Dr Carty Lecture 3: Nucleotide Metabolism 2024-25 PDF

UNIT 4: Nitrogen metabolism Nucleotide metabolism (Chapter 22, Lippincott Biochemistry) 1  What are amino acids required for in the body? 1. Majority of amino acids 3. used in protein synthesis...

UNIT 4: Nitrogen metabolism Nucleotide metabolism (Chapter 22, Lippincott Biochemistry) 1  What are amino acids required for in the body? 1. Majority of amino acids 3. used in protein synthesis Proportion used as precursors of other. Proportion is: nitrogen i) completely metabolised compounds or energy or ii) used to form glucose r ketone bodies 2 mino acids as precursors of other nitrogen-containing compounds Compound Amino acid(s) required Heme Glycine Dopamine Tyrosine Histamine Histidine Nitric oxide Arginine Creatine Arginine, Glycine Thyroxine (thyroid hormone) Tyrosine Serotonin Tryptophan Glutathione (anti-oxidant) Glycine, Cysteine, Glutamate Purines Glutamine, Aspartate, Glycine Pyrimidines Glutamine, Aspartate 3 rines and pyrimdines bases are important nitrogen-containing mole Purines: A and G Pyrimidines: T, C and U Bases found in DNA and RNA Also important for ATP: energy cAMP, cGMP: signalling 4  The sugar is different between RNA and DNA Lack of 2’OH makes DNA more stable than RNA enzyme ribonucleotide reductase converts ribonucleotides to deoxyribonucle 5 Nucleotide: base plus sugar plus phosphate group 6  There are two ways humans obtain pyrimidine and purine nucleotides 1. Synthesis in cells 2. Salvage of bases from nucleic acids in diet 7 Pyrimidine biosynthesis 8  Pyrimidine biosynthesis 4 3 5 2 6 1 The precursors of the pyrimidine ring are: Glutamine CO2 Aspartate  The two nitrogen atoms in the ring come from amino 9 acid  Biosynthesis of pyrimidines MAIN POINTS 6 steps Carbamoyl phosphate is synthesised and condensed with aspartate Then ring closure occurs; this generates orotate A ribose sugar (PRPP) is then condensed with orotate. UMP is produced Note: PRPP = phosphoribosyl pyrophosphate 10  Defect in pyrimidine synthesis: orotic aciduria Orotate Pathway is regulated by feedback inhibition intermediate 11 TP is converted to CTP by addition of an amino grou 12  Summary Amino acids are also precursors of pyrimidine bases. In pyrimidine synthesis, pyrimidine ring is made first, then the ribose sugar (PRPP) is attached. UTP is converted to CTP. Pyrimidine biosynthesis is regulated by feedback inhibition. 13  Anti-cancer drugs targeting pyrimidine biosynthesis 5-Fluorouracil dUMP is converted to dTMP by addition of a methyl group, carried by tetrahydrofolate Methotrexate Drugs inhibit key enzymes: reduce supply of dTTP available for cell divi Side effects due to killing rapidly growing epithelial cells. 14  Summary of pyrimidine biosynthesis Amino acids are precursors of pyrimidine bases. In pyrimidine synthesis, pyrimidine ring is made first, then the ribose sugar (PRPP) is attached. UTP is converted to CTP; regulation by feedback inhibition. dUDP is converted to dTTP. Target for anti-cancer drugs. 15 Purine biosynthesis 16 Purine biosynthesis 17  Purine biosynthesis Y POINTS he purine ring is built up, one or a few atoms at a time, tached to the ribose sugar (PRPP) throughout the process. e nitrogen atoms in purine nucleotides come from amino acids ycine, partate and utamine e precursors of the nitrogen atoms in the purine ring ong, complex pathway: Nitrogen atoms derived from amino acids Tetrahydrofolate acts as a carrier of one-carbon groups. mportance of folic acid to maintain supply of nucleotides for cell division 18 rine biosynthesis produces inosine monophosphate (IM P is converted to AMP and to GMP, by addition of an amino g Feedback Feedback inhibition inhibition 19 Nucleosides are phosphorylated to give rise to nucleoside triphos 20  Summary of purine biosynthesis 1. Nitrogen atoms in purine ring come from amino acids 2. Purine ring built up by attaching atoms to sugar PRPP 3. Complex series of reactions generates IMP 4. IMP is converted to GMP and AMP 5. Pathway regulated by feedback inhibition 21  There are two ways humans obtain pyrimidines and purines 1. Synthesis in cells Energy-requiring process 2. Salvage of bases from nucleic acids in diet 22  Salvage pathway for purines We can also get purines from nucleic acids taken in e.g. in diet. Purine bases that are released during degradation of nucleic acids can be converted back to purine nucleotides 23 Nucleic acids in food are broken down to nucleosides, and then to base Nucleic acids present in food are degraded The bases released can be either: a) used (‘salvaged’) to regenerate nucleotides or b) excreted 24 xamples of enzymes that convert bases back to nucle 25  Example: Salvage pathway for guanine Base Sugar Disease: Lesch-Nyhan syndrome HGPRT deficiency (hypoxanthine guanine phosphoribosyltransferase) 26  Lesch-Nyhan syndrome X-linked disease, affects boys Approximately 1/380,000 Severe neurological and developmental effects Biting fingers and lips characteristic: self-injury Due to deficiency in HGPRT, caused by mutation of the HGPRT gene Overproduction of uric acid; increased PRPP and purines Having less than 1.5% of the normal level of HGPRT enzyme activity is associated with the disease 27 Purine degradation 28 ic acids in food are broken down to nucleosides, and then to Nucleic acids present in food are degraded The bases released can be either: a) used (‘salvaged’) to regenerate nucleotides or b) excreted 29  Purine degradation (catabolism) Nucleotides degraded to release bases Purines are broken down to uric acid (urate) Diseases of purine degradation: 1. GOUT is a disease of purine breakdown High serum levels of urate cause the disease gout Overproduction of uric acid, resulting from alterations in enzymes of the purine breakdown pathway or increased availability of purine Defects in uric acid excretion also cause gout. 2. Adenosine deaminase deficiency causes Severe combined immunodeficiency (SCID) Affects B and T cell proliferation Gene therapy to treat condition 30  Purine degradation AMP IMP Hypoxanthine AMP deaminase 31  GOUT is a disease of purine degradation Overproduction of uric acid, or reduced excretion, leads to Gout Solubility of urate is 7mg/100ml Above this level sodium urate precipitates Due to precipitation of sodium urate crystals Leads to inflammation of the joints Damage to kidneys Treated by (i) reducing nucleotide-rich foods in diet (ii) by drug allopurinol, an inhibitor of xanthine oxidase 32 Gout leads to inflammation and swelling of joints Uric acid crystals deposited 33 Diseases of purine degradation are medically signifi ADA deficiency: SCID. Gene therapy Gout 34  Summary of nucleotide metabolism 1. Nucleotides can be synthesised de novo, or salvaged from diet 2. Nitrogen atoms in pyrimidine ring come from glutamine and aspartate 3. Pyrimidine biosynthesis targeted by anti-cancer drugs 4. Purine ring built up by attaching atoms to sugar PRPP in a complex series of reactions generating IMP; IMP converted to AMP and GMP 5. Salvage pathway for nucleotides important: Defect in this pathway leads to Lesch-Nyhan syndrome 6. Gout is a disease of purine degradation 35