Nucleotide Metabolismlibvolume6.xyz/molecularbiology/bsc/semester2/...biosynthesis • Synthesis of...
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Nucleotide MetabolismNucleotide Metabolism
Nucleotide SynthesisNucleotide Synthesis
• De Novo Pathway– Synthesize purine and pyrimidine nucleotides
from low M.W. precursors• Salvage Pathway
– synthesize nucleotides from nucleosides of nucleobases
NB: important targets for therapy of microbial or parasitic diseases
Nucleic Acid DegradationNucleic Acid Degradation
• Intracellular Degradation– Turnover of unstable RNA; cell death; ingested
nucleic acids
• Extracellular Degradation– Major route by which nucleosides or
nucleobases become available in animals
Nucleic Acid DegradationNucleic Acid Degradation
– endonucleases → oligonucleotides
– phosphodiesterases → mononucleotides
– nucleotidases → ortho PO4; nucleosides
– nucleoside phosphorylase → base; ribose-1-P
Nucleoside Nucleoside PhosphorylasePhosphorylase
PRPP is a Central Metabolite in De Novo and Salvage PathwaysPRPP is a Central Metabolite in De Novo and Salvage Pathways
• An intermediate in histidine and tryptophan biosynthesis
• Synthesis of nucleoside-5’-phosphate (nucleotide) from free bases
LowLow––Molecular Weight Precursors to the Molecular Weight Precursors to the PurinePurine RingRing
• Feedback regulation of early steps– PRPP synthetase (–) by AMP, ADP, GDP– PRPP amidotransferase (–) by AMP, GMP
(synergistic inhibition)
Pathways from Pathways from inosinic inosinic acid to GMP and acid to GMP and AMPAMP
• Energy drive is from GTP/ATP– A way to control
the proportion of IMP that go to adenine and guanine nucleotide synthesis
• (reciprocal substrate relation)
Conversion of Nucleoside Conversion of Nucleoside Monophosphate Monophosphate to the to the TriphosphateTriphosphate
• Step 1- Specific ATP-Dependent Kinase
– Guanylate kinase– Adenylate kinase
• Step 2 – Non-Specific ATP-Dependent
– Nuceloside diphosphokinase
Control of Control of Purine Purine BiosynthesisBiosynthesis
• Primates → uric acid• Most mammals further oxidize the purine
ring to allantoin and to allantoic acid or further to urea
Enzymatic abnormalities that lead to Enzymatic abnormalities that lead to hyperuricemia hyperuricemia and gout and gout by elevating the rate of de novo by elevating the rate of de novo purine purine nucleotide biosynthesisnucleotide biosynthesis
GoutGout
• Excessive accumulation of uric acid
• 3/1000 suffer from HYPERURICEMIA– Urate ppt. causes inflammation in the joints→ Painful arthritis
• Allopurinol inhibits xanthine oxidase• Solubility: hypoxanthine/xanthine > uric acid
De Novo Synthesis of De Novo Synthesis of Pyrimidine Pyrimidine NucleotidesNucleotides
• Pyrimidine ring is assembled as a free base
• Unbranched pathway
• Aspartate transcarbamoylase(–) by CTP(+) by ATP
Pyrimidine Pyrimidine RingRing
Control of Control of Pyrimidine Pyrimidine BiosynthesisBiosynthesis
Catabolic Pathways in Catabolic Pathways in Pyrimidine Pyrimidine Nucleotide Nucleotide MetabolismMetabolism
Overview of Overview of Deoxyribonucleotide Deoxyribonucleotide BiosynthesisBiosynthesis
Overview ofOverview of DeoxyribonucleotideDeoxyribonucleotide BiosynthesisBiosynthesis
• Close regulatory relationships between DNA synthesis and dNTP metabolism
• Conversion of ribose to deoxyribose
• Conversion of uracil to thymine
Mechanism for Reduction of a Mechanism for Reduction of a Ribonucleoside Ribonucleoside Diphosphate Diphosphate by by rNDP ReductaserNDP Reductase
• Replacement of the 2’-hydroxyl moiety of the sugar by a hydride ion – retention of configuration
• Ribonucleoside diphosphate reductase (rNDP)
Reductive electron transport sequences in the Reductive electron transport sequences in the action of action of ribonucleoside diphosphate reductaseribonucleoside diphosphate reductase
Ribonucleoside Diphosphate ReductaseRibonucleoside Diphosphate Reductase
• Both activity and specificity being regulated– To maintain balanced pools of DNA precursors
(1) Activity Sites• Low affinity binding of ATP or dATP• ATP binding → (+)• dATP binding → (–)
Ribonucleoside Diphosphate ReductaseRibonucleoside Diphosphate Reductase
(2) Specificity Sites• High affinity binding of ATP, dATP, dGTP or dTTP
• Modulates the activities of enzyme toward different substrates
– Maintain a balanced rate of production of dNTPs
Salvage and De Novo Synthetic Pathways to Salvage and De Novo Synthetic Pathways to Thymine Thymine NucleotidesNucleotides
Relationship between Relationship between thymidylate synthase thymidylate synthase and and enzymes of enzymes of tetrahydrofolate tetrahydrofolate metabolismmetabolism
Thymidylate Thymidylate Synthesis: A Target Site for Synthesis: A Target Site for Cancer ChemotherapyCancer Chemotherapy
• Rapidly dividing cells (eg. cancer cells) require an abundant supply of deoxythymidylate for DNA synthesis
• Thymidylate synthase & Dihydrofolate reductase – TARGET ENZYMES
ThymidylateThymidylate Synthesis: A Target Site for Synthesis: A Target Site for Cancer ChemotherapyCancer Chemotherapy
• Fluorouracil → fluorodeoxyuridylate (F-dUMP)
↓
suicide inhibition of thymidylate synthesis
ThymidylateThymidylate Synthesis: A Target Site for Synthesis: A Target Site for Cancer ChemotherapyCancer Chemotherapy
• Aminopterin and methotrexate inhibit dihydrofolate reductase
• Acute leukemia• Choriocarcinoma
–Rapidly growing tumors