Chapter 8 Metabolism of Nucleotides The biochemistry and molecular biology department of CMU
Degradation of nucleic acid Nucleoprotein Nucleic acid Protein Nuclease Nucleotide Nucleotidase Phosphate Nucleoside Nucleosidase Base Ribose
Significances of nucleotides 1. Precursors for DNA and RNA synthesis 2. Essential carriers of chemical energy, especially ATP 3. Components of the cofactors NAD+, FAD, and coenzyme A
Significances of nucleotides (continue) 4. Formation of activated intermediates such as UDP-glucose and CDP-diacylglycerol. 5. cAMP and cGMP, are also cellular second messengers.
There are two pathways leading to nucleotides De novo synthesis: The synthesis of nucleotides begins with their metabolic precursors: amino acids, ribose-5-phosphate, CO2, and one-carbon units. Salvage pathways: The synthesis of nucleotide by recycle the free bases or nucleosides released from nucleic acid breakdown.
§ 2.1 De novo synthesis • Site: in cytosol of liver, small intestine and thymus • Characteristics: a. Purines are synthesized using 5-phosphoribose as the starting material step by step. b. PRPP is active donor of R-5-P. c. AMP and GMP are synthesized further at the base of IMP.
4. Formation of NDP and NTP AMP + ATP 2 ADP NDP + ATP NTP + ADP Adenylate kinase nucleoside diphosphate kinase
5. Regulation of de novo synthesis The significance of regulation: (1) Fulfill the need of the body, without wasting. (2) [GTP]=[ATP]
§ 2.2 Salvage pathway The significance of salvage pathway : a. Save the fuel. b. Some tissues and organs such as brain and bone marrow are only capable of synthesizing nucleotides by a salvage pathway.
HGPRT: Hypoxanthine-guanine phosphoribosyl transferase • APRT: Adenine phosphoribosyl transferase • Absence of activity of HGPRT leads to Lesch-Nyhan syndrome.
§ 2. 4 Formation of deoxyribonucleotide • Formation of deoxyribonucleotideinvolves the reduction of the sugar moiety of ribonucleoside diphosphates (ADP, GDP, CDP or UDP). • Deoxyribonucleotide synthesis at the nucleoside diphosphate level.
§ 2. 5 Antimetabolites of purine nucleotides • Antimetabolites of purine nucleotides are structural analogs of purine, amino acids and folic acid. They can interfere, inhibit or block synthesis pathway of purine nucleotides and further block synthesis of RNA, DNA, and proteins.
1. Purine analogs • 6-Mercaptopurine (6-MP) is a analog of hypoxanthine.
- - - - - de novo synthesis • 6-MP nucleotide is a analog of IMP amidotransferase IMP 6-MP 6-MP nucleotide AMP and GMP HGPRT salvage pathway
2. Amino acid analogs • Azaserine (AS) is a analog of Gln.
3. Folic acid analogs • Aminopterin(AP)andMethotrexate (MTX)
Uric acid is the excreted end product of purine catabolism in primates, birds, and some other animals. • The rate of uric acid excretion by the normal adult human is about 0.6 g/24 h, arising in part from ingested purines and in part from the turnover of the purine nucleotides of nucleic acids.
The disease gout, is a disease of the joints, usually in males, caused by an elevated concentration of uric acid in the blood and tissues. The joints become inflamed, painful, and arthritic, owing to the abnormal deposition of crystals of sodium urate. The kidneys are also affected, because excess uric acid is deposited in the kidney tubules.
§ 4.1 De novo synthesis Characteristics: • The enzymes mostly lie in cytosol, but some enzymes exist in mitochondria. • The pyrimidine ring is first synthesized, then combines with PRPP. • UMP is first synthesized, then UMP is used for synthesizing other pyrimidine nucleotides.
Difference of carbamoyl phosphate synthetaseⅠand Ⅱ
3. Synthesis of CTP Amidation at the nucleoside triphosphate level.
4. Formation of dTMP The immediate precursor of thymidylate (dTMP) is dUMP. The formation of dUMP either by deamination of dCMP or by hydrolyzation of dUDP. The former is the main route.