Nucleotide Metabolism II Flashcards
How are pyrimidines made?
Synthesized as free bases and then transferred to ribose
First three steps catalyzed by multifunctional protein CAD
What is the first pyrimidine synthesized?
UMP
What are the first three steps of pyrimidine synthesis?
Glutamine and HCO3- and 2 ATP are converted to carbamoyl phosphate by carbamoyl phosphate synthetase II
Carbamoyl phosphate is converted to N-carbamoyl aspartate by aspartate transcarbamoylase
N-carbamoyl aspartate is converted to dihydroorotate by dihydroorotase
How is UMP synthesized?
Dihydroorotate is converted to orotate by dihydroorotate dehydrogenase
Orotate with PRPP is converted to orotidine 5’-monophosphate (OMP) by orotate phosphoribosyltransferase
Orotidine 5’-monophosphate is converted to Uridine 5’-monophosphate (UMP) by OMP decarboxylase
Where is dihydroorotate dehydrogenase found?
In mitochondria
Where are orotate phosphoribosyltransferase and OMP decarboxylase activites carried on?
Multifunctional protein UMP synthase (cytoplasm and nucleus)
What converts UMP to UTP?
Nucleoside 5’-monophosphate kinases and nucleoside diphosphate kinases
What converts UTP to CTP?
CTP synthase
Glutamine provides amino group
What does carbamoyl phosphate synthetase II do and how is it regulated?
Converts CO2, glutamine, and 2 ATP to carbamoyl phosphate
Inhibited by UTP - first step of pyrimidine synthesis inhibited by high levels of pyrimidine nucleotides
Activated by PRPP - high levels of precursor stimulates pyrimidine synthesis
What does CTP synthase do and how is it regulated?
CTP synthase converts UTP to CTP
Inhibited by CTP
Activated by UTP
Self regulating system
What do we need more of when cells divide?
Deoxyribonucleotides
Via Ribonucleotide reductase which converts nucleoside 5’-diphosphates to 2’-deoxyribonucleoside 5’-diphosphates
How is ribonucleotide reductase regulated?
Transcriptional control - both induction and repression
Allosteric modulators - NTPs and dNTPS function as activators and inhibitors. dATP inhibits reduction of ADP, CDP, GDP, and UDP
What targets ribonucleotide reductase?
Antineoplastic agent hydroxyurea
Inhibits ribonucleotide reductase
Prevents dNTP synthesis
Blocks cell division
What are deoxyribonucleotides of thymine?
dTMP, dTDP, dTTP
How is dTMP made?
dUMP is converted to dTMP by thymidylate synthase
How does 5-flourouracil inhibit thymidylate synthase?
5-fluourouracil converts to F-flurouridine monophosphate (FUMP)
FUMP becomes FUDP
FUDP becomes FdUDP
FdUDP them becomes FdUMP which irreversibly inhibits thymidylate synthase
What does degradation of pyrimidine nucleotides yield?
Beta-amino acids
UMP, CMP, and dCMP - metabolized to acetyl-CoA
dTMP catabolyzed to Beta-aminoisobutyrate which is metabolized to succinyl-CoA
These are readily soluble and some eliminated in the urine
What is unique to pyrimidine metabolism?
the production of Beta-aminoisobutyrate
Found in urine of cancer patients undergoing radiation or chemotherapy
What are the salvage pathways?
Conversion of free bases and nucleosides to nucleotides
What are the sources of free bases and nucelosides?
Purine and pyrimidine nucleotide degradation and diet.
What is the path of ingested DNA and RNA to red blood cells?
Ingested DNA and RNA is converted to free nucleotides by pancreatic nucleases
Free nucleotides are converted to free nucleosides by phosphatases
Free nucleosides are taken up by intestinal epithelial cells and are mostly metabolized
Some free bases and nucleosides released in to circulation where they are trafficked to red blood cells
What are the enzymes associated with purine base salvage pathway? What do they do?
Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) - converts Hypoxanthine and PRPP to IMP and PPi and is reversed
Adenine phosphoribosyltransferase (APRTase) - converts Guanine and PRPP to GMP and PPi (reverse rxn too). Also converts Adenine and PRPP to AMP and PPi (reverse rxn too)
How does the salvage of purine bases affects the synthesis of purines?
Reduces the rate of de novo synthesis
PRPP consumed by HGPRTase and APRTase so less PRPP available for amidophophoribosyltransferase. Less PRA formed
What can be directly phosphorylated to its corresponding nucleotide?
Adenosine
Adenosine kinase converts adenosine to AMP
What is the path of the salvage of pyrimidine bases?
Free uracil (and orotate) salvaged by pyrimidine phosphoribosyltransferase which converts Pyrimidine and PRPP to Pyrimidine nuceloside 5’monophosphate and PPi
What is the outcome of the salvage of pyrimidine bases?
Consumes PRPP
Less PRPP available for stimulation of carbamoyl phosphate synthetase II
Less PRPP for orotate phosphoribosyltransferase
Impacts de novo pyrimidine synthesis
How are pyrimidine nucleosides converted to nucleotides?
Specific kinases
Uridine-Cytidine kinase converts Uridine to UMP and Cytidine to CMP
Thymidine kinase converts thymidine to dTMP
