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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

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