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3.2: TCA-siklus - Biologie

3.2: TCA-siklus - Biologie


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3.2: TCA -siklus

'n Omgekeerde TCA-siklus 2-oksosuur:ferredoksienoksidoreduktase wat C-C-bindings van CO maak 2

2-oksoglutaraat: ferredoksienoksidoreduktase (OGOR) is 'n tiamienpyrofosfaat (TPP) en [4Fe-4S] groepsafhanklike ensiem uit die reduktiewe trikarboksielsuur (rTCA) siklus wat CO regmaak2 tot suksiniel-CoA, wat 2-oksoglutaraat en CoA vorm. Hier rapporteer ons 'n OGOR uit die rTCA -siklus van Magnetococcus marinus MC-1, saam met al drie potensiële ferredoksien (Fd) redoksvennote. Ons demonstreer MmOGOR werk tweerigting (beide CO2-binding en 2-oksoglutaraat oksideerbaar), en dat slegs een Fd (MmFd1) ondersteun doeltreffende katalise. Ons 1.94-Å en 2.80-Å resolusie kristalstrukture van inheemse en substraatgebonde vorms van MmOGOR onthul die determinante van substraatspesifisiteit en CoA-binding in 'n OGOR, en belig die [4Fe-4S] -groepomgewing, wat die elektroniese kanaal uitbeeld MmFd1 moet aan die gebonde-TPP gekoppel word. Strukturele en biochemiese data identifiseer Glu45α verder as 'n mobiele residu wat katalitiese vooroordeel teenoor CO beïnvloed2-fiksering, hoewel dit geen direkte kontak met TPP-gebonde tussenprodukte maak nie, wat daarop dui dat reaksierigting kan ingestel word deur interaksies met die tweede laag. (149 van 150 woorde beperk).

Sleutelwoorde: 2-oksoglutaraat: ferredoksienoksidoreduktase CO2-fiksasie Magnetococcus marinus MC-1 katalitiese vooroordeel elektron-oordrag ferredoksien yster-swael trosse reduktiewe trikarboksielsuur (rTCA) siklus tiamienpyrofosfaat.

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Figuur 1.. 2-oksosuur:ferredoksienoksidoreduktase (OFORs) en die...

Figuur 1 .. 2-oksosuur: ferredoksienoksidoreduktases (OFOR's) en die domeinreëlings van struktureel gekenmerkde OFOR's.

Figuur 2.. Spektroskopiese eienskappe van Mm OGOR.

Figuur 2.. Spektroskopiese eienskappe van Mm OGOR.

(A) UV-vis spektra van 5 μM soos gesuiwer Mm OGOR…

Figuur 3 .. Die struktuur van Mm OGOR…

Figuur 3 .. Die struktuur van Mm OGOR en die vergelyking van aktiewe terreine tussen OFORs.

Figuur 4 .. Die omgewing van die proksimale ...

Figuur 4 .. Die omgewing van die proksimale [4Fe-4S] -groep in struktureel gekarakteriseerde OFOR's.

Figuur 5.. Struktuur van Mm OGOR met...

Figuur 5 .. Struktuur van Mm OGOR met 2-oksoglutaraat en suksiniel-CoA gebind.


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TCA -siklusdefekte en kanker: wanneer metabolisme die toestand van Redox afstem

Ingebore defekte van die trikarboksielsuur (TCA) siklusensieme is al meer as twintig jaar bekend. Tot onlangs is slegs resessiewe mutasies beskryf, wat, hoewel dit gelei het tot ernstige multisisteme sindrome, nie geneig was tot die aanvang van kanker nie. In die afgelope tien jaar is 'n oorsaaklike rol in karsinogenese gedokumenteer vir oorgeërfde en verworwe veranderinge in drie TCA-siklus-ensieme, suksinaatdehidrogenase (SDH), fumaraathidrase (FH), en isositraatdehidrogenase (IDH), wat dui op metaboliese veranderinge as die onderliggende kenmerk van kanker. Hierdie vraestel som die neoplastiese veranderinge van die TCA-siklus-ensieme op met die fokus op die generering van pseudohipoksiese fenotipe en die verandering van epigenetiese homeostase as die belangrikste tumor-bevorderende effekte van die TCA-siklus wat defekte beïnvloed. Verder debatteer ons oor die vermoë van hierdie mutasies om sellulêre redokstoestand te beïnvloed en om karsinogenese te bevorder deur 'n impak op redoksbiologie.

1. Inleiding

Kankerselle verskil van normale as gevolg van 'n oorvloed aan onkogenes-aangedrewe biochemiese veranderinge wat ontwerp is om 'n hoë groei en verspreiding te handhaaf [1]. Die eerste tumor-spesifieke verandering in metabolisme is aan die begin van die 20ste eeu deur Warburg [2] aangemeld. Sy waarnemings het getoon dat die metabolisme van kankerselle staatmaak op 'n verhoogde glikolitiese vloed wat selfs in die teenwoordigheid van suurstof gehandhaaf word ('aërobiese glikolise' of 'Warburg -effek'), sonder 'n gepaardgaande toename in die oksidatiewe fosforileringstempo. Die oorskakeling van asemhaling na glikolise word gewoonlik beskou as 'n gevolg, eerder as 'n oorsaak, van kanker. In die afgelope dekade het die ontdekking dat geërfde en verworwe veranderings in sommige ensieme van trikarboksielsuur (TCA) siklus 'n oorsaaklike rol in karsinogenese het egter hierdie siening verander, wat wys op veranderde metabolisme as die onderliggende kenmerk van neoplastiese transformasie. Hierdie veranderinge bestaan ​​uit kiemlyndefekte in gene wat subeenhede van SDH en FH kodeer, sowel as somatiese mutasies in koderingsvolgorde vir IDH. Saam met metabolomiese studies wat die verandering van HIF-afhanklike seinweg en epigenetiese dinamika as die belangrikste tumorbevorderende effek van hierdie mutasies dokumenteer, ondersteun 'n toenemende hoeveelheid bewyse ook hoe veranderinge in die TCA-siklusensieme tumorigenese kan bevoordeel deur 'n invloed op die sellulêre redoksietoestand te hê. Daarom vat ons in hierdie artikel die prooncogeniese defekte in die TCA-siklusensieme op wat hul betrokkenheid by die afstemming van redoksomgewing en die betrokkenheid van redoksafhanklike tumorigeniese sein bespreek.

2. Grondbeginsels van die TCA -siklus

Die TCA -siklus is 'n kernweg vir die metabolisme van suikers, lipiede en aminosure [3]. Dit word gewoonlik aangebied in 'n naïewe perspektief van 'n sikliese mitochondriale roete wat die asetielgroep van asetiel-koënsiem A voortdurend oksideer2, genereer NADH en FADH2, wie se elektrone die mitochondriale respiratoriese ketting aanvuur vir ATP-generering. Die TCA-siklus begin met die kondensasie van asetiel-CoA met oksaloasetaat om sitraat te vorm, gekataliseer deur sitraat sintase. Sitraat kan na die sitoplasma uitgevoer word, waar dit as voorloper vir lipiedbiosintese gebruik word of in die mitochondria agterbly, waar dit deur akonitase na isositraat omgeskakel word. In die volgende stap, α-ketoglutaraat (α-KG), gevorm deur die oksidatiewe dekarboksilering van isositraat wat deur IDH gekataliseer word, word omgeskakel na succinyl-CoA deur 'n verdere dekarboksilering deur die α-KG dehidrogenase kompleks. Succinyl-CoA word dan omskep in suksinaat deur die succinyl-CoA sintetase. Fumaraat, geproduseer deur suksinaatoksidasie wat deur die SDH -kompleks gekataliseer word, word deur FH gehidreer om te malateer. Oksidasie van malaat, gekataliseer deur malaatdehidrogenase, regenereer uiteindelik oxaloacetate en verseker sodoende die voltooiing van die siklus (Figuur 1). Op die blote biochemiese standpunt is die TCA-siklus in nie-tumorcelle in twee fases verdeel: (i) dekarboksilering, waarin sitraat omgeskakel word na succinyl-CoA wat twee CO vrystel2 molekules (ii) reduktief, wat die opeenvolgende oksidasies van suksinaat na oksaloasetaat behels. Dit is interessant dat opkomende bevindings van die afgelope jaar die hipotese ondersteun dat (in) selstelsels soos (i) kankerselle wat mutasies in kompleks I of kompleks III van die elektrontransportketting (ETC) bevat, (ii) pasiënt-afgeleide nierkarsinoom selle met mutasies in FH, (iii) selle met normale mitochondria onderworpe aan akute farmakologiese ENS, inhibisie, sowel as (iv) tumorselle wat aan hipoksie blootgestel is, kan die eerste fase van die siklus in die teenoorgestelde rigting voortgaan deur die reduktiewe karboksilering van α-KG om sitraat te vorm. Dit stel selle in staat om asetiel-koënsiem A te produseer De novo lipogenese en hul lewensvatbaarheid [4-6]. Alhoewel mitochondria in fisiologiese en rustende toestande nodig en voldoende is om die siklus uit te voer, is isovorme van sommige van sy ensieme ook in die sitosol gevind. Dit verseker 'n dubbele kompartementalisering (sitosolies en mitochondriale) van reaksies en metaboliete, wat die siklus kan reageer op omgewings- en ontwikkelingsseine, waardeur die anaboliese reaksies sowel as die aanvulling van die ATP-vervaardigende masjinerie. Die TCA-siklus is ook 'n belangrike pad vir interomsetting van metaboliete wat voortspruit uit transaminering en deaminering van aminosure en verskaf die substrate vir aminosure-sintese deur transaminering, sowel as vir glukoneogenese en vetsuursintese. Regulering van die TCA-siklus hang hoofsaaklik af van 'n aanbod van geoksideerde kofaktore: in weefsels waar die primêre rol daarvan energieproduksie is, is 'n respiratoriese beheer bemiddel deur respiratoriese ketting en oksidatiewe fosforilering werksaam. Hierdie aktiwiteit maak staat op beskikbaarheid van NAD + en ADP, wat weer afhang van die tempo van benutting van ATP in chemiese en fisiese werk.


Redoksveranderinge veroorsaak deur TCA -siklusdefekte. Redoksveranderinge wat veroorsaak word deur mutasies in SDH, FH en IDH word getoon. Verlies aan funksie van SDH verhoog ROS-vlakke wat lei tot DNA-mutasies en HIF-1α stabilisering. IDH1 en IDH2 (nie getoon nie) mutasies verlaag GSH en NADPH vlakke. (R)-2-HG, geproduseer deur onkogene mutasies in IDH1 en IDH2, veroorsaak ROS-akkumulasie. Defekte in FH stimuleer kerntranslokasie van Nrf2 en die transkripsie van antioksidante ensieme deur die suksinasie van Keap1. Ensieme en metaboliete betrokke by gewasvorming en redoksveranderinge is in rooi. Blou pyle dui TCA -siklusreaksies aan. Gestippelde pyle dui paaie aan wat die selredoksietoestand moduleer.

3. Genetiese defekte in die TCA-siklus

Genetiese defekte wat die TCA-siklus ensieme beïnvloed, is al meer as twee dekades bekend. Tot onlangs is slegs resessiewe mutasies gedokumenteer waarvan die kliniese gevolge soortgelyk was aan veranderinge in die elektrontransportketting (ETC) en oksidatiewe fosforilering [7]. Hierdie defekte was geassosieer met multisisteemafwykings en ernstige neurologiese skade, maar geen kanker-aanleg, as gevolg van baie aansienlike verswakte ATP-vorming in die sentrale senuweestelsel. In die afgelope tien jaar is dominante defekte wat met onkogenese geassosieer word beskryf in sitoplasmiese en mitochondriale isovorme van drie kerngekodeerde ensieme, SDH, FH en IDH, wat dit moontlik maak om die ekstrametaboliese rolle van die TCA-siklusmetaboliete en hul sein na tumorvorming te ondersoek.

3.1. Suksinaat dehidrogenase

Die SDH-kompleks (ook bekend as suksinaat:ubikinoonoksidoreduktase of mitochondriale kompleks II) is 'n hoogs gekonserveerde heterotetrameriese tumoronderdrukker, saamgestel uit twee katalitiese subeenhede (SDHA en SDHB), wat in die mitochondriale matriks uitsteek, en twee hidrofobiese subeenhede (SDHC en SDHD) ), wat die katalitiese komponente aan die innerlike mitochondriale membraan veranker en ook die ubiquinoon bind [8]. Al die subeenhede word deur kerngenoom gekodeer en, anders as die meeste van die TCA-siklus ensieme, het geen sitosoliese eweknieë nie. SDH kataliseer die oksidasie van suksinaat na fumaraat in die TCA-siklus met die gelyktydige reduksie van ubiquinone na ubiquinol in die ENS. 'N Dekade gelede is mutasies in SDHB-, SDHC- en SDHD -subeenhede geïdentifiseer by pasiënte met oorerflike paragangliome (hPGL's) en feochromocytome (PCC's), 'n seldsame neuro -endokriene neoplasma van die chromaffienweefsel van die adrenale medulla of afkomstig van die parasimpatiese weefsel van die kop en nek paraganglioom, onderskeidelik [9-12]. Meer onlangs is mutasies in SDHA en die SDH -samestellingsfaktor 2 (SDHAF2), wat nodig is vir flavinasie van SDH [13, 14], geassosieer met hPGL/PCC -sindroom [15]. Die genetiese defekte in die SDH -gene wat vir die hPGL sowel as PCC vatbaar is, is heterosigotiese kiemlynmutasies, wat die inaktivering van die proteïen veroorsaak en die neoplastiese transformasie ontwikkel as gevolg van verlies aan heterosigositeit, wat veroorsaak word deur die volledige verlies van die ensiemfunksie met 'n sekonde mutageniese treffer (gewoonlik verwydering) [16]. Benewens hPGL en PCC, is 'n aantal ander neoplasmas geassosieer met mutasies in SDH -gene, insluitend gastro -intestinale stromale gewasse, nierselkanker, skildkliergewasse, neuroblastome en testikulêre seminoom [8].

3.2. Fumaraat Hidratase

FH is 'n homotetramere TCA-siklusensiem wat die stereospesifieke en omkeerbare hidrasie van fumaraat na L-malaat kataliseer. Homosigotiese FH -tekorte lei tot fumaric aciduria [17], wat gekenmerk word deur 'n vroeë aanvang van ernstige enkefalopatie en psigomotoriese vertraging inteendeel, heterosigotiese FH -mutasies vatbaar vir veelvuldige kutane en uteriene leiomyome (MCUL), sowel as oorerflike leiomyomatose en nierkanker ( HLRCC) [18, 19]. In die besonder is die niertumore in HLRCC, waarvan die morfologiese spektrum papillêre tipe II, tubulopapilar, buisvormige, versamelkanaal en helder selkarsinoom insluit, besonder aggressief. Toenemende bewyse dui daarop FH mutasies kan ook betrokke wees by die patogenese van bors-, blaas- en Leydig -selgewasse [20, 21]. Die mees algemene tipes tumor-predisponerende genetiese defekte is missense mutasies (57%), gevolg deur raamverskuiwing en nonsens mutasies (27%), sowel as grootskaalse delesies, invoegings en dupliserings [22]. Soos SDH, is ensiematiese aktiwiteit van FH heeltemal afwesig in HLRCC as gevolg van die verlies van die wildtipe alleel in die getransformeerde sel.

3.3. Isositraat dehidrogenase

IDH is 'n lid van die β-dekarboksilerende dehidrogenase familie van ensieme en kataliseer die oksidatiewe dekarboksilering van isositraat om 2-oksoglutaraat te produseer (α-KG) en CO2 in die TCA -siklus. Kerne -genoom kodeer vir drie isoforme van IDH: IDH1 en IDH2 is NADP + -afhanklike homodimere, terwyl IDH3 'n NAD + -betrokke heterotetrameriese ensiem is. Terwyl IDH1 in sitoplasma en peroksisome voorkom, word IDH2 en IDH3 uitsluitlik in die mitochondriale matriks gelokaliseer, en hoewel al drie isoforme isositraat kan dekarboksileer, is IDH3 die belangrikste vorm van IDH -funksionering in die TCA -siklus onder fisiologiese toestande, terwyl IDH1 en IDH2 is veral betrokke by die reduktiewe glutamienmetabolisme, onder hipoksie en ETC -veranderinge [4, 5, 23]. Alhoewel dit 'n sentrale rol in energieproduksie speel, was daar tot dusver geen verslae van kankergeassosieerde mutasies in enige van die IDH3-subeenhede nie. Omgekeerd het genome wye mutasie-ontledings en hoë-deurset diep volgordebepaling die teenwoordigheid van mutasies in IDH1 of die mitochondriale eweknie IDH2 in 70% van graad II-III gliomas en sekondêre glioblastomas [24, 25] onthul. Sedert hierdie aanvanklike verslae is mutasies in IDH1 en IDH2 by 16-17% van die pasiënte met akute myeloïde leukemie, in 20% van die angioimmunoblastiese T-sel limfome [26] geïdentifiseer en in 'n verskeidenheid ander kwaadaardighede by laer frekwensies opgemerk [ 27, 28] soos B-akute limfoblastiese leukemieë, skildklier-, kolorektale en prostaatkanker [29, 30]. Anders as SDH en FH mutasies in onderskeidelik hPGL en HLRCC, IDH1 en IDH2 mutasies is somaties en monoallelies. Verder, terwyl mutasies in SDH en FH kom dwarsdeur die geen voor, die meerderheid IDH -mutasies wat in gliomas en AML geïdentifiseer word, is veranderinge in die aminosuurreste R132 in IDH1 en óf R172 óf R140 in IDH2 [31]. As gevolg van hierdie veranderinge kan gemuteerde IDH's nie die oksidatiewe dekarboksilering van isositraat doeltreffend kataliseer nie en 'n neomorfe katalitiese aktiwiteit verkry wat 'n NADPH-afhanklike vermindering van α-KG in die onkometaboliet (R)-2-hidroksiglutaarsuur ((R) -2HG) [31, 32].

4. Meganismes van tumorenese wat veroorsaak word deur die TCA -siklusdefekte

Die bevinding dat baie gewasse opwek uit mutasies in beide SDH en FH gene word gekenmerk deur hipoksiese kenmerke het voorgestel dat die aktivering van die hipoksie-induseerbare transkripsiefaktor-1α (HIF-1α) kan 'n ondersteunende rol speel in die tumorigeniese prosesse wat veroorsaak word deur disfunksies van die TCA -siklus. Inderdaad, HIF-1α Dit is bekend dat dit die biochemiese herprogrammering van kankerselle koördineer wat daarop gemik is om hul groei en verspreiding sowel as tumorvaskularisasie [33–35] te handhaaf. Die oorsaaklike verband tussen TCA-siklus disfunksie en HIF-1α aktivering is aanvanklik deur Selak en kollegas voorgestel wat demonstreer dat die ophoping van suksinaat in SDH-tekorte selle die inhibisie van prolyl 4-hidroksilases (PHDs) veroorsaak, 'n negatiewe reguleerder van die stabiliteit van die α subeenheid van HIF [36]. Die PHD's is lede van die superfamilie van α-KG-afhanklike hidroksilases, wat die hidroksilasie van die substrate koppel met die oksidasie van α-KG om op te som in reaksies wat afhanklik is van O2 en Fe 2+ [37]. In normoksiese toestande hidroksileer PHD's twee prolienresidue in die suurstofafhanklike afbraakdomein van HIF-1α, wat dit moontlik maak om poliubikitineer en afgebreek te word via proteasoom. Die opgehoopte suksinaat in SDH-tekorte of SDH-onaktiewe selle benadeel PHDs aktiwiteit wat lei tot HIF-1α stabilisering onder normoksiese toestande (pseudohypoksie) [36]. Net soos suksinaat, is ook gedemonstreer dat fumaraat, wat ophoop in gewasse wat verlies aan FH-funksie bevat, kragtige inhibeerders van PHD's [38] is. Interessant genoeg is daar waargeneem dat fumaraat-gemedieerde stabilisering van HIF die opregulering van verskeie HIF-teikengene veroorsaak, insluitend dié wat selgroei en angiogenese stimuleer, wat die pseudohypoksie-reaksie moontlik maak as 'n aanneemlike meganisme vir HLRCC-aanvang [38]. Ten spyte daarvan toon hierdie groot hoeveelheid bewyse 'n direkte verband tussen HIF-1α uitdrukking en tumorgenese, het onlangse bevindings 'n paar vrae laat ontstaan ​​oor die protumorigene rol van pseudohipoksiese aanpassing in alle soorte gewasse wat uit TCA-siklusdefekte ontstaan. Die eerste vraag is gestel uit die studie van Adam en kollegas. Hulle het getoon dat nie die teenwoordigheid van HIF of die afwesigheid van PHD's nodig is vir die vorming van hiperplastiese nier siste (tipiese kenmerk van HLRCC) in 'n nierspesifieke Fh1 (die ortholoog van menslike FH) uitklopmuise wat baie kenmerke van die menslike siekte herkapitel [39], wat daarop dui dat alternatiewe onkogene aksies van fumaraat verantwoordelik kan wees vir HLRCC-generering (sien volgende paragraaf). Benewens hierdie studie, wat HIF uitbeeld as 'n soort 'omstanderspeler' by die aanvang van gewasse wat FH-mutasies bevat, het 'n ander verslag hierdie transkripsiefaktor aangedui as 'n tumoronderdrukker-proteïen in gewasse wat IDH1/2-mutasies dra. Inderdaad, soos bewys deur Koivunen en kollegas, in plaas daarvan om te succinate en fumarate, (R)-2HG stimuleer PHDs aktiwiteit, bestuur, op so 'n manier, HIF-1α vir proteasoom-gemedieerde degradasie [40]. Verder het hulle daarop gewys dat HIF-1α afregulering verhoog die verspreiding van menslike astrasiete en bevorder hul transformasie, wat 'n regverdiging bied vir die ondersoek van PHD's inhibisie as 'n potensiële behandelingstrategie vir gewasse wat IDH1/2-mutasies huisves [40].

As lid van die α-KG-afhanklike hidroksilases, PHD's kataliseer die hidroksilering van 'n wye reeks substrate, behalwe HIF-1α [37]. Daarom kan die verminderde hidroksilering van PHD-doelwitte bydra tot tumorigenese, ongeag HIF-1α aktiwiteit en die verkryging van 'n hipoksiese handtekening. Daar is byvoorbeeld voorgestel dat SDH-tekort 'n PHD-afhanklike geprogrammeerde seldood van neurone kan benadeel, en daarom die weg gebaan vir neoplastiese transformasie van neuronale selle. Hierdie hipotese vind ondersteuning in die onlangse studies wat toon dat die proapoptotiese aktiwiteit van die prolylhidroksilase EglN3 'n funksionele SDH vereis, synde terugvoer geïnhibeer deur suksinaat [41, 42]. Aangesien EglN3 tydens ontwikkeling nodig is om die geprogrammeerde seldood van sommige simpatieke neuronale voorgangerselle moontlik te maak, kan die remming daarvan, veroorsaak deur die verhoging van suksinaatvlakke, 'n rol speel in die patogenese van gewasse wat veroorsaak word deur 'n gebrekkige ontwikkelingsapoptose, soos feochromocytomas.

Beklemtoon die HIF-1α-afhanklike tumorigeniese meganismes, die groeiende bewyse plaas die verandering van die TCA -vloed ook stroomop van die epigenetiese dinamika. Histoonmetilering is 'n belangrike epigenetiese modifikasie wat gedemonstreer is om geenuitdrukking te reguleer deur chromatienstruktuur te wysig en sodoende die binding van transkripsiefaktore te verfyn [43, 44]. Een van die mees bestudeerde ensieme wat histoonmetileringshandtekening reguleer, is die Jumonji C-terminale domein (JmjC) familie van histoondemetielases [45]. As hulle die metielgroepe op die arginien- en lysienresidue van histone verwyder nadat hulle 'n α-KG- en suurstofafhanklike hidroksilering, dit is ingesluit in die α-KG-afhanklike hidroksilase familie. Daar is getoon dat opeenhoping van succinaat in selle met 'n gebrek aan SDH 'n negatiewe uitwerking op die aktiwiteit van baie lede van so 'n klas histoon demetielase het. Suksinaat-gemedieerde JMJD3-inhibisie lei byvoorbeeld tot veranderinge in die metileringsmerk van histoon H3 op arginien [46]. Verder, in 'n gismodel van paraganglioom, is bevind dat die histoon demetielase, Jhd1, belemmer word deur opeenhoping van succinaat in 'n α-KG-mededingende wyse [47]. Net so toon onlangse studies dat, behalwe SDH-veranderinge, ook IDH1/2-defekte geassosieer word met hipermetileerde fenotipe. Inderdaad, in selle wat huisves IDH1/2 mutasies, intrasellulêre (R)-2-HG-vlakke kan die waarde van 10 mM bereik. Hierdie konsentrasies bevorder die mededingende inhibisie van die α-KG-afhanklike histoon N ε -lysien demetielase JMJD2A, en die tien-elf translokasie (TET) familie van 5-metielcitosien (5mC) hidroksilases, 'n klas proteïene wat die α-KG-afhanklike verwydering van metielmerk uit 5-metielcitosiene, wat lei tot 'n verbeterde histoon- en DNA-metilering, onderskeidelik [48, 49]. Interessant genoeg, alhoewel fumaraat in staat is om HIF-regulerende PHD's te inhibeer, net soos suksine, is daar tot dusver geen bewyse getoon wat die vermoedelike vermoë daarvan om sy verwante metaboliet suksinaat te weerspieël om histoonmetilering te beïnvloed, tot dusver bewys nie. Daarbenewens, as TET ensieme is lede van die α-KG-afhanklike hidroksilases-familie, 'n vermeende vermoë van beide suksinaat en fumaraat in hul remming kan redelik aangevoer word. Op grond van die vermoë van die epigenetiese veranderings om afstammingspesifieke differensiasie te beïnvloed en die aktivering van onkogenes of stilmaak van tumoronderdrukkers [44, 50, 51] te veroorsaak, veroorsaak die mededingende remming van histoon- en DNA-demetielases wat veroorsaak word deur defekte in vloei van TCA -siklusmetaboliete kan tumorigenese dryf deur seltransformasie en onbeheerde verspreiding te bevorder.

5. Redoksafhanklike tumorige veranderinge wat veroorsaak word deur die TCA-siklusdefekte

Afgesien van die blote metaboliese standpunt, dui oortuigende bewyse daarop dat die reaktiewe suurstofspesies (ROS), geproduseer deur 'n gedereguleerde mitochondriale funksionering, die onkogene sein kan veroorsaak of ten minste kan deelneem aan die vordering van gewasse wat gekenmerk word deur defekte in die TCA -siklusensieme. (Figuur 1). Hierdie aanname vind steun in die waarneming dat, in vergelyking met hul normale eweknieë, baie soorte kankerselle 'n verhoogde vlak van ROS het wat veroorsaak word deur 'n gebrekkige mitochondriale elektron-transportketting [52-54]. Deur hul chemiese reaktiwiteit te benut met biomolekules, soos nukleïensure, is dit bekend dat ROS verskeie soorte DNA-skade veroorsaak, insluitend ontbinding en depirimidinasie, enkel- en dubbelstrengs DNA-breuke, basis- en suikerveranderinge, en DNA-proteïen-kruisbindings. Op so 'n manier dryf permanente modifikasies van DNA, as gevolg van volgehoue ​​prooksidant toestande, die mutageniese gebeure onderliggend aan karsinogenese.

Die waarneming dat spesifieke SDHC mutant (mev-1) van die C. elegans aalwurm kon superoksied genereer

[55, 56] het die moontlikheid voorgestel dat ROS 'n oorsaaklike rol kan speel in die patogenese van gewasse wat defekte in die TCA -siklus dra. Hierdie hipotese is verder versterk deur die bewyse dat muisfibroblaste met 'n muriene ekwivalent van die mev-1 mutant is gekenmerk deur 'n volgehoue ​​ROS produksie en 'n aansienlik hoër DNA mutasie frekwensie as wild-tipe eweknieë [57]. Alhoewel hierdie bewysstukke die mutagene rol van ROS wat deur gebrekkige SDH -kompleks gegenereer word, ondersteun, is geen opspoorbare DNA -skade, ondanks 'n verhoogde produksie van ROS en proteïenoksidasie, beskryf in 'n S. cerevisiae stam wat Sdh2 ontbreek (die gis-ortoloog van soogdier-SDHB) [47]. Om die prooksidantstoestand wat deur SDH-disfunksies veroorsaak word, te koppel aan tumorigenese, het Guzy en kollegas voorgestel dat die ROS 'n ondersteunende rol in die onkogene proses kan speel deur by te dra tot die aktivering van HIF-1α [58]. Vertrou inderdaad op 'n voorheen gekenmerkde rol van respiratoriese ketting-afgeleide ROS as seine vir HIF-1α stabilisering onder hipoksie [59, 60], is getoon dat selle wat mutante SDHB uitdruk, maar nie mutante SDHA nie, gekenmerk word deur beduidende mitochondriale ROS-produksie wat saam met suksinaat benodig word vir 'n volledige inaktivering van PHD's en HIF-1α stabilisering [58]. Daarom versterk hierdie resultate die rol van ROS as versterker van die pseudohipoksiese respons, waargeneem in alle selle wat SDH-defekte dra, wat 'n biochemiese rasionaal verskaf vir die erns van SDHB-mutasies wat gewoonlik met aggressiewe PCC geassosieer word.

Die vermoëns van die TCA-siklusdefekte in die afstemming van sellulêre redokstoestand is ondersteun deur die bewyse dat ook onkogene mutasies in IDH1/2-gene geassosieer word met die oksidasie van intrasellulêre milieu. Normaalweg word die beheer van sellulêre redoksstoestand by aërobiese organismes verseker deur die balans tussen die prooksidantspesies, hoofsaaklik geproduseer deur mitochondria, NADPH -oksidases of as byproduk van die intermediêre metabolisme, en die opruiming daarvan deur die sinergistiese werking van die antioksidant -ensieme en die tioolbevattende antioksidante. Onder laasgenoemde speel die tripeptide glutathione (GSH) 'n deurslaggewende rol in die bepaling van die steady-state waarde van die intrasellulêre redokspotensiaal. Inderdaad, sy intrasellulêre oorvloed (1–10 mM) laat GSH toe om as elektronskenker deel te neem aan die ensiematiese vermindering van waterstofperoksied en lipiedperoksiede en aan die generering van omkeerbare S-glutathionilated addukte met proteïentiole, wat verhoed dat hulle onomkeerbare vorme van oksidasie ondergaan [61]. Die vermoë van IDH-mutasies om oksidatiewe intrasellulêre toestande te veroorsaak, is gekoppel aan 'n afname in GSH-vlakke, waargeneem in beide IDH1-R132H- en IDH2-R172K-wat glioomselle uitdruk met betrekking tot hul wt eweknieë [62]. GSH word gesintetiseer in twee ATP-afhanklike stappe: (i) sintese van γ-glutamylcysteïne, van L-glutamaat en cysteïne via die tempo-beperkende ensiem glutamaat-sisteïen ligase (GCL) (ii) byvoeging van glisien aan die C-terminaal van γ-glutamilcysteïne via die ensiem glutathion sintetase. Intrasellulêre glutamaat, wat nodig is vir die eerste reaksie van GSH -biosintese, word hoofsaaklik geproduseer deur die oksidatiewe deaminering van glutamien wat deur die ensiem glutaminase gekataliseer word [63]. As IDH1/2 mutant selle word gekenmerk deur laer vlakke van glutamaat met betrekking tot hul wt by ooreenstemmende onderdele [62], is dit moontlik dat onkogene defekte in IDH 'n verswakte GSH -sintese tot gevolg het as gevolg van 'n laer beskikbaarheid van glutamaat, wat die prooksidanttoestande wat in glutaminase -gebrekkige selle waargeneem word, fenokopieer [64]. Die gedempte glutamaatvlakke kan die gevolg wees van 'n verbeterde α-KG -vraag na IDH1/2 -mutante selle wat die biosintese van die onkometaboliet moontlik maak (R) -2-HG. Hierdie aanname word ondersteun deur die bewyse dat die behandeling van gliomaselle met (R)-2-HG deputeer nie glutamaat- of glutathionvlakke nie [62], wat daarop dui dat baie metaboliese veranderinge wat in IDH-gemuteerde selle waargeneem word nie te wyte is aan die direkte werking van (R) -2-HG maar 'n gevolg van die onkogene produksie daarvan. Die betrokkenheid van IDH1/2 -mutasies by die opwekking van prooksidante toestande hou nie net verband met die verandering van intrasellulêre GSH -inhoud nie. Inderdaad, die oksidatiewe dekarboksilering van isocitraat, wat benadeel is in alle mutante van IDH1 en IDH2 proteïene, is gekoppel aan 'n verminderde vermoë om NADPH te genereer. Boonop hou die gebrek aan intracellulêre NADPH -produksie verband met 'n verhoogde NADPH -oksidasie, wat nodig is om die reduktiewe biosintese van (R) -2-HG [31, 32, 65]. Aangesien GSH en die op tiool gebaseerde antioksidant proteïen thioredoksien NADPH benodig as 'n bron om ekwivalente vir hul eie wedergeboorte te verminder [61], kan die veranderde ewewig van NADP + /NADPH wat deur IDH1 /2 mutasies veroorsaak word, bydra tot die verskuiwing van die intrasellulêre redoks toestand na meer oksiderende toestande. Alhoewel hierdie bewysstukke die vermoë van mutante IDH1/2 tot gevolg het om onafhanklik prooksidante toestande te veroorsaak as gevolg van die direkte optrede van (R) -2-HG op menslike redoks-metabolome, is voorgestel dat hierdie onkometaboliet homself kan bydra tot die oksidasie van die intrasellulêre omgewing. Inderdaad, sommige verslae demonstreer sy vermoë om oksidatiewe skade in serebrale korteks van jong rotte te veroorsaak [66] en om ROS-generasie te ontlok deur die stimulasie van NMDA-reseptor [67]. Although these findings support prooxidant capability of (R)-2-HG, to date no striking evidence has been provided attesting its mutagenic role.

Whereas accumulating pieces of evidence support the capability of oncogenic mutations in SDH as well as IDH genes to oxidize intracellular milieu, conflicting findings do not allow for defining a clear role of FH deficiency in cellular redox state modulation. The most convincing evidence showing the capability of FH-deficient cells to promote intracellular ROS accumulation comes from the work of Sudarshan and colleagues [68]. This study demonstrated that inactivating mutations of FH in an HLRCC-derived cell line result in glucose-induced NADPH oxidases-mediated generation of and ROS-dependent HIF-1α stabilization. On the contrary, O’Flaherty and colleagues provided clear evidence that accumulation of fumarate, due to the absence of a functional FH, is the sole mechanism responsible for the inhibition of HIF-1α prolyl hydroxylation, independently on defect in mitochondrial oxidative metabolism [69]. Indeed, the complete correction of HIF-1α pathway activation in Fh1 −/− MEFs by extra-mitochondrial FH expression suggests that, at least in tumors harboring FH defects, neither impaired mitochondrial function nor the consequent dependence of energy metabolism on glycolysis contributes significantly to HIF-1α engagement. The most substantial pieces of evidence, depicting the elevation of fumarate levels as a condition linked to the reduction of intracellular redox state, came from two recent studies demonstrating that FH loss results in the activation of nuclear factor erythroid 2-related factor 2 (Nrf2) [39, 70], the pivotal transcription factor responsible for the induction of the antioxidant-responsive-element- (ARE-) driven genes, which codify for phase II detoxification enzymes and antioxidant proteins such as glutathione S-transferases and GCL [71]. Both studies demonstrated that reconstitution of FH-deficient cells with wild-type FH or an extra-mitochondrial FH decreased fumarate levels and restored Nrf2 regulation [39, 70]. In addition, elevation of intracellular fumarate content by a membrane-permeable fumarate ester was found sufficient to induce Nrf2 and its orchestrated antioxidant program [70]. According to the current view, in resting conditions, Nrf2 is retained in the cytoplasm through its interaction with Keap1 which prevents its nuclear translocation and rules its ubiquitin-proteasome-mediated turnover, as well. However, in the presence of electrophiles as well as during redox unbalance, Keap1 is modified at several reactive cysteine residues, resulting in Nrf2 stabilization and the activation of the protective gene expression program [71, 72]. In line with this accepted model, both groups revealed by mass spectroscopy analyses that fumarate was able to succinate several cysteine residues previously shown to be electrophile targets, including Cys 151 and Cys 288 , thereby providing a mechanistic explanation of the fumarate-induced Nrf2 activation [39, 70]. Although ROS can promote carcinogenesis by inducing oxidative damages to DNA, a recent outstanding study demonstrates that oncogene-induced Nrf2 activation promotes tumorigenesis by lowering ROS levels and conferring a more reduced intracellular environment [73]. Therefore, on the basis of these evidence, it is possible to hypothesize that the fumarate-mediated activation of the Nrf2-antioxidant pathway might drive the oncogenic signal for tumors characterized by defects in the FH enzyme. Although this assumption has not been demonstrated yet, the observation that heme oxygenase 1, one of the best defined target genes of Nrf2, is upregulated in FH-deficient cells allowing their survival [74] supports the putative causal role of Keap1 succination in the onset of tumors carrying FH defects. Furthermore, mounting bodies of evidence show that Nrf2 and its downstream genes are overexpressed in many cancer cell lines and human cancer tissues conferring them advantage for survival and growth as well as acquired chemoresistance [75, 76]. Therefore, it is possible to speculate that besides driving renal tumorigenesis, fumarate-induced succination of Nrf2 could contribute to the reduced sensitivity of particularly aggressive and recurrent forms of kidney cancer, such as HLRCC [77], to many chemotherapeutic approaches. The enhanced activation of Nrf2 observed both by Pollard and Furge groups contributes to explain the results obtained by Raimundo and coworkers in nontumor cells [78]. Indeed, they documented that FH-deficient diploid human fibroblasts are characterized by a highly reduced redox state with increased GSH levels, as result of increased expression of the GSH biosynthetic enzyme GCL. As highly reducing environment has been shown to stimulate cell proliferation [79], it is possible to hypothesize that the reduced redox state elicited by FH mutations could favor the doublings of stem-cell-like populations promoting thus the initial event of tumor formation. This assumption finds support in the observation that lower ROS levels have been found in many cancer stem cells with respect to the nontumorigenic counterparts, allowing them to maintain a high proliferative status and to prevent their differentiation [80].

6. Concluding Remarks

The direct involvement of TCA cycle enzymes in tumor formation has been arousing from a decade. In tumors associated with defects of SDH, FH, and IDH enzymes, the underlying mechanisms of tumorigenesis involve the accumulation of metabolites (succinate, fumarate, and (R)-2-HG) that convey oncogenic signals (oncometabolites). Large amount of evidence points towards the generation of pseudohypoxic phenotype and the alteration of epigenetic homeostasis as the main cancer-promoting effects of the TCA cycle affecting mutations. Besides inhibiting the α-KG-dependent hydroxylases, mounting body of evidence supports the ability of these oncometabolites to alter cellular redox state in precancerous as well as transformed cells. Therefore, alternatively or concomitantly to the generation of pseudohypoxic phenotype and the alteration of epigenetic dynamics, the oncometabolites-induced engagement of redox-dependent signaling pathways could contribute both to the neoplastic transformation of healthy cells as well as to the progression of malignancies characterized by germline mutations in SDH and FH and of somatic defects in IDH. These emerging findings reveal a dynamic interaction between the genetic profile, the metabolic status, and the redox tuning of the cell. Moreover, the different impact of oncogenic mutations of the TCA cycle on cellular redox state could contribute to explain the differences in the clinical phenotype and outcome of their associated tumors, opening new perspectives in the comprehension of the molecular mechanisms of oncogenesis and therapeutic targeting of these neoplastic alterations.

Erkennings

This work was partially supported by Grants from AIRC (no. IG 10636) and from Ministero dell’Università e della Ricerca (MIUR).

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Tricarboxylic acid cycle

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Tricarboxylic acid cycle, (TCA cycle), also called Krebs siklus en sitroensuur siklus, the second stage of cellular respiration, the three-stage process by which living cells break down organic fuel molecules in the presence of oxygen to harvest the energy they need to grow and divide. This metabolic process occurs in most plants, animals, fungi, and many bacteria. In all organisms except bacteria the TCA cycle is carried out in the matrix of intracellular structures called mitochondria.

The TCA cycle plays a central role in the breakdown, or catabolism, of organic fuel molecules—i.e., glucose and some other sugars, fatty acids, and some amino acids. Before these rather large molecules can enter the TCA cycle they must be degraded into a two-carbon compound called acetyl coenzyme A (acetyl CoA). Once fed into the TCA cycle, acetyl CoA is converted into carbon dioxide and energy.

The TCA cycle consists of eight steps catalyzed by eight different enzymes (see Figure ). The cycle is initiated (1) when acetyl CoA reacts with the compound oxaloacetate to form citrate and to release coenzyme A (CoA-SH). Then, in a succession of reactions, (2) citrate is rearranged to form isocitrate (3) isocitrate loses a molecule of carbon dioxide and then undergoes oxidation to form alpha-ketoglutarate (4) alpha-ketoglutarate loses a molecule of carbon dioxide and is oxidized to form succinyl CoA (5) succinyl CoA is enzymatically converted to succinate (6) succinate is oxidized to fumarate (7) fumarate is hydrated to produce malate and, to end the cycle, (8) malate is oxidized to oxaloacetate. Each complete turn of the cycle results in the regeneration of oxaloacetate and the formation of two molecules of carbon dioxide.

Energy is produced in a number of steps in this cycle of reactions. In step 5, one molecule of adenosine triphosphate (ATP), the molecule that powers most cellular functions, is produced. Most of the energy obtained from the TCA cycle, however, is captured by the compounds nicotinamide adenine dinucleotide (NAD + ) and flavin adenine dinucleotide (FAD) and converted later to ATP. Energy transfers occur through the relay of electrons from one substance to another, a process carried out through the chemical reactions known as oxidation and reduction, or redox reactions. (Oxidation involves the loss of electrons from a substance and reduction the addition of electrons.) For each turn of the TCA cycle, three molecules of NAD + are reduced to NADH and one molecule of FAD is reduced to FADH2. These molecules then transfer their energy to the electron transport chain, a pathway that is part of the third stage of cellular respiration. The electron transport chain in turn releases energy so that it can be converted to ATP through the process of oxidative phosphorylation.

The German-born British biochemist Sir Hans Adolf Krebs proposed this cycle, which he called the citric acid cycle, in 1937. For his work he received the 1953 Nobel Prize in Physiology or Medicine. Although Krebs elucidated most of the reactions in this pathway, there were some gaps in his design. The discovery of coenzyme A in 1945 by Fritz Lipmann and Nathan Kaplan allowed researchers to work out the cycle of reactions as it is known today.


1 Antwoord 1

Oxygen is actually not needed in the Krebs cycle - it is needed in the electron transport chain that is downstream of the Krebs cycle to regenerate NAD + from NADH. NAD + is a co-enzyme and acts as an electron carrier in oxidizing reactions at various positions in the Krebs cycle. However, note that without O2, NADH accumulates and the cycle cannot continue as it needs NAD + to run.

Krebs cycle - No O2 benodig:

Electron transport chain - O2 needed to regenerate NAD + essential for the Krebs cycle:


Glycolysis, Pyruvic Oxidation, Krebs cycle, and Respiratory Chain

Out of these stages, the very first happens in the cytosol for which oxygen is not essential, while the other three occur within the mitochondria where the presence of oxygen is necessary.

Glikolise

Glycolysis is the breakdown of glucose up to the formation of pyruvic acid. Glycolysis can happen both in the absence of oxygen (anaerobic condition) or in the presence of oxygen (aerobic condition). In both, the completed product of glucose breakdown is pyruvic acid.

The breakdown of glucose occurs in a series of actions, each catalyzed by a specific enzyme. All these enzymes are present dissolved in the cytosol. In addition to the enzymes, ATP and coenzyme NAD (nicotinamide adenine dinucleotide) are also important.

Glycolysis is divided into two phases, a preparatory phase, and an oxidative phase. In the preparatory phase breakdown of glucose occurs and energy is expended. In the oxidative phase, high energy phosphate bonds are formed and energy is stored.

Preparatory phase

The first step in glycolysis is the transfer of a phosphate group from ATP to glucose. As a result, a molecule of glucose-6 -phosphate is formed. An enzyme catalyzes the conversion of glucose-6-phosphate to its isomer, fructose-6 – phosphate.

At this stage, another ATP molecule transfers a second phosphate group. The product is fructose 1,6-bisphosphate. The next step in glycolysis is the enzymatic splitting of fructose 1,6 -bisphosphate into two fragments. Each of these molecules contains 3 carbon atoms. One is called 3 – phospo-glyceraldehyde, 3-PGAL, or Glyceraldehyde 3-phosphate (G3P) while the other is dihydroxyacetone phosphate. These two molecules are isomers and in fact, are readily interconverted by yet another enzyme of glycolysis.

Oxidative (pay off) phase

The next step in glycolysis is important to this procedure. Two electrons or two hydrogen atoms are removed from the molecule of 3- phosphoglyceraldehyde (PGAL) and transferred to a molecule of NAD. This is naturally, an oxidation-reduction reaction, with the PGAL being oxidized and the NAD being reduced.

During this reaction, a 2nd phosphate group is contributed to the molecule from inorganic phosphate present in the cell. The resulting molecule is called 1,3 Bisphosphoglycerate (BPG).

The oxidation of PGAL is an energy yielding procedure. Therefore a “high energy” phosphate bond is created in this molecule. At the very next step in glycolysis, this phosphate group is transferred to a molecule of adenosine diphosphate (ADP) transforming it into ATP.

The end product of this reaction is 3-phosphoglycerate (3-PG). In the next action, 3-PG is transformed into 2-Phosphoglycerate (2PG). From 2PG a particle of water is removed and the product is phosphoenolpyruvate (PEP). PEP then quits its ‘high energy’ phosphate to transform the 2nd molecule of ADP to ATP. The product is pyruvate, pyruvic acid (C3 H.4 O3). It is equivalent to half a glucose molecule that has been oxidized to the extent of losing two electrons (as hydrogen atoms).

Pyruvic oxidation

Pyruvic acid (pyruvate), the completed product of glycolysis, does not go into the Krebs cycle directly. The pyruvate (3- carbon particle) is first become 2-carbon acetic acid molecule. One carbon is released as CO2 (decarboxylation). Acetic acid ongoing into the mitochondrion unites with coenzyme-A (Co A) to form acetyl Co A (active acetate). In addition, more hydrogen atoms are moved to NAD.

Krebs cycle or citric acid cycle
Discovery

The citric acid cycle was identified in 1937 by Hans Adolf Krebs and William Arthur Johnson. That is why sometimes it is also known as the Krebs siklus.

Definisie

The citric acid cycle (CAC)– also called the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle is a series of chemical reactions utilized by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA stemmed from carbs, fats, and proteins. In addition, the cycle supplies precursors of specific amino acids, along with the reducing representative NADH, that is used in numerous other reactions.

Verduideliking

Acetyl CoA now enters a cyclic series of chemical reactions during which the oxidation procedure is finished. This series of reactions is called the Krebs cycle or the citric acid cycle. The first step in the cycle is the union of acetyl CoA with oxaloacetate to form citrate. In this procedure, a molecule of CoA is regenerated and one molecule of water is utilized. Oxaloacetate is a 4-carbon acid. Citrate thus has 6 carbon atoms.

After two steps that simply result in forming an isomer of citrate, isocitrate another NAD- mediated oxidation happens. This is accompanied by the removal of a molecule of CO2. The outcome is a-ketoglutarate. It, in turn, undergoes further oxidation (NAD + 2H– > NADH2) followed by decarboxylation (CO2) and the addition of a molecule of water.

The product then has one carbon atom and one oxygen atom less. It is succinate. The conversion of α-ketoglutarate into succinate is accompanied by a complimentary energy change which is made use of in the synthesis of an ATP molecule. The next step in the Krebs cycle is the oxidation of succinate to fumarate. Once again, 2 hydrogen atoms are eliminated, however this time the oxidizing agent is a coenzyme called flavin adenine dinucleotide (FAD), which is reduced to FADH2.

With the addition of another molecule of water, fumarate is transformed into malate. Another NAD moderated oxidation of malate produces oxaloacetate, the original 4-carbon molecule. This completes the cycle. The oxaloacetate may now combine with another molecule of acetyl CoA to go into the cycle and the whole procedure is repeated.

Respiratory chain

In the Krebs cycle, NADH and H + are produced from NAD + . NADH then moves the hydrogen atom to the respiratory chain (also called electron transport chain)where electrons are transported in a series of oxidation-reduction steps to react, ultimately, with molecular oxygen.

The oxidation-reduction substances which participate in the respiratory chain are:

  1. A coenzyme called coenzyme Q.
  2. A series of cytochrome enzymes (b, c, a, a3).
  3. Molecular oxygen (O2).

Cytochromes are electron transport intermediates containing haem of associated prosthetic groups, that undergo valency changes of the iron atom. Haem is the same iron consisting of a group that is oxygen-carrying pigment in hemoglobin. The path of electrons in the respiratory chain seems as follows.

NADH is oxidized by coenzyme Q. This oxidation yields enough totally free energy to permit the synthesis of a molecule of ATP from ADP and inorganic phosphate. Coenzyme Q is in turn oxidized by cytochrome b which is then oxidized by cytochrome c. This step likewise yields adequate energy to permit the synthesis of a molecule of ATP.

Cytochrome c then reduces a complex of two enzymes called cytochrome a and a3 (for the benefit the complex is referred to as cytochrome a). Cytochrome is oxidized by an atom of oxygen and the electrons get to the bottom end of the respiratory chain. Oxygen is the most electronegative substance and the final acceptor of the electrons. A molecule of water is produced. In addition, this final oxidation supplies enough energy for the synthesis of the 3rd molecule of ATP.

Oxidative phosphorylation

The synthesis of ATP in the presence of oxygen is called oxidative phosphorylation. Generally, oxidative phosphorylation is combined with the respiratory chain. As currently explained ATP is formed in 3 steps of the respiratory chain. The formula for this procedure can be expressed as follows:

Where Pi is inorganic phosphate.

The molecular system of oxidative phosphorylation occurs in conjunction with the respiratory chain in the inner membrane of the mitochondrion. Here likewise, as in photosynthesis, the system involved is chemiosmosis by which electron transport chain is coupled with the synthesis of ATP.

In this case, nevertheless, the pumping/movement of protons (H + ) is throughout the inner membrane of mitochondrion folded into cristae, in between matrix of mitochondrion and mitochondrion’s intermembrane space. The coupling factors in respiration are also different from those in photosynthesis.

Gereelde vrae

Q1: What are obligative anaerobic?

Antwoorde: Obligative anaerobic organisms include certain types of bacteria. These survive only in the complete absence of molecular oxygen.

Q2: Which electron carriers are used in Krebs cycle?

Antwoorde: Nicotinamide adenine dinucleotide (NAD) and Flavin adenine dinucleotide (FAD).

Q3: What is electron transport chain?

Antwoorde: The collection of molecules embedded in the inner membrane of the mitochondrion is called electron transport chain.

Q4: How electrons are transported in respiratory chain?

Antwoorde: NADH or FADH2 brings electron to electron transport system in mitochondria. The electrons are transported from NADH to FMN, cytochrome b, cytochrome c, cytochrome a and cytochrome a3.


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Hierdie finale Krebs-siklusreaksie regenereer die molekule, oksaloasetaat, wat asetiel-CoA in die siklus in stap 1 aanvaar het. Die ensiem malaat dehidrogenase kataliseer die verwydering van die paar waterstofatome (blou) wat op koolstowwe #1 en #2 geplaas is deur water in die vorige reaksie. Let op dat die suurstof (groen) van die hidroksielgroep op koolstof #1 agtergelaat word.

NAD + is die oksideermiddel en, soos gewoonlik, word een van die waterstofatome as 'n hidriedioon verwyder (nie getoon nie) om NADH te vorm en die ander waterstofioon word deel van die waterstofioonpoel.


Kyk die video: Interfaza,pojam haploidne celije (Oktober 2022).