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Nucleotides Purine Pyrimidine Metabolism
Chemistry of Nitrogenous bases and Nucleotides. Sources of Carbon and Nitrogen in purine and pyrimidine metabolism
Nucleotides Purine Pyrimidine Metabolism
- 1. Chemistry of Nitrogenousbases and Nucleotides. Sources of Carbon and Nitrogen in Purine and Pyrimidine Metabolism Dr. Satyaki Basu
- 2. QUESTION • A middleaged lady presenting with acute pain in small and large joints and nodular swelling of joints. • H/o diet rich in meat, sea food and ethanol. • Provisional Diagnosis? GOUT
- 3. Biomedical Importance ofNucleotides • Building blocks of nucleic acids • Which is responsible for genetic information and its transmission in all species. • Co enzymes • NAD+, NADP+, FAD etc. • Sources of high energy • ATP, GTP ADP etc. • Role in various metabolic reactions • Oxidative phosphorylation • Synthetic analogues • t/t of cancer, AIDS, immunosupression.
- 4. STRUCTURE OF NUCLEOTIDES •3 components • Nitrogenous base (N) • A sugar (S) • Phosphate group (P) • N + S = nucleoSide • N+S+P = nucleoTide
- 5. Nitrogenous Bases • Purines •Pyrimidines • Heterocyclic Nitrogenous ring structures
- 6. Purines • Larger molecule- shorter name • 9 member ring
- 7. Pyrimidine • Smaller molecule– Larger name • 6 member ring
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- 10. Uracil is absentin DNA primarily because cytosine, a DNA base, can spontaneously deaminate into uracil, which would lead to mutations if uracil were a standard DNA component. Thymine, which is present in DNA, is more stable and resistant to photochemical damage, making it a better choice for long-term genetic information storage.
- 11. Purines and pyrimidinebases have two important structural features. • They are planar in nature and this helps in the stacking of bases one above the other. This property is useful in stabilization of DNA structure. • Due to the presence of oxy and amino groups, they show keto-enol and amino-imino tautomerism
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- 13. • Linkage withRibose • = Ribonucleosides • Linkage with deoxy- ribose • = Deoxyribonucleosides • Linkage in both is by beta-N-glycosidic bond
- 14. NUCLEOTIDES (N+S+P) • Phosphorylated nucleosides. •When 5’-OH group of ribose of a ribonucleoside or of a 2’-deoxyribonucleoside is phosphorylated, Ribonucleotide or a 2’deoxyribonucleotide is formed AMP Ribonucleotide 2’d AMP Deoxyribonucleoti de
- 15. • Additional phosphategroups are also added to the terminal 5'-phosphate group to give rise to diphosphate and tri- phosphate derivatives all of which are nucleotides.
- 16. • So differentribonucleotides and deoxyribonucleotides differ from each other only in the purine or pyrimidine base attached to them
- 17. ATYPICAL AND MODIFIEDBASES • Besides five main bases (A, G, U, C, T), nucleic acid also contains some other bases which are modified or atypical. • These atypical bases have a role in recognition of oligonucleotides containing them and regulation of their half lives. • Examples: • Dihydrouracil in tRNA • Methylated xanthines are present as caffeine – in coffee
- 18. Synthetic Nucleotide Analogues •Used in treatment of cancer: • 5 flurouracil • 6 mercaptopurine • Treatment of other diseases: • Allopurinol-A purine analogue is used in the treatment of gout. • Azathioprine—A purine derivative used as immuno- suppressant following the organ transplants.
- 19. PURINE METABOLISM • Synthesis •Biosynthesis occurs in two major pathways : • De novo purine biosynthesis occurs from basic precursors and a new purine ring is synthesized using various metabolic intermediates as sources of carbon, nitrogen etc. • Salvage pathways re-utilize preformed (not dietary) free purine bases or nucleosides to make nucleotides.
- 20. De novo PurineBiosynthesis Pathway • Occurs primarily in Liver. • De novo synthesis is an expensive process for the cell and uses many important metabolic intermediates in the synthesis of purine ring. Sources of Atoms of Purine ring
- 21. De novo PurineBiosynthesis Pathway cont.d • In de novo purine biosynthesis pathway alpha-D-ribose5- phosphate is used to synthesize a nucleotide, inosine monophosphate (IMP). • This IMP is then converted into AMP and GMP which are the end products of this pathway.
- 22. Formation of Diphosphateand Triphosphate Nucleotides • AMP and GMP are phosphorylated using ATP as the source of phosphate to first make nucleoside diphosphate and then nucleoside triphosphate i.e. ADP, ATP, GDP and GTP
- 23. Formation of DeoxyRibonucleotides from Ribonucleotides • Deoxyribonucleotides are required for DNA synthesis.They are formed from ribonucleoside diphosphates. • This reduction occurs in actively DNA synthesizing cells.
- 24. Purine Salvage Pathway •In purine salvage pathway, free purine bases or nucleosides are re-utilized to make nucleotides. • No new nucleotides are synthesized from precursors. • The advantage is that this requires much less energy and metabolic intermediates than the de novo synthesis. • In Brain, RBCs, Neutrophils
- 25. Purine Salvage Pathwaycont.d • This takes place in two ways: • 1) Conversion of free purines into nucleotides • Hypoxanthine guanine phosphoribosyl transferase(HGPRTase) and Adenine phosphoribosyl transferase(APRTase) are the enzymes. • This is the main pathway
- 26. Purine Salvage Pathwaycont.d • 2) Conversion of nucleosides into nucleotides • This pathway uses kinase enzymes, adenosine kinase and deoxycytidine kinase, etc.
- 27. Purine Catabolism • Uricacid is the end product.
- 28. Disorders of PurineMetabolism • 1) GOUT • At pH of 5.75 and above uric acid forms a salt; monosodium urate which is 10 times more soluble in plasma than uric acid(very little solubility in aqueous Plasma). • In renal tubules where acidification of urine occurs, pH is usually below 4.0. At such a low pH, uric acid is in the undissociated uric acid form which is much less soluble. • Hence when in excess, it is precipitated and deposited as uric acid stones in kidneys. • In the soft tissues, it is precipitated and deposited as monosodium urate crystals as pH of synovial fluid is 7.4 • Any condition that decreases blood pH(Acidosis) therefore promotes formation of uric acid.
- 29. GOUT cont.d • Increasein blood uric acid level above the normal value of 7 mg% is called Hyperuricemia. • Chronic hyperuricemia leads to renal stone formation and deposition of monosodium urate crystals into joints. This leads to chronic inflammation and arthritis of joints specially the big toe. • This arthritis resulting from hyperuricemia is called gout. It is a debilitating painful condition leading to deformity of joints.
- 30. GOUT cont.d • Jointinflammation is caused by deposition of monosodium urate crystals as aggregates in joint spaces. These manifest as small nodules called tophi.
- 31. GOUT cont.d • Typesof Gout: Primary & Secondary • Primary gout: due to metabolic defect where uric acid synthesis is increased. • Secondary gout: due to increased nucleotide turnover wherein more uric acid is formed. Uric acid metabolism is normal.
- 32. GOUT cont.d Treatment: isaimed at the following: • removal of stones, if any • enhancing uric acid excretion by alkalinization of urine • inhibiting the uric acid formation by medicines e.g. febuxostat, allopurinol, which inhibits xanthine oxidase and decreases the uric acid production.
- 33. • 2) HGPRTasedeficiency-Lesch-Nyhan Syndrome • Hyperuricemia • Neurological deficit manifesting as spasticity, mental retardation, etc • Self mutilation-a unique feature in which children often bite their lips and finger tips. • The deficiency of HGPRTase enzyme is the cause. • No definitive treatment. • Patient dies of kidney failure.
- 34. • 3) SCID •This autosomal-recessive deficiency causes a type of severe combined immunodeficiency (SCID),involving T-cell, B-cell, and natural killer-cell depletion (lymphocytopenia). • Untreated ADA-deficient children usually die before age 2 years from overwhelming infection • Treatments include: BMT, ERT, and gene therapy. • 4) PURINE NUCLEOSIDE PHOSPHORYLASE (PNP) DEFICIENCY • This autosomal-recessive deficiency is rarer and less severe than ADA deficiency. • It affects T cell development, primarily. • PNP-deficient individuals have recurrent infections and neurodevelopmental delay
- 36. PYRIMIDINE METABOLISM • Synthesis •Synthesis of pyrimidine nucleotides occurs both by denovo pathway and salvage pathway. • The enzymes of synthetic pathway are both cytosolic and mitochondrial- only one enzyme is mitochondrial. • The enzymes of pyrimidine synthesis pathway are not independent Proteins. They are arranged as multi enzyme protein complex.
- 37. Sources of atomsof pyrimidine ring • The constituent atoms of pyrimidine ring are derived from glutamine, CO2 and aspartate
- 38. Disorders of PurineMetabolism • Hereditary Orotic Aciduria • Hereditary orotic aciduria is a metabolic defect of denovo- synthesis of pyrimidine nucleotides. • The defective enzymes are orotidyl phosphoribosyltransferase and orotidyl decarboxylase. • Type I has deficiency of both the enzymes while Type II has only decarboxylase deficiency. • The disorder is characterized by excessive urinary excretion of orotic acid. Children usually have severe anemia and retarded body growth. • Treatment: diet rich in Uridine
- 39. Extra Points • Alcoholand gout • precipitation of acute attack of gout occurs following alcohol ingestion. • Alcohol on metabolism leads to increase in NADH+ concentration after its conversion to acetate and then oxidation in TCA. • Increased NADH+ promotes uric acid formation rather than monosodium urate.
- 40. • Carbamoyl phosphatesynthetase