Biosynthesis of purine nucleotides:

1. METABOLISMOFPURINE AND PYRIMIDINE NUCLEOTIDESM.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

2. Biosynthesis of purine nucleotides: • The three processes that contribute to purine nucleotide biosynthesis are. • Synthesis from amphibolic intermediates ( synthesis de novo ). • Phosphoribosylation of purines. • Phosphorylation of purine nucleosides.

3. Purinesare synthesized by most of the tissues ,the major site is liver.Subcellularsite -- cytoplasm • Denovosynthesis:Major pathway • Synthesis of purine nucleotides from various small molecules derived as intermediates of many metabolic pathways in the body. • Salvage pathway: Minor pathway

4. DENOVO SYNTHESIS Purine ring is built on ribose -5- phosphate

5. Parent purinenucliotide first synthesised is INOSINE MONO PHOSPHATE (IMP) It is a nucleotide composed of (HYPOXANTHINE + RIBOSE + PHOSPHATE ) • From IMP other purine nucleotides are synthesized, like • AMP(adenosine mono phosphate) • GMP(guanosine mono phosphate)

7. Formation of PRPP Addition of N9 Addition of C4 , C5 and N 7 Addition of C8

8. Addition of N3 Cyclisation (closure of ring) Addition of C6 Addition of N1

9. Removal of fumarate Addition of C 2 Cyclisation

11. Biosynthesis of PurineRibonucleotides 1. Ribose 5-phosphate, produced in the hexosemonophosphate shunt of carbohydrate metabolism is the starting material for purine nucleotide synthesis. It reacts with ATP to form phsophoribosyl pyrophosphate (PRPP). PRPP Synthetase is inhibited by PRPP

12. Glutamine transfers it’s amide nitrogen to PRPP to replace pyrophosphate and produce 5-phosphoribosylamine. The enzyme PRPP glutamylamidotransferaseis controlled by feedback inhibition of nucleoltides (IMP, AMP and GMP,).

13. 3.Phosphoribosylamine reacts with glycine in the presence of ATP to form glycinamideribosyl 5-phosphate or glycinamideribotide (GAR).

14. 4. N5,N10 formyltetrahydrofolatedonates the formyl group and the product formed is formylglycinamideribosyl 5-phosphate.

15. 5. Glutamine transfers the second amido amino group to produce formylglycinamideineribosyl 5-phosphate. GlnGlu ATP mg+ Synthetase

16. 6. The imidazole ring of the purine is closed in an ATP dependent reaction to yield 5-aminoimidazole ribosyl 5-phosphate H2O Ring closure ATP mg+ SYNTHETASE

17. 7. Incorporation of CO2 (carboxylation) occurs to yield aminoimidazolecarboxylateribosyl 5-phosphate. This reaction does not require the vitamin biotin and /or ATP which is the case with most of the carboxylation reaction. CO2 • Carboxylase

18. Aspartate condenses with aminoimidazolecarboxylateribosyl 5-phosphate. to form aminoimidazole 4-succinyl carboxamideribosyl 5-phosphate. aspertate H2O synthetase

19. Adenosuccinasecleaves off fumarte and only the amino group of aspartate is retained to yield aminoimidazole 4-carboxamide ribosyl 5-phosphate. • f Fffumarate arginosuccinase

20. 10. N10formyltetrahydrofolatedonates a one-carbon moiety to produce formimidoimidazole 4-carboxamide ribosyl 5-phosphate. With this reaction, all the carbon and nitrogen atoms of purine ring are contributed by the respective sources.

21. The final reaction catalysed by cyclohydrolaseleads to ring closure with an elimination of water molecule from formimidoimidazole ribosyl-5-P by Inosine - monophosphate (IMP) cyclohydrolase forms IMP.

22. Synthesis of AMP and GMP from IMP • Inosinemonophosphate is the immediate precursor for the formation of AMP & GMP • Aspertatecondences with IMP in the presence of GTP to produce sdenylosuccinate which on cleavage forms AMP. • For the synthesis of GMP, IMP undergoes NAD+ dependent dehydrogenation to form Xanthosinemonophosphate ( XMP). Glutamine then transfers amide nitrogen to XMP to produce GMP.

24. Inhibitors of purine synthesis. • Sulfonamides are the structural analogs of paraaminobenzoic acid (PABA). these sulfa drugs can be used to inhibit the synthesis of folic acid by microgranisms. this indirectly reduces the synthesis of purines and therefore, the nucleic acids (DNA and RNA). sulfonamides have no influence on humans, since folic acidf is not synthesized and is supplied through diet.

25. The structural analogs of folic acid (eg: methotrexate) are widely used to control cancer. They inhibit the synthesis of purine nucleotides and thus nucleic acids. These inhibitors also affect the proliferation of normally growing cells. This causes many side-effects including anemia, baldness, scaly skin etc.

26. SALVAGE PATHWAY FOR PURINES • The free purines ( adenine, guanine & hypoxanthine ) are formed in the normal turnover of nucleic acids & also obtained from the dietary sources. • The purines can also be converted to corresponding nucleotides, & this process is known as ‘salvage pathway’.

27. Adenine phosphoribosyltransferase catalyses the formation of AMP from adenine. • Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) converts guanine & hypoxanthine respectively, to GMP & IMP. • Phosphoribosyl pyrophosphate (PRPP) is the donor of ribose 5 phosphate in the salvage pathway. • The salvage pathway is perticularly important in certain tissues such as erythrocytes & brain where denovo synthesis of purine nucleotides is not operative.

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