Purines synthesis and catabolism process

Synthesis of nucleotides

1. Synthesis of purines

2. A: Pathways De novo synthesis Salvage pathway

3. a: De novo synthesis P urines are not initially synthesized as free bases Purines like pyrimidines needs PRPP but for purines PRPP rovides foundation on which purines assembles step by step. PRPP is activated ribose which is formed in pentose phosphate pathway and ready to accept nucleotide base. First purine derivative formed is Inosine Mono-phosphate (IMP) The purine base is hypoxanthine AMP and GMP are formed from IMP

4. Step 1: The starting material for purine biosynthesis is Ribose-5-P, a product of the Hexose MonoPhosphate Shunt or Pentose Phosphate pathway (HMP Shunt). The ribose-5-P is converted into phosphoribosyl pyrophosphate by Pyrophospho Kinase in this reaction ATP is consumed . Step 2: PRPP is converted into Phosphoribosyl amine in the presence of the enzyme Amidophosphoribosyl transferase. Here amide group donor is Glutamine . Step 3: The phosphoribosyl amine is condensed with glycine it forms Glycinamide ribotide (GAR). This reaction is catalyzed by GAR Synthase . Step 4: Glycinamide ribotide is converted into Formyl glycine amide ribotide (FGAR). This reaction is catalyzed by transformylase . Here Formyl donor is N10-Formyl-THF . Step 5 : Formyl Glycine ribotide is converted into Formylglycinaidine ribotide (FGAM) in the presence of the enzyme FGAM synthetase . Here amide donor is Glutamine and it is ATP consumed reaction . Step 6: FGAM is converted into 5-amino imidazole ribotide (AIR). This reaction is catalyzed by AIR Synthetase . Here ATP is consuming.

5. Step 7: Amino imidazole ribotide is converted into CarboxyAmino Imidazole Ribotide (CAIR). This reaction catalyzed by AIR carboxylase. In this reaction Carbonic acid is substituted on 4th carbon atom as in the form of Carboxyl group (CAIR ). Step 8: Carboxy Amino Imidazole is converted into 5-AminoImidazole (N- Succinylocarboxamide ) ribotide (SACAIR). This reaction is catalyzed by SACAIR synthatase . In this reaction one Aspartic acid linked with Carboxyl group ATP is consumed . Step 9:SACAIR is converted into 5-AminoImidazole-4-CarboxyAmide Ribotide (AICAR). This reaction is catalyzed by Adenosuccinate Lyase . The linked Aspartic acid hydrolyzed as Fumarate, which directly enter into TCA cycle . Step 10: AICAR is converted into 5-FormaminoImidazole-4-Carboxamide Ribotide (FAICAR). This reaction is catalyzed by Transformylase . Here formyl group donor is N10-Formyl THF . Step 11: FAICAR is converted into Inosine Mono Phosphate (IMP) by the catalyzation process. This reaction is catalyzed by IMP Cyclohydrolase .

6. Reaction 1: Formation of PRPP

7. Reaction 2: Formation of 5-phosphoribosyl-1-phosphate Glutamine phosphoribosyl amidotransferase

8. Reaction 3: Formation of glycinamide ribonucleotide

9. Reaction 4: Formation of formylglycinamide ribonucleotide Formyltransferase

10. Reaction 5: Formation of formylglycinamidine ribonucleotide VI synthetase

11. Reaction 6: Formation of 5-Aminoimidazole ribonucleotide VII synthetase H 2 O

12. Reaction 7: Formation of carboxyaminoimidazole ribonucleotide VII carboxylase

13. Reaction 8: Formation of 5-Aminoimidazole-4-(N- succinylcarboxamide ) ribonucleotide IX synthetase

14. Reaction 9: Formation of 5-Aminoimidazole-4-carboxamide ribonucleotide Adenylosuccinase

15. Reaction 10: Formation of 5-formaminoimidazole-4-carboxamide ribonucleotide Formyltransferase

16. Reaction 11: Inosinate IMP cyclohydrolase

17. Conversion of IMP into AMP and GMP Adenylosuccinate synhetase IMP dehydrogeanse Adenylosuccinase Transamidinase

18. i:Conversion into AMP Step 1: The IMP is converted into adenyloSuccinate by taking Aspartate and GTP, Which gives the power by the UTP to GTP and inorganic phosphate. This reaction is catalyzed by Adenylo Succinate synthatase . Step 2: Adenylo Succinate is converted into AMP by releasing Aspartate as in the form of Fumarate. This reaction is catalyzed by Adenylo Succinate Lyase .

19. ii: Conversion into GMP Step 1: IMP is converted into Xanthosine Monophosphate in the presence if the enzyme IMP-dehydrogenase. This is the dehydrogenation . Step 2: XMP is converted into GMP by the enzyme GMP synthase. Here Amino group donor is Glutamate.

20. Organ and tissues where synthesis occur Liver is major site of synthesis and provides purines and purine nucleosides for: Salvage Tissues incapable of synthesis Human brain has low level of PRPP glutamyl amidotransferase and depends upon exogenous purines. RBC and polymorphonuclear WBC cannot synthesize 5-phosphoribosylamine and needs exogenous purines.

21. b: Salvage pathway Requires less energy then de novo synthesis. Convert purines, their ribonucleosides and deoxyribonucleosides into their nucleotides. Free bases derived from: Turnover of nucleotides Diet Involves two ways: Phosphoribosylation of purines by PRPP Phosphoryl transfer from ATP to nucleosides

22. Phosphoribosylation of purines by PRPP Most important method Catalyzed by Adenosine and Hypoxanthine phosphoribosyl transferase.

23. Phosphoryl transfer from ATP to nucleosides Less common Catalyzed by adenosine kinase or deoxycytidine kinase. Pu-R + ATP  Pu-R-P + PPi

24. Synthesis of Ribonucleosides diphosphates and triphosphates NMP and NDP must be converted into NTP for participation in DNA and RNA chain. NDP is synthesized from NMP by phosphoryl transfer from ATP. It is catalyzed by monophosphate kinase. NTP is synthesized from NDP by 2 nd phosphoryl transfer from ATP. It is catalyzed by diphosphate kinase. ADP to ATP conversion is mainly done b oxidative phosphorylation

25. AMP + ATP ADP + ADP GMP + ATP GDP + GTP GDP + ATP GTP + ADP

26. B: Conversion into deoxyribonucleotides Conversion of ribonucleotides into deoxyribonucleotides involves replacement of hydroxyl group at C2 with hydrogen atom. Substrate= NDP or NTP Enzyme= rinonucleotide reductase The R1 subunit contains contains 3 cysteine and a glutamate residues. All these four participate in reduction of ribose. Reductant= NADPH

27. Mechanism The reaction begins with transfer of electron from a cys residue on R1 to tyrosyl radical on R2. The loss of electron generates a highly reactive cys thiyl radical within active site of R1. This radical abstracts a hydrogen atom from C3 of ribose unit and generating radical at that carbon atom. The radical at C3 promotes release of OH ion on C2 atom. Protonated by a 2 nd cys residue and departed OH ion leaves as a water. A hydride ion is then transferred from a third cys residue to complete reduction of C2 position, forming disulfide bond and reforms C3 radical. This C3 radical recaptures the same hydrogen atom originally abstracted by first cys residue and deoxyribonucleotides are free to leave enzyme. The disulfide bond regenerated in enzyme active site is then reduced by specific disulfide containing proteins such as thioredoxin to regenerate the active enzyme.

28. C: Regulation Inhibition The essential rate limiting steps in purine biosynthesis occur at the first two steps of the pathway. The major determinant is PRPP concentration which depends upon availability of ribose-5-phosphate and activity of PRPP synthase. The synthesis of PRPP by PRPP synthetase is feed-back inhibited by purine-5'-nucleotides (predominantly AMP and GMP). Combinatorial effects of those two nucleotides are greatest, e.g., inhibition is maximal when the correct concentration of both adenine and guanine nucleotides is achieved . The amidotransferase reaction catalyzed by PRPP amidotransferase is also feed-back inhibited allosterically by binding ATP, ADP and AMP at one inhibitory site and GTP, GDP and GMP at another. Conversely the activity of the enzyme is stimulated by PRPP . Additionally, purine biosynthesis is regulated in the branch pathways from IMP to AMP and GMP. AMP and GMP feedback inhibits adenylosuccinate synthetase and IMP dehydrogenase. Activation The activator molecule for Purine synthesis is PRPP, which activates the enzyme AmidoPhospho Ribosyl transferase The initiator molecule for the synthesis is Ribose-5-Phosphate. The optimum concentration of Ribos-5-Phosphate is maintained by the enzyme Glucose-6-Phosphate dehydrogenase, which is regulatory enzyme of Hexose Mono Phosphate Shunt . The accumulation of excess ATP leads to accelerated synthesis of GMP, and excess GTP leads to accelerated synthesis of AMP. It is because conversion of IMP into AMP requires GTP and similarly conversion of XMP into GMP requires ATP.