Pyrimidine Metabolism Flashcards
Can pyrimidine derivatives be absorbed form the diet?
They are poorly absorbed from the diet
What is the main site of de novo pyrimidine ring synthesis?
Liver
The liver secretes pyrimidine _____ and ______.
How are they utilized by the blood and other tissue after
- bases and nucleosides
- via salvage pathways
What is the rate limiting step in assembling the pyrimidine ring?
The first step - carbamoyl phosphate catalyzed by aspartate
What is the first step of pyrimidine synthesis
Assembly of the ring structure, then combining it with PRPP to form a nucleotide structure
What forms the ring structure in pyrimidine synthesis?
Condensation of carbamoyl phosphate with aspartate
What are the AA needed in pyrimidine synthesis?
Glutamine, Aspartate
All de novo pyrimidine nucleotide synthesis proceeds through which nucleosides?
UMP, UDP. UTP
How is UTP converted to CTP
From the NH2 from glutamine
What must happen to CTP to form pyrimidine deoxyribonucleotides?
What is the round-about pathway that happens next?
We go backwards: CTP is dephosphorylated to CDP CDP is reduced to dCDP dCDP is dephosphorylated to dCMP dCMP is deaminated to dUMP dUMP is methylated to dTMP dTMP is re-phosphorylatted to dTDP --> dTTP --> DNA
How is carbamoyl phosphate formed?
- From CO2 and an amino group from glutamine
- 2 ATPs needed to provide energy for the reaction (one of the P groups goes to the carbamoyl phosphate)
- Catalyzed by carbamoyl phosphate synthetase II (CPSII)
How is CPSII regulated?
Allosterically inhibited by UTP and activated by PRPP
What is the pathway for CPS-I vs CPS-II?
Urea cycle vs pyrimidine biosynthesis
What is the source of nitrogen for CPS-I vs CPS-II?
NH4+ vs glutamine
What is the location for CPS-I vs CPS-II?
Mitochondria vs cytosol
What is the activator for CPS-I vs CPS-II?
N-acetylglutamte vs PrPP
What is the inhibitor for CPS-I vs CPS-II?
nothing vs UTP
Why is compartmentation important for CPS-I and CPS-II?
Makes it possible to regulate two pathways differently even though they share a common intermediate
When carbamoyl phosphate and aspartate combine what do they form?
Pyrimidine ring structure, orotate
Orotate combines with PRPP to form ______
What happens after it forms?
a nucleotide
it is decarboxylated to yield UMP
Where do the atoms in th pyrimidine ring come from?
Glutamine and aspartate (and CO2)
Which enzyme is involved in the conversion of CDP to dCDP?
ribonucleotide reductase
it also makes dADP and dGDP
Allosteric effectors change the substrate specificity of the enzymes. What does this mean for the enzyme?
The enzyme can exist in several different conformations, each of which favors binding of a specific substrate
Allosteric effectors change the substrate specificity of the enzymes. What is the purpose of this?
Helps make sure that balanced levels of the t4 deoxynucleotide triphosphates are maintained for DNA synthesis
The allosteric (regulatory) site binds to
triphosphates
The catalytic site binds to
diphosphates
How does the triphosphate binding affect diphosphate binds?
It determines which diphosphate can bind in the catalytic site
ATP activates reduction of
CDP to dCDP
dTTP activates reduction of
GDP to dGDP
dGTP activates reduction of
ADP tp dADP
aADTP inibitis
reduction of all substrates
What are two substrates for pyrimidine nucleoside phosphorylase?
cytosine and uracil
Can pyrimidine ring structure be broken down in human cells?
yes
What are cytosine and uracil degraded to
b-alanine
Cancer chemotherapeutic agents work on non-proliferating or proliferating cells?
Why?
Proliferating cells.
B/c of DNA synthesis
What is the key to inhibiting DNA synthesis?
blocking dTMP synthesis
The synthesis from dTMP from dUMP requires addition of a _________ to the pyrimidine ring
methyl group
obtained from tetrahydrofolate
Which enzyme carries out the methyl transfer reaction
thymidylate synthase
Why is the methyl transfer reaction unique?
It oxidized FH4 to FH2
Which enzyme reduces FH2 back to FH4
dihydrofolate reductase
What is the importance of dihydrofolate reductase and thymidylate synthase?
Important in cancer chemotherapy
Inhibition of these enzymes selectively kills/inhibits proliferating cells including cancer cells
(but also some normal proliferating cells eg epithelial cells in stomach and intestine, bone marrow cells that produce RBC and WBC and hair follicle cells)
What is 5-FU?
5-flurouracil
an analogue of uracil and thymine
a fluorine atom in place of methyl group of thymine
What does e5-FU inhibit?
What does this inhibition cause?
thymidylate synthase -
reduces the availability of dTMP and dTTP which slows or prevents cell proliferation
What does methotrexate inhibit?
What does inhibition cause?
dihydrofolate reductase
prevents conversion of FH2 back to FH4
In proliferating cells, all single cell metabolism is shut down, killing the cell
Does methotrexate affect proliferating or non-proliferating cells?
Why?
Proliferating cells
proliferating cells - lots of dTMP being made for DNA synthesis therefore FH2 accumulates and FH4 is depleted
non-proliferating cells - very little dTMP made, FH2 does not accumulate
What does Thio IMP do to the de novo purine synthesis pathway?
inhibits several key enzymes
All thymidine kinase (TK) genes are evolutionarily related except?
herpes TK genes
Herpes TK can phosphorylate purine nucleoside analogues known as _________
acyclovirs
How do acyclovir nucleotides work?
kill cells by inhibiting all DNA synthesis
In gout, what is the gene defect, metabolite that accumulates and clinical symptoms?
multiple causes
uric acid
painful joints
In SCID, what is the gene defect, metabolite that accumulates and clinical symptoms?
adenosine deaminase (purine salvage pathway)
deoxyadenosine and derivatives thereof
loss of immune system, including no T or B cells
In immunodeficiency disease, what is the gene defect, metabolite that accumulates and clinical symptoms?
purine nucleoside phosphorylase (purine salvage pathway)
purine nucleosides
Partial loss of immune system
In Lesch-Nyhan syndrome, what is the gene defect, metabolite that accumulates and clinical symptoms?
hypoxanthine-guanine phosphoribosyltransferase (purine salvage pathway)
purines, uric acid
mental retardation, self mutilation
In hereditary orotic aciduria, what is the gene defect, metabolite that accumulates and clinical symptoms?
UMP synthase (de novo pyrimidine synthesis)
Orotic acid
Growth retardation
