Nucleic acids metabolism Dr. Gamal Gabr, College of Pharmacy

Nucleotides serve as building blocks for RNA and DNA. OVERVIEW Nucleotides serve as building blocks for RNA and DNA. Nucleotides can serve as sources of energy (i.e., ATP), physiological signaling mediators (i.e., adenosine in control of coronary blood flow), secondary messengers (cAMP and cGMP), and allosteric enzyme effectors. Nucleotides can be synthesized de novo, or from components “salvaged” from the degradation products of nucleic acids. Dr. Gamal Gabr, College of Pharmacy

de novo and salvaged Pathway Nucleotides can be synthesized de novo, or from components “salvaged” from the degradation products of nucleic acids. When in excess, nucleotides are degraded to products that can either be consumed by other pathways or excreted. Defects in the pathways for de novo synthesis, salvage, and nucleotides degradation result in clinical disorders, and many drugs target these pathways. Dr. Gamal Gabr, College of Pharmacy

(1) Biosynthesis of purine nucleotides  The purine nucleotides are synthesized by most of the tissues.  The major site is the liver. This pathway operates in the cytoplasm.  The major pathway is denoted as de novo synthesis, because the purine ring is synthesize from different small components and from different sources.  During de novo synthesis, purine ring is build up on a ribose-5- phosphate molecule. Dr. Gamal Gabr, College of Pharmacy

Dr. Gamal Gabr, College of Pharmacy

The purine ring is constructed by a series of reactions that add the donated carbons and nitrogen to a preformed ribose 5-phosphate. Dr. Gamal Gabr, College of Pharmacy

PURINE NUCLEOTIDE SYNTHESIS Dr. Gamal Gabr, College of Pharmacy

(2) De novo synthesis of pyrimidine nucleotides Dr. Gamal Gabr, College of Pharmacy

(3): Degradation of purine nucleosides The end product of purine nucleotide catabolism is uric acid (Urate). This degradation is taking place mainly in the liver. Normal blood level of uric acid ranges from 2-5 mg/dl in females, and 3-7 mg/dl in males. Nucleic acid content is more in non vegetarian diet Dr. Gamal Gabr, College of Pharmacy

NUCLEOTIDE CATABOLISM AND SALVAGE Nucleotide turnover occurs continuously in cells. Breakdown of DNA and RNA releases nucleoside 5’-monophosphates, which can be hydrolyzed by 5’-nucleotidases to yield nucleosides. Although both purine and pyrimidine nucleosides can be degraded to waste products that are excreted, the catabolic pathways have branch points in most cells at which the components of nucleotides can be salvaged. Having shared catabolism and salvage pathways saves metabolic energy while preventing nucleotide pools from reaching toxic levels. Dr. Gamal Gabr, College of Pharmacy

NUCLEOTIDE CATABOLISM AND SALVAGE Dr. Gamal Gabr, College of Pharmacy

Salvage pathway This pathway ensures the recycling of purines formed by degradation of nucleotides. Nucleosides and deoxy-nucleosides can also be salvaged.  PRPP is the starting material in this pathway. It is also a substrate for de novo synthesis. Hence , these two pathways are closely inter-related.  The pathway is of importance in tissues like RBCs and brain where the de novo pathway is not operating. Dr. Gamal Gabr, College of Pharmacy

DE NOVO RIBONUCLEOTIDE SYNTHESIS Purine and pyrimidine nucleoside 5’-monophosphates can be synthesized de novo from PRPP and various carbon and nitrogen donors. A few important drugs inhibit key steps in de novo purine nucleotide synthesis and as a result kill rapidly dividing cells. Dr. Gamal Gabr, College of Pharmacy

PURINE SALVAGE Dr. Gamal Gabr, College of Pharmacy

Diseases associated with purine degradation Gout Gout is a disorder characterized by high levels of uric acid, the end product of purine catabolism in blood (hyperuricemia), as a result of either the overproduction or under excretion of uric acid. The hyperuricemia leads to the deposition of monosodium urate crystals in the joints, and an inflammatory response to the crystals, causing first acute and then to chronic gouty arthritis. Dr. Gamal Gabr, College of Pharmacy

Diseases associated with pyrimidine metabolism Orotic aciduria Excessive excretion of orotic acid in urine. It causes a characteristic form of anemia and may be associated with mental and physical retardation.. Dr. Gamal Gabr, College of Pharmacy

Diseases associated with pyrimidine metabolism The major control of de novo pyrimidine nucleotide synthesis in man is competitive inhibition of carbamoyl phosphate synthetase II AMP is first deaminated to IMP. Then it loses the phosphate and the sugar to form the free base, hypoxanthine. Both hypoxanthine and xanthine are oxidized by xantine oxidase. So, The following enzymes are involved in the catabolism of AMP to uric acid in order: A deaminase , A nucleotidase , A nucleoside phosphorylase , Xanthine oxidase Dr. Gamal Gabr, College of Pharmacy

Diseases associated with pyrimidine metabolism In the catabolism of CTP: Both the side-chain and ring nitrogen are released as ammonia. So: nitrogen will be released in the form of ammonia (ammonium ion). The formation of dATP for DNA synthesis occurs primarily by: converting ADP to dADP using thioredoxin. Inhibition of xanthine oxidase would NOT be expected to contribute to hyperuricemia (gout) Direct sources of purine ring atoms in the de novo synthesis of IMP include: glutamine, aspartate, glycine. Dr. Gamal Gabr, College of Pharmacy

Diseases associated with pyrimidine metabolism A nucleoside phosphorylase is: cleaves a nucleoside with the production of ribose 1-phosphate, is necessary for the major salvage pathway for pyrimidines, is used in the degradation of purine nucleotides. If a cell has an adequate supply of adenine nucleotides but requires more guanine nucleotides for protein synthesis: ATP will stimulate the production of GMP from IMP Major controls of de novo AMP synthesis include: allosteric inhibition by GMP allosteric inhibition by AMP availability of PRPP stimulation by GTP Dr. Gamal Gabr, College of Pharmacy

Diseases associated with pyrimidine metabolism Aspartate plays a role in de novo synthesis of AMP, de novo synthesis of orotic acid, the synthesis of most proteins but not in conversion of UTP to CTP Allopurinol is an inhibitor of xanthine oxidase. Administration of allopurinol to a patient with gout and normal HG-PRT levels would be expected to lead to: decreased de novo synthesis of IMP decreased urate in the urine an increase in hypoxanthine in the blood Not increased levels of PRPP Dr. Gamal Gabr, College of Pharmacy