheenolateformofpyruvateattacksthecarboxylgroupofcarboxybiotinformingoxaloacetateand
regeneratingbiotin.
CoenzymeA
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VitaminC
Ascorbatemetabolizingpathwaysandtheirreagulationbystressinanimalcells
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Physiologicalandexperimentalagentsaffectingascorbatemetabolismareshowninitalicsthemostimportantmediatorsofstressresponseareunderlined.
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Tocopherolrecyclingbyascorbate
RBC:humanerythrocyte
ASC:ascorbate
T:alphatocopherol
X:freeradicalspecies
Recyclingoflipidperoxylradicalsbyalphatocopherol,andofalphatocopherolbyascorbate
Biosynthessisandassemblyofcollagen
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Step14occursinthecytoplasmofcollagensynthetizingcellsSteps6amd7occurintheextracelluralregionGal=galactoseGlc=glucose
Moretoprint:Structuressummary