Purine and Pyrimidine Nucleotide Metabolism Flashcards
What are the three components of a nucleotide?
Base- purine or pyrimidine
Pentose- ribose or deoxyribose
Phosphoryl group/s- one or more
Differentiate the structure of the two purine bases
Adenine: amino group (NH2)
Guanine: NH2 and =O
Differentiate the structure of the three pyrimidine bases
Cytosine: NH2 and =O
Uracil: two =O
Thymine: two =O and a methyl group
Which bases are found in DNA vs RNA?
AGCT= DNA AGCU= RNA
Differentiate base, nucleoside and nucelotide
Base: ring bases only (AGCTU)
Nucleosides: base + sugar
Nucleotide: Base, sugar and phosphate
Differentiate ribose from deoxyribose
Deoxyribose: one less -OH group
What are the three sources of atoms in the purine ring structure?
Amino acids- glycine, aspartate and glutamine
CO2
Tetrahydrofolate (FH4) derivative
In the synthesis of purines, is the purine ring synthesized first and then attached to ribose, or is the purine ring synthesized on top of ribose?
The purine ring is built up atom by atom on top of the ribose
What is the precursor molecule for purines?
Ribose-5-phosphate
What are the first two enzymatic steps in purine biosynthesis starting with ribose-5-phosphate?
Step 1: activation of ribose-5-phosphate to give PRPP (Pi from ATP)
Step 2: formation of 5-phosphoribosylamine (NH2 from glutamine)
What is the source of ribose-5-phosphate?
Pentose-phosphate pathway
What enzyme catalyzes the committed and major regulated step of purine synthesis?
The second step, the formation of 5-phosphoribosylamine, is the committed step and major regulated step
Catalyzed by Glutamine PRPP amidotransferase
Which nucleotide stands at the branch point leading to the synthesis of AMP and GMP?
IMP
IMP becomes the precursor for both AMP and GMP
What are the major allosteric regulators (negative feedback) in purine synthesis?
High levels of IMP, AMP and GMP negatively inhibit purine biosynthesis
Why might an inhibitor of purine biosynthesis be useful in slowing tumor growth?
Tumors are very dependent on the purine biosynthetic pathway to produce purine nucleotides for DNA and RNA synthesis.
In contrast, most normal tissues are less dependent on de novo synthesis and instead recycle existing purine bases (salvage pathway)
What tumor pathway is inhibited by the anti-tumor agent, 6-mercaptopurine
6-mercaptopurine inhibits enzymes in the purine biosynthetic pathway.
It is used to treat acute lymphocytic leukemia (ALL)
What three molecules contribute atoms to the pyrimidine ring?
aspartate, glutamine and CO2
What is the regulated step in pyrimidine biosynthesis?
Step 1: glutamine + CO2 + 2ATP
Catalyzed by carbamoyl phosphate synthetase II
Indicate whether the pyrimidine ring is synthesized first and then attached to the ribose, or whether the pyrimidine ring is synthesized on top of ribose.
The pyrimidine ring structure is formed first and then attached to a ribose-5-phosphate (activated form)
Name the pyrimidine nucleotide that is biosynthesized first and becomes a precursor for the synthesis of the other pyrimidine nucleotides
UMP is the precursor for synthesis of the other pyrimidine nucleotides
Indicate which metabolic pathway is blocked by the rheumatoid arthritis drug, leflunomide
Leflunomide, an immunosuppressive drug used to treat rheumatoid arthritis, blocks pyrimidine biosynthesis by inhibiting dihydroorotate dehydrogenase
Briefly describe the synthesis of dTMP from dUMP
dTMP is synthesized from dUMP and N5,N10-methylene tetrahydrofolate by thymidylate synthase.
Describe how 5-fluorouracil acts as an antitumor agent.
5-fluorouracil is converted to f-dUMP which then acts as a pseudosubstrate and covalently binds to, and inhibits, thymidylate synthase
TLDR: Inhibition of thimidylate synthase
Describe how methotrexate acts as an antitumor agent
Methotrexate is a potent competitive inhibitor of dihydrofolate reductase and increases dihydrofolate and causes an intracellular tetrahydrofolate deficiency.
TLDR: Tetrahydrofolate deficiency
Name the enzymes responsible for the synthesis of the deoxyribonucleotides and indicate the substrates for this enzyme
Ribonucleotide reductase
Substrates: ribonucleoside 5’-diphosphates (ADP, GDP, CDP or UDP)
They form their corresponding dADP, dGDP, dCDP or dUDP
Indicate which hydroxyl group is removed from the ribose in this process
2’-hydroxyl group of ribose is replaced with a hydrogen atom
What is the mechanism of the antitumor effect of hydroxyurea
See small group problem
When are the enzymes responsible for the synthesis of deoxyribonucleotides upregulated?
Late G1/ early S phase
Describe how deoxyribonucleotide biosynthesis is regulated during the cell cycle
Enzymes responsible for purine and pyrimidine biosynthesis are kept at low concentrations in the cell until late G1/ early S where they are upregulated.
Name the classes of enzymes that are involved in the degradation of DNA and RNA into free purine and pyrimidine bases
nucleases, nucleotidases, nucleoside phosphorylases
Indicate which bases are formed by degradation of DNA and RNA
Uracil, Thymine, Guanine and hypoxanthine (from adenine)
What happens to the majority of the free purine bases produced by cellular degradation of RNA?
They are reutilized in the salvage pathway
Describe the enzymatic reaction involved in salvaging hypoxanthine and guanine.
Hypoxanthine + PRPP –> IMP + PPi
Guanine + PRPP –> GMP + PPi
What is the genetic deficiency in Lesch-Nyhan syndrome?
Genetic deficiency in HGPRTase
Hypoxanthine-guanine phosphoribosyltransferase
What happens to the purine bases that are not salvaged?
They are degraded to uric acid and excreted
What is the enzyme that converts hypoxanthine to xanthine, and xanthine to uric acid?
Xanthine oxidase
Indicate the forms of uric acid present in blood and urine
Uric acid exists in two tautomeric forms - one of which can be deprotonated to form urate.
At the pH of blood, urate is the predominant form, while at the pH of urine, substantial amounts of uric acid exists.
Describe the consequences of overproduction or underexcretion of uric acid
Overproduction or underexcretion of uric acid leads to gout
Describe the mechanism by which allopurinol relieves the symptoms of gout
allopurinol inhibits xanthine oxidase
