Asam nuukleat

Purin, Pirimidin dan Asam NukleatRETNO SINTOWATI

Purin, Pirimidin, Asam nukleat I. Pendahuluan II. Basa Purin dan Basa Pirimidin III. Nukleosida IV. Nukleotida V. Asam Nukleat

Kepentingan biomedis

kemampuan nukleotid menyerap sinar UV menjadikan sinar UV sbg unsur mutagen yg potentsenyawa analog purin & pirimidin disintesis secara kimiawi digunakan untuk terapi penyakit kankerSel yg akan membelah, selnya mengalami replikasiKatabolisme purin xantin asam urat

I. PendahuluanNukleoprotein protein majemuk terdapat dalam inti sel penyusun material inti sel tersusun atas : protein: histon / protamingugus non protein semua sel hidup mengandung nukleoproteinKromatinKonstituen penting dalam sel, tersusun atas nukleoprotein kelainan pembentukan nukleoprotein perubahan dlm pertumbuhan /reproduksi sel sebab kromatin pembelahan sel

Nukleoprotein Enzim proteolitik - Protein sederhana - Asam nukleat trtm: histon & protamin (RNase / DNase)

polinukleotida polinukleotidase (fosfoesterse) mononukleotida purin/pirimidin nukleotidase (fosfatase) nukleosida asam fosfat (fosforilase)basa purin/basa pirimidin gula (ribosa/deoksi ribosa)

Nukleosida Purin

Adenosin Guanosin adenosin deaminase purin nukleosida fosforilase NH3 Inosin Guanin purin nukleosida fosforilase Hipoxantin Xantin oksidase Xantin + O2 asam urat urin Xantin oksidase

Penyelamatan Purin Untuk pembentukan nukleotida bbrp jaringan manusia tgt pd purin /purin ribonukleosida eksogen Ada 2 jalan penyelamatan :Fosforibosilasi purin bebas dgn enzim yang membutuhkan PRPP sbg donor ribosa fosfatFosforilasi nukleosida purin pd gugus 5-hidroksi oleh enzim adenosin kinaseContoh :Adenin menjadi AMP dg enzim adenin fosforibosa transferaseHipoxantin dan Guanin menjadi IMP dan GMP dengan enzim hipoxantin-guanin fosforibosa transferase

Nukleosida pirimidin Sitidin Deoksisitidin deaminasi Uridin deaminasi Deoksiuridin Deoksitimidin fosforilse Urasil dan Timin degradasi -alanin + NH4+ +CO2 degradasi As. -amino isobutirat + NH4+ +CO2 urine Banyak tdp pd penderita tumor yang dikhemoterapi ( DNA degradasi )

II. Basa Purin Dan Basa Pirimidin

Basa PurinBasa heterosiklis , molekul planar , aromatik sebag. besar tdp. dlm sel dlm. bentuk asam nukleatdisintesis secara de novo, dg sumber :N1 dari as. Aspartat, C6 dr CO2 respirasi, N7 dari glisin, C8 dari N5,N10-metenilFH4, N3 dan N9 dari nitrogen amida der. Glutamin danC2 dari N-formil -tetrahidroksi folatDiberi nomor mulai atom N1 kearah kebalikan jarum jam

Basa Pirimidinnama lebih panjang dari purin ttp hy mgd 6 atom yang heterosiklis, sedang purin nama pendek mgd 9 atom heterosiklis

Sitosin (S): tdp dlm semua asam nukleat, kecuali DNA virus ttt Timin (T) : trtm tdp dlm as nukleatyg mgd gula deoksiribosa (DNA); sedikit tdp dlm tRNA Urasil (U): tdp dalam RNA

Nitrogenous BasesPlanar, aromatic, and heterocyclicDerived from purine or pyrimidineNumbering of bases is unprimed

Nucleic Acid Bases

PurinesPyrimidines

SugarsPentoses (5-C sugars)Numbering of sugars is primed

Sugars D-Ribose and 2-Deoxyribose

*Lacks a 2-OH group

NucleosidesResult from linking one of the sugars with a purine or pyrimidine base through an N-glycosidic linkage

Purines bond to the C1 carbon of the sugar at their N9 atomsPyrimidines bond to the C1 carbon of the sugar at their N1 atoms

Nucleosides

Phosphate GroupsMono-, di- or triphosphates

Phosphates can be bonded to either C3 or C5 atoms of the sugar

NucleotidesResult from linking one or more phosphates with a nucleoside onto the 5 end of the molecule through esterification

III. Nukleosida dan nukleotidaNukleosida:tdr dari basa + d-ribosa gula ribosa terikat pada N9 basa purin dan pada N1 basa pirimidin (kecuali U pd C5 ikatan C-C) dan semuanya mrpk ikatan -N-glikosidik Nukleosida yang dimanfaatkan untuk menghambat pertumbuhan sel:5-fluorourasil - 5-iodo-2-deoksiuridin6-tioguanin - 6-merkaptopurin6-azauridin - arabinosil sitosin

NukleotidaMrpk unit struktural as nukleat, mrpk komponen seluler yg mudah dpt diketahui krn menyerap kuat sinar UV, der. basa purin absorbsinya lebih kuat dari der. basa pirimidinTersusun: - basa purin/ basa pirimidin - gula (ribosa atau 2-deoksiribosa), - asam fosfatPengikatan fosfat: RNA pd at C3 dari gulanya DNA pd at C5 dari gulanyaBasa Nukleosida Nukleotida Singkatan RNA DNAAdenin Adenosin Asam adenilat A dAGuanin Guanosin Asam guanilat G dGSitosin Sitidin Asam Sitidilat C dCUrasil Uridin Asam uridilat U dUTimin Timidin Asam timidilat T dT

Lanjutan nukleotidabentuk nukleotida lebih mudah larut dalam air dp nukleosida dan basa bebasnya

Fungsi nukleotida 1. Berperan dlm metabolisme energi Contoh: ATP - bentuk energi kimia yang diperlukan untuk sel, dihasilkan dari fosforilasi oksidatif atau fosforilasi tingkt substrat - terlibat dalam kontraksi otot - transport aktif - mempertahankan gradien/ion - membantu sbg donor fosfat untuk sin- tesis nukleosida 5-trifosfat substrat untuk reaksi yg dikatalisis oleh RNA dan DNA polimerase

Lanjutan fungsi nukleotida2. Monomer unit asam nukleat: DNA dan RNA3. Mediator fisiologik: nukleosida dan nukleotida membantu sbg kunci mediator fisiologik proses metabolik: - adenosin penting dlm kontrol aliran darah koroner - ADP critical dlm agregasi platelet dan koagulasi darah - cAMP & cGMP: bekerja sbg second messenger - GTP: diperlukan untuk capping mRNA4. Precursor function: GTP adalah prekursor pemben- tukan kofaktor, tetrahidrobiopterin, untuk reaksi hidroksilasi dan generasi oksida nitrat5. Komponen koenzim: NAD, NADP, FAD

Lanjutan fungsi nukleotida6. Activated intermediate - nukleotida membantu sbg kunci activated intermediate, diperlukan untuk berbagai reaksi - UDP-glukosa-kinase intermidiate sintesis glikogen, glikoprotein dll.7. Allosteric effectors pd metabolisme nukleotida

Sintesis nukleotida Di dalam sel mamalia disintesis secara de novoTidak semua sel mampu mensintesis nukleotid purin Contoh; sel-sel darah merahPada akhir reaksi menghasilkan ITP prekursor adenosin 5-monofosfat (AMP) dan guanosin 5-mono-Fosfat (GMP)

Lanjutan sintesis nukleotidaIMP AMP energinya dari GTPIMP GMP energinya dari ATPApabila terjadi defek dlm reaksi metabolik menyebabkan kehilangan pengaturan sintesis nukleotida purin over produksi purin asam urat meningkat Artritis gout Pembentukan deoksi ribonukleotida pada keadaan sel tidak proliferasi rendah pada saat replikasi sintesis DNA sintesis deoksiribonukleotida meningkat untuk mensuport sintesis DNA

IV. Asam nukleat (Polinukleotid)dibentuk dari ikatan fosfodiester yg terikat pada C3- dan C5 monomer yg berdekatanada mol. yg berakhir 5 dan 3masing-masing mol. memp. struktur primernya, contoh 5-3p Gp Gp Ap Tp Cp AAda 2 macam asam nukleat besar: DNA & RNA

Nucleotida: Synthesis dan Degradasi

Nucleotide MetabolismPURINE RIBONUCLEOTIDES: formed de novoi.e., purines are not initially synthesized as free basesFirst purine derivative formed is Inosine Mono-phosphate (IMP)The purine base is hypoxanthineAMP and GMP are formed from IMP

Purine NucleotidesGet broken down into Uric Acid (a purine) Buchanan (mid 1900s) showed where purine ring components came from:

N1: Aspartate AmineC2, C8: FormateN3, N9: GlutamineC4, C5, N7: GlycineC6: Bicarbonate Ion

Formyl-THF

Formyl-THF

NNCCCCCNN

HHHHHH

Glutamine

Glutamine

Aspartate

CO2

Glycine

HNNCCCCCNN

HHHHHH

PurineNNCCCC

HHHHPyrimidineHHGlycineGlutamine (2)AspartateN10-Formyl-THF (2)CO2GlutamineAspartateCO2(C5H7N4)(C4H6N2)

Purine Nucleotide Synthesis

AMPGMPIMP is converted to AMP or GMPIMPXMP

Regulatory Control of Purine Nucleotide BiosynthesisGTP is involved in AMP synthesis and ATP is involved in GMP synthesis (reciprocal control of production)PRPP is a biosynthetically central molecule (why?)ADP/GDP levels negative feedback on Ribose Phosphate Pyrophosphokinase Amidophosphoribosyl transferase is activated by PRPP levelsAPRT activity has negative feedback at two sitesATP, ADP, AMP bound at one siteGTP,GDP AND GMP bound at the other siteRate of AMP production increases with increasing concentrations of GTP; rate of GMP production increases with increasing concentrations of ATP

Regulatory Control of Purine BiosynthesisAt level of IMP production:Independent controlSynergistic controlFeedforward activation by PRPPBelow level of IMP productionReciprocal control

Total amounts of purine nucleotides controlledRelative amounts of ATP, GTP controlled

Purine Catabolism and SalvageAll purine degradation leads to uric acid (but it might not stop there)Ingested nucleic acids are degraded by pancreatic nucleases, and intestinal phosphodiesterases in the intestineGroup-specific nucleotidases and non-specific phosphatases degrade nucleotides into nucleosidesDirect absorption of nucleosides Further degradation Nucleoside + H2O base + ribose (nucleosidase) Nucleoside + Pi base + r-1-phosphate (n. phosphorylase)

NOTE: MOST INGESTED NUCLEIC ACIDS ARE DEGRADED AND EXCRETED.

Intracellular Purine CatabolismNucleotides broken into nucleosides by action of 5-nucleotidase (hydrolysis reactions)Purine nucleoside phosphorylase (PNP)Inosine HypoxanthineXanthosine XanthineGuanosine GuanineRibose-1-phosphate splits offCan be isomerized to ribose-5-phosphateAdenosine is deaminated to Inosine (ADA : Adeosine deaminase)

Intracellular Purine CatabolismXanthine is the point of convergence for the metabolism of the purine bases

Xanthine Uric acidXanthine oxidase catalyzes two reactions

Purine ribonucleotide degradation pathway is same for purine deoxyribonucleotides

Adenosine Degradation

Xanthosine Degradation

Ribose sugar gets recycled (Ribose-1-Phosphate R-5-P ) can be incorporated into PRPP (efficiency) Hypoxanthine is converted to Xanthine by Xanthine Oxidase Guanine is converted to Xanthine by Guanine Deaminase Xanthine gets converted to Uric Acid by Xanthine Oxidase

GMPAMP GuanosinIMP Inosin GuaninHipoxantin Xantin

Asam urat URIN tmp kerja xantin oxidase, dihambat oleh Alopurinol

Uric Acid ExcretionHumans excreted into urine as insoluble crystalsBirds, terrestrial reptiles, some insects excrete insoluble crystals in paste form (hewan urikotelik)Excess amino N converted to uric acidOthers further modification :

Uric Acid Allantoin Allantoic Acid Urea AmmoniaEkskresi asam urat total man 400-600mg/24jamAspirin dosis tinggi hambat ekskresi & reabsorpsi urat.

Purine Salvage(Jalur penyelamatan purin)Adenine phosphoribosyl transferase (APRT)Adenine + PRPP AMP + PPi

Hypoxanthine-Guanine phosphoribosyl transferase (HGPRT)Hypoxanthine + PRPP IMP + PPiGuanine + PRPP GMP + PPi

(NOTE: THESE ARE ALL REVERSIBLE REACTIONS)

AMP,IMP,GMP do not need to be resynthesized de novo !

Di hati : nukleotida defosforilasi nukleosidaSering nukleosida diputus basa bebas + gulanukleosida & basa dibawa ke jar lain refosforilasi mjd nukleotidaBasa bebas diselamatkan dg mereaksikan dg PRPP nukleotida baruEnzim penghemat basa tsb HGPRTReaksi analog juga terjadi utk pirimidin

A CASE STUDY : GOUTA 45 YEAR OLD MAN AWOKE FROM SLEEP WITH A PAINFUL AND SWOLLEN RIGHT GREAT TOE. ON THE PREVIOUS NIGHT HE HAD EATEN A MEAL OF FRIED LIVER AND ONIONS, AFTER WHICH HE MET WITH HIS POKER GROUP AND DRANK A NUMBER OF BEERS.HE SAW HIS DOCTOR THAT MORNING, GOUTY ARTHRITIS WAS DIAGNOSED, AND SOME TESTS WERE ORDERED. HIS SERUM URIC ACID LEVEL WAS ELEVATED AT 8.0 mg/dL (NL < 7.0 mg/dL).THE MAN RECALLED THAT HIS FATHER AND HIS GRANDFATHER, BOTH OF WHOM WERE ALCOHOLICS, OFTEN COMPLAINED OF JOINT PAIN AND SWELLING IN THEIR FEET.

A CASE STUDY : GOUTTHE DOCTOR RECOMMENDED THAT THE MAN USE NSAIDS FOR PAIN AND SWELLING, INCREASE HIS FLUID INTAKE (BUT NOT WITH ALCOHOL) AND REST AND ELEVATE HIS FOOT. HE ALSO PRESCRIBED ALLOPURINOL. A FEW DAYS LATER THE CONDITION HAD RESOLVED AND ALLOPURINOL HAD BEEN STOPPED. A REPEAT URIC ACID LEVEL WAS OBTAINED (7.1 mg/dL). THE DOCTOR GAVE THE MAN SOME ADVICE REGARDING LIFE STYLE CHANGES.

GoutImpaired excretion or overproduction of uric acidUric acid crystals precipitate into joints (Gouty Arthritis), kidneys, ureters (stones)Lead impairs uric acid excretion lead poisoning from pewter drinking gobletsFall of Roman Empire?Xanthine oxidase inhibitors inhibit production of uric acid, and treat goutAllopurinol treatment hypoxanthine analog that binds to Xanthine Oxidase to decrease uric acid production

ALLOPURINOL IS A XANTHINE OXIDASE INHIBITOR

A SUBSTRATE ANALOG IS CONVERTED TO AN INHIBITOR, IN THIS CASE A SUICIDE-INHIBITOR

Pyrimidine Ribonucleotide Synthesis Uridine Monophosphate (UMP) is synthesized firstCTP is synthesized from UMPPyrimidine ring synthesis completed first; then attached to ribose-5-phosphate

N1, C4, C5, C6 : AspartateC2 : HCO3-N3 : Glutamine amide Nitrogen

Pyrimidine Synthesis

UMP UTP and CTPNucleoside monophosphate kinase catalyzes transfer of Pi to UMP to form UDP; nucleoside diphosphate kinase catalyzes transfer of Pi from ATP to UDP to form UTP

CTP formed from UTP via CTP Synthetase driven by ATP hydrolysis Glutamine provides amide nitrogen for C4 in animals

Regulatory Control of Pyrimidine SynthesisAnimals regulation at carbamoyl phosphate synthetase IIUDP and UTP inhibit enzyme; ATP and PRPP activate itUMP and CMP competitively inhibit OMP Decarboxylase*Purine synthesis inhibited by ADP and GDP at ribose phosphate pyrophosphokinase step, controlling level of PRPP also regulates pyrimidines

Orotic AciduriaCaused by defect in protein chain with enzyme activities of last two steps of pyrimidine synthesisIncreased excretion of orotic acid in urine Symptoms: retarded growth; severe anemiaOnly known inherited defect in this pathway (all others would be lethal to fetus)Treat with uridine/cytidine HOW DOES URIDINE AND CYTIDINE ADMINISTRATION WORK TO TREAT OROTIC ACIDURIA?

Degradation of PyrimidinesCMP and UMP degraded to bases similarly to purines DephosphorylationDeaminationGlycosidic bond cleavageUracil reduced in liver, forming b-alanine Converted to malonyl-CoA fatty acid synthesis for energy metabolism

Deoxyribonucleotide FormationPurine/Pyrimidine degradation are the same for ribonucleotides and deoxyribonucleotidesBiosynthetic pathways are only for ribonucleotide production Deoxyribonucleotides are synthesized from corresponding ribonucleotides Pd tingkat difosfat,gugus ribosa direduksi mjd deoksiribosa o/ enz Ribonukleotida reduktase.ex. CTP defosforilasi CDP reduksi dCDPdCTP defosforilasi dan deaminasi dUMP

Katabolisme pirimidin menghasilkan metabolit yg dpt larut dalam air, yaitu : CO2, NH3, B-alanin, B-aminoisobutiratProduk katab purin tidak begitu mudah larut, kelarutan ditingkatkan dg alkalinisasi.