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Nfletffillfl Sm of Nuelieotfl Dles Nfletffillflsm of Nuelieotfl dles ucleotides \f consistof a nitrogenousbase, a | \ pentose and a phosphate. The pentose sugaris D-ribosein ribonucleotidesof RNAwhile in deoxyribonucleotides(deoxynucleotides) of i Aspariaie--'N.,,,t .J . DNA, the sugaris 2-deoxyD-ribose. Nucleotides t participate in almost all the biochemical processes/either directly or indirectly.They are the structuralcomponents of nucleicacids (DNA, Y RNA), coenzymes, and are involved in tne Glutamine regulationof severalmetabolic reactions. Fig. 17.1 : The sources of individuat atoms in purine ring. (Note : Same colours are used in the syntheticpathway Fig. lZ.2). n T. C4, C5 and N7 are contributedby glycine. Many compoundscontribute to the purine ring of the nucleotides(Fig.t7.l). 5. C6 directly comes from COr. 1. purine N1 of is derivedfrom amino group It should be rememberedthat purine bases of aspartate. are not synthesizedas such,but they are formed as ribonucleotides. The purines 2. C2 and Cs arise from formate of N10- are built upon a formyl THF. pre-existing ribose S-phosphate. Liver is the major site for purine nucleotide synthesis. 3. N3 and N9 are obtainedfrom amide group Erythrocytes,polymorphonuclear leukocytes and of glutamine. brain cannot producepurines. 388 BIOCHEMISTF|Y m-gg-o-=_ |l Formylglycinamide ribosyl S-phosphate Kn H) Glutam H \-Y OH +ATt OH OH Glutame cl-D-Ribose-S-phosphate + ADP orr-l t'1 PRPPsYnthetase ,N o"t*'] \cH + Hrcl-itl HN:C-- O EO-qn2-O.- H -NH l./ \l KH H) I u \.]_j^/ r,\-iEl-/^\-td Ribose5-P II Formylglycinamidineribosyl-s-phosphate OH OH I S-Phosphoribosylo-pyrophosphate ATP\l ) Synthetase ADP+ PiYl PRpp glutamyl amidotrahsfera-se +H2O + E-o-gH4.o-\ NHz 1../ \ | H) \H I HtsrH Ribose5-P OH OH S-Amlnoimidazoleribosyl-S-phosphate p-S-Phosphoribosylamine I aor--l ,, I Carborytase ;;,.:i!:r+ ATp_.rl I I Synthetaee I ADP+ Pia'l o+ + tl CH2 f.iHZ ;-; -/-/ E-o-cHz-o.- NH D/ \l f\H H) H\-Jn Ribose5-P tt 5-Aminolmidazolecarborylate OH OH rlbosyl s-phosphate Glycinamiderlbosyl S-phosphate Aspartate+Al rrmyltransferase ADP+I n -ooc li | H -51 f.l HC- ir, l\_ .PH t,, --'\cH CH, r- / ,.1::r.. il co( O I NH Ribose5-P I Ribose S-Aminoimidazole 5-p 4-succinylcarboxamide Formylglycinamideribosyl S-phosphate ribosyl 5-phosphate Fag. l?.2 contd. next column Fag. 17.2 contd. next page Ghapter 17 : METABOLISMOF NUCLEOTIDES 389 5-Aminoimidazole 1. Ribose 5-phosphate, produced in the 4-succinylcarboxamide ribosyl 5-phosphate hexose monophosphateshunt of carbohydrate metabolism is the starting material for purine nucleotidesynthesis. lt reactswith ATP to form phosphoribosylpyrophosphate (PRPP). 2. Clutamine transfersits amide nitrogento ll PRPP to replace pyrophosphateand produce 5-phosphoribosylamine.The enzyme PRPP glutamyl amidotransferase is controlled by feedback inhibition of nucleotides (lMP, AMP and GMP). This reaction is the 'committed step' Ribose5-P in purine nucleotidebiosynthesis. S-Aminoimidazole 4-carboramide ribosyl S-phosphate 3. Phosphoribosylaminereacts with glycine in the presenceof ATP to form glycinamide ribosyl 5-phosphate or glycinamide ribotide (cAR). 4. N10-Formyltetrahydrofolate donates the group product tl formyl and the formed is formyl- glycinamideribosyl 5-phosphate. 5. Clutamine transfers the second amido amino group to produce formylglycinamidine ribosyl 5-phosphate. 5-Formaminoimidazole 6. The imidazole ring of the purine is closed 4-carboxamide yield ribosyl 5-phosphate in an ATP dependentreaction to 5-amino- I imidazole ribosyl S-phosphate. I u n) Cyclohydrolase 7. Incorporation of COz (carboxylation) + occurs to yield aminoimidazole carboxylate o ribosyl 5-phosphate. This reaction does not tl require the vitamin biotin and/or ATP which is the case with most of the carboxvlation reactions. 8. Aspartatecondenses with the product in Ribose5-P reaction 7 to form aminoimidazole4-succinvl lnoaane monophosphate carboxamideribosyl S-phosphate. Adenosuccinatelyase fumarate Fig. 17.2 : The metabolicpathway for the 9. cleavesoff synthesisof inosine monophosphate,the parent purine and only the amino group of aspartateis retained n ucleotide (P R PP-Phosphori bosyl pytophosph ate ; to yield aminoimidazole4-carboxamide ribosyl PPi-Pyrophosphate). 5-phosphate. 10. N1O-Formyltetrahydrofolate donates a The pathway for the synthesis of inosine one-carbon moiety to produce formamino- monophosphafe (lMP or inosinic acid), the imidazole 4-carboxamide ribosyl 5-phosphate. 'parent' purine nucleotide is given in Fig.l7.2. With this reaction,all the carbon and nitrogen The reactionsare brieflv described in the next atoms of purine ring are contributed by the column. resoectivesources. 390 BIOCHEMISTFIY reaction o 1'l. The final tl catalysed by cyclo- Hr.r/-YN\ hydrolase leads to ring |il) closurewith an elimination \rlT, of water molecule. The Ribose5-P productobtained is inosine InosinemonophosPhate (lMP) monophosphate(lMP), the Asoartate+ GTP parent purine nucleotide IMP dehydrogenase from which other purine NAD: GDP+ Pi r HrO nucleotidescan be synthe- Adenylsuccinate synthetase sized. NADH + H* rcoc-cH2-?H-coo- o lnhibitors of NH purine synthesis I Folic acid (THF) is ru---YNr\ essentialfor the synthesis |il) of purine nucleotides \*tT, XanthosinemonophosPhate (reactions 4 and 10). Ribose5-P (XMP) Sulfonamides are the Adenylsuccinate Glutamine structuralanalogs of para- + ATP+ HrO I aminobenzoic acid Adenylsuccinase Fumarate4 (PABA).These sulfa drugs I Glutamate+ can be used to inhibit the + AMP + PPi synthesis of folic acid by NHz o microorganisms. This N. the \\ indirectlv reduces ) synthesisof purines and, / H2 N' therefore,the nucleicacids I Ribose5-P (DNA and RNA). AdencinemonophcPhab GuanosinemonoPhosPhate Sulfonamides have no (AMP) (GMP) influence on humans, acid is not since folic Fig. 17.3 : Synthesisof AMP and GMP from inosine monophosphate. svnthesized and is suppliedthrough diet. The structural analogs of folic acid (e.9. (Fig.l7.3).Aspartate condenses with IMP in the methotrexate) are widely used to control cancer. presence of CTP to produce adenylsuccinate They inhibit the synthesisof purine nucleotides which, on cleavage,forms AMP. (reaction4 and 10)and, thus, nucleic acids. Both For the synthesisof CMP, IMP undergoes these reactionsare concernedwith the transferof NAD+ dependent dehydrogenation to form one-carbon moiety (formyl group). These xanthosine monophosphate(XMP). Glutamine inhibitorsalso affectthe proliferationof normally then transfersamide nitrogento XMP to produce growing cells. This causes many side-effects CMP. includinganemia, baldness, scaly skin etc. 6-Mercaptopurineis an inhibitor of the of AMF $yrrthesis synthesis of AMP and GMP. lt acts on and GMP from IMP the enzyme adenylsuccinase (of AMP lnosine monophosphatets the immediate pathway) and IMP dehydrogenase(of GMP precursorfor the formationof AMP and GMP pathway). Shapt*rr 17 : METABOLTSM OF NUCLEOTTDES 391 Nucleosidemonophosphate the metabolicreactions. (AMP,cMP) This is achievedby the I transferof phosphategroup from ATp, ATP\l catalvsed by nucleosidemonophosphate (NMp) NMPkinase kinases ,l and nucleoside diphosphate (NDp) kinases ADPT- (Fig Yi 17.4). Nucleosidediphosphate (ADP,cDP) Salvage E**'B0?wayfor p*rfine*s I ATP\l The free purines (adenine, guanine and NDpkinase J hypoxanthine)are formed in the normalturnover ADP*- I + of nucleic acids (particularlyRNA), and arso Nucleotidetriphosphate obtained from the dietary sources.The purines (ATE GTP) can directly _be converted to the corresponding nucleotides, Fig" 17.4 : Conversionof nucteosidemonophosphates and this process is known as to di- and triphosphates(NMp-Nucleoside mono- 'salvage pathway' (Fig.l 7.fl. phosphate; NDP-Nucleosjde diphosphate). Adenine phosphoribosyltransferase catalyses the formation of AMp from adenine. Forrnatiem of p*erime n*86;Ee#side Hypoxanthine-guaninephosphoribosyl trans_ ferase (HCPRT) riipFros;phaBes em# trfrea[ao*pfu ote* converts guanine ano hypoxanthine,respectively, to CMp and lMp. The nucleosidemonophosphates (AMp ano Phosphoribosylpyrophosphate (pRpp) is rne CMP) haveto be converted to the corresponding donor of ribose 5-phosphate in the salvaee di- and triphosphatesto participatein most of pathway. Adeninephosphoribosyl- Iransterase Adenine 4p1p Ribose5-P oo Hypoxanthine-guanine H^r/^\yN\"'l' ^, * t'),n,-/---N., II \ pnosphoribosyltransferase+ tf \ l- tt / Hlr\*A^rl ffi rr*\*Ar) Guanine GMP Ribose5-p Fig' 17-5 : Salvagepathways of purine nucteotide synthesis(PRPP-phosphoribosyt pyrophosphate; PPi-lnorganic pyrophosphate; AMP-Adenosine monophosphate: GMp-Guanosine monophosphate; IMP-lnosinemonophosphate; * Detieiencyof HGpRT causesLesch-Nyhan "Vni[oiJ". 392 BIOCHEMISTF|Y Base O-o-O-o-H2c //'o\Base RibonucreotidereducraseO-o-O-o-t,? r,'o- ry-juffiOH OH fi;).OHH Ribonucleoside Thioredoxin Thioredoxin Ribonucleoside diphosphate (2SH,reduced) (S-S-, oxidized) diphosphate (ADP,GDP, (ADP,GDP, CDEUDP) cDe UDP) NADP* NADPH+ H* Fig. 17.6 : Formation of deoxyribonucleotides frcm ribonucleotides. The salvagepathway is particularlyimportant moiety (Fig.t7.6).This reactionis catalysedby a in certaintissues such as erythrocytesand brain multisubunit (two B1 and two 82 subunits) where de novo (a new) synthesisof purine enzvme, rihonucleotide reductase. nucleotidesis not operative. Supply of reducing equivalents : The enzyme A defect in the enzyme HGPRTcauses Lesch' ribonucleotide reductase itself provides the Nyhan syndrome (details given later). hydrogen atoms needed for reduction from its sulfhydrylBroups. The reducingequivalents, in Regulation of purine
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