Ch 26 - Nucleotides

What are nucleotides used for?

DNA, RNA, energy transfer, and signaling

What are the two nucleotide synthesis pathways?

De novo and salvage pathways

What is the de novo pathway?

Synthesis of nucleotides from small precursors

What is the salvage pathway?

Recycling free bases with PRPP

Why is the salvage pathway important?

Saves energy

What happens if salvage pathways fail?

Nucleotide deficiency and disease

What molecule activates ribose for nucleotide synthesis?

PRPP

What does PRPP stand for?

Phosphoribosyl pyrophosphate

What happens if PRPP levels are too high?

Purine overproduction and gout

What are de novo pathways dependent on?

ATP, amino acids, CO2, and PRPP

How are pyrimidines synthesized differently than purines?

Pyrimidine ring made first, purine ring built on sugar

What do de novo pathways initially produce?

Ribonucleotides

How are deoxyribonucleotides made?

Reduction of ribonucleotides

What produces thymidine nucleotides?

Methylation of dUMP

What are the precursors for pyrimidine synthesis?

HCO3-, NH3, and aspartate

What is the first committed step in pyrimidine synthesis?

Formation of carbamoylaspartate

What enzyme catalyzes committed pyrimidine synthesis?

Aspartate transcarbamoylase (ATCase)

What happens if ATCase is inhibited?

Pyrimidine synthesis decreases

What inhibits ATCase?

CTP

What activates ATCase?

ATP

Why does ATP activate ATCase?

Balances purine and pyrimidine synthesis

What happens if CTP accumulates?

Feedback inhibition of pyrimidine synthesis

What is carbamoyl phosphate synthase (CPS)?

Enzyme that synthesizes carbamoyl phosphate

What substrates does CPS use?

Bicarbonate, glutamine, and ATP

How many ATP are required by CPS?

2 ATP

What happens if CPS is defective?

Impaired pyrimidine synthesis

Why is channeling important in CPS?

Prevents intermediate loss and speeds reactions

What intermediate does CPS form first?

Carboxyphosphate

What amino acid donates nitrogen in pyrimidine synthesis?

Glutamine

What happens if glutamine is deficient?

Nucleotide synthesis decreases

What is CAD?

Multifunctional mammalian pyrimidine synthesis protein

What happens if CAD is defective?

Impaired pyrimidine biosynthesis

What is orotate?

Pyrimidine intermediate

What happens if orotate accumulates?

Orotic aciduria

What disease involves excess orotate?

Orotic aciduria

What symptoms occur in orotic aciduria?

Growth retardation and anemia

How is UMP formed?

Orotate reacts with PRPP

What happens if PRPP is unavailable?

Nucleotide synthesis decreases

What converts UMP to UDP?

UMP kinase

What converts UDP to UTP?

Nucleoside diphosphate kinase

What enzyme synthesizes CTP?

CTP synthetase

What does CTP synthetase use as nitrogen source?

Glutamine

What happens if CTP synthetase is inhibited?

CTP synthesis decreases

What nucleotide is precursor to CTP?

UTP

What happens if UTP is low?

CTP synthesis decreases

How is the purine ring synthesized?

Built stepwise on ribose

What are the atom donors in purine synthesis?

Gln, Gly, Asp, THF, and CO2

What is the committed step of purine synthesis?

Formation of phosphoribosylamine

What happens if committed step is inhibited?

Purine synthesis decreases

What enzyme uses glutamine in committed step?

Glutamine PRPP amidotransferase

What inhibits purine synthesis?

IMP, AMP, and GMP

What happens if purine feedback inhibition fails?

Overproduction of purines

What disease is linked to excess purines?

Gout

What is IMP?

Common precursor for AMP and GMP

Is IMP used directly in DNA or RNA?

No

What ring does IMP contain?

Hypoxanthine

How is AMP synthesized from IMP?

Aspartate added then fumarate released

What nucleotide is required to make AMP?

GTP

Why is GTP required for AMP synthesis?

Balances nucleotide pools

How is GMP synthesized from IMP?

Oxidation then glutamine addition

What nucleotide is required for GMP synthesis?

ATP

Why is ATP required for GMP synthesis?

Balances nucleotide pools

What happens if AMP accumulates?

Inhibits AMP branch synthesis

What happens if GMP accumulates?

Inhibits GMP branch synthesis

What are purinosomes?

Purine synthesis enzyme complexes

Why are purinosomes important?

Increase efficiency of synthesis

What are thiopurines?

Purine analog drugs

What are examples of thiopurines?

6-mercaptopurine and 6-thioguanine

How do thiopurines work?

Inhibit purine synthesis

What diseases are thiopurines used for?

Leukemia and autoimmune disorders

What happens if purine synthesis is inhibited in cancer cells?

DNA synthesis decreases

What enzyme makes deoxyribonucleotides?

Ribonucleotide reductase (RR)

What reaction does RR catalyze?

Removes 2'-OH from NDPs

What substrates does RR use?

NDPs and NADPH

What happens if RR (ribonucleotide reductase) is inhibited?

No DNA synthesis

Why is RR clinically important?

Target for anticancer drugs

What are the RR subunits?

R1 and R2

What does R1 contain?

Active site and allosteric sites

What does R2 contain?

Tyrosyl radical

Why is the tyrosyl radical important?

Initiates reduction reaction

What metal center stabilizes RR radical?

Diiron center

What happens if radical formation fails?

No deoxyribonucleotide synthesis

What reduces RR after catalysis?

Thioredoxin

What reduces thioredoxin?

Thioredoxin reductase

What reduces thioredoxin reductase?

NADPH

What happens if NADPH is low?

dNTP synthesis decreases

What happens if thioredoxin is defective?

RR cannot reset

What inhibits overall RR activity?

dATP

What activates RR overall activity?

ATP

Why does dATP inhibit RR?

Prevents excess dNTP accumulation

What happens if dATP accumulates excessively?

DNA synthesis stops

What controls RR substrate specificity?

Allosteric specificity site

What happens when TTP binds RR?

GDP reduction increases

What happens when dGTP accumulates?

ADP reduction stimulated

Why is RR regulation important?

Balances dNTP pools

What happens if dNTP pools become imbalanced?

Mutation risk increases

What cancer drug inhibits RR as suicide inhibitor?

Gemcitabine

What cancer is gemcitabine used for?

Pancreatic cancer

How does gemcitabine work?

Converted to inhibitory NDP analog

What drugs mimic dATP to inhibit RR?

Clofarabine and cladribine