Pyrimidine Metabolism

Question 1

Can pyrimidine derivatives be absorbed form the diet?

Answer 1

They are poorly absorbed from the diet

Question 2

What is the main site of de novo pyrimidine ring synthesis?

Answer 2

Liver

Question 3

The liver secretes pyrimidine _____ and ______.

How are they utilized by the blood and other tissue after

Answer 3

• bases and nucleosides

- via salvage pathways

Question 4

What is the rate limiting step in assembling the pyrimidine ring?

Answer 4

The first step - carbamoyl phosphate catalyzed by aspartate

Question 5

What is the first step of pyrimidine synthesis

Answer 5

Assembly of the ring structure, then combining it with PRPP to form a nucleotide structure

Question 6

What forms the ring structure in pyrimidine synthesis?

Answer 6

Condensation of carbamoyl phosphate with aspartate

Question 7

What are the AA needed in pyrimidine synthesis?

Answer 7

Glutamine, Aspartate

Question 8

All de novo pyrimidine nucleotide synthesis proceeds through which nucleosides?

Answer 8

UMP, UDP. UTP

Question 9

How is UTP converted to CTP

Answer 9

From the NH2 from glutamine

Question 10

What must happen to CTP to form pyrimidine deoxyribonucleotides?
What is the round-about pathway that happens next?

Answer 10

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

Question 11

How is carbamoyl phosphate formed?

Answer 11

• 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)

Question 12

How is CPSII regulated?

Answer 12

Allosterically inhibited by UTP and activated by PRPP

Question 13

What is the pathway for CPS-I vs CPS-II?

Answer 13

Urea cycle vs pyrimidine biosynthesis

Question 14

What is the source of nitrogen for CPS-I vs CPS-II?

Answer 14

NH4+ vs glutamine

Question 15

What is the location for CPS-I vs CPS-II?

Answer 15

Mitochondria vs cytosol

Question 16

What is the activator for CPS-I vs CPS-II?

Answer 16

N-acetylglutamte vs PrPP

Question 17

What is the inhibitor for CPS-I vs CPS-II?

Answer 17

nothing vs UTP

Question 18

Why is compartmentation important for CPS-I and CPS-II?

Answer 18

Makes it possible to regulate two pathways differently even though they share a common intermediate

Question 19

When carbamoyl phosphate and aspartate combine what do they form?

Answer 19

Pyrimidine ring structure, orotate

Question 20

Orotate combines with PRPP to form ______

What happens after it forms?

Answer 20

a nucleotide

it is decarboxylated to yield UMP

Question 21

Where do the atoms in th pyrimidine ring come from?

Answer 21

Glutamine and aspartate (and CO2)

Question 22

Which enzyme is involved in the conversion of CDP to dCDP?

Answer 22

ribonucleotide reductase

it also makes dADP and dGDP

Question 23

Allosteric effectors change the substrate specificity of the enzymes. What does this mean for the enzyme?

Answer 23

The enzyme can exist in several different conformations, each of which favors binding of a specific substrate

Question 24

Allosteric effectors change the substrate specificity of the enzymes. What is the purpose of this?

Answer 24

Helps make sure that balanced levels of the t4 deoxynucleotide triphosphates are maintained for DNA synthesis

Question 25

The allosteric (regulatory) site binds to

Answer 25

triphosphates

Question 26

The catalytic site binds to

Answer 26

diphosphates

Question 27

How does the triphosphate binding affect diphosphate binds?

Answer 27

It determines which diphosphate can bind in the catalytic site

Question 28

ATP activates reduction of

Answer 28

CDP to dCDP

Question 29

dTTP activates reduction of

Answer 29

GDP to dGDP

Question 30

dGTP activates reduction of

Answer 30

ADP tp dADP

Question 31

aADTP inibitis

Answer 31

reduction of all substrates

Question 32

What are two substrates for pyrimidine nucleoside phosphorylase?

Answer 32

cytosine and uracil

Question 33

Can pyrimidine ring structure be broken down in human cells?

Answer 33

yes

Question 34

What are cytosine and uracil degraded to

Answer 34

b-alanine

Question 35

Cancer chemotherapeutic agents work on non-proliferating or proliferating cells? Why?

Answer 35

Proliferating cells. | B/c of DNA synthesis

Question 36

What is the key to inhibiting DNA synthesis?

Answer 36

blocking dTMP synthesis

Question 37

The synthesis from dTMP from dUMP requires addition of a _________ to the pyrimidine ring

Answer 37

methyl group | obtained from tetrahydrofolate

Question 38

Which enzyme carries out the methyl transfer reaction

Answer 38

thymidylate synthase

Question 39

Why is the methyl transfer reaction unique?

Answer 39

It oxidized FH4 to FH2

Question 40

Which enzyme reduces FH2 back to FH4

Answer 40

dihydrofolate reductase

Question 41

What is the importance of dihydrofolate reductase and thymidylate synthase?

Answer 41

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)

Question 42

What is 5-FU?

Answer 42

5-flurouracil an analogue of uracil and thymine a fluorine atom in place of methyl group of thymine

Question 43

What does e5-FU inhibit? | What does this inhibition cause?

Answer 43

thymidylate synthase - reduces the availability of dTMP and dTTP which slows or prevents cell proliferation

Question 44

What does methotrexate inhibit? | What does inhibition cause?

Answer 44

dihydrofolate reductase prevents conversion of FH2 back to FH4 In proliferating cells, all single cell metabolism is shut down, killing the cell

Question 45

Does methotrexate affect proliferating or non-proliferating cells? Why?

Answer 45

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

Question 46

What does Thio IMP do to the de novo purine synthesis pathway?

Answer 46

inhibits several key enzymes

Question 47

All thymidine kinase (TK) genes are evolutionarily related except?

Answer 47

herpes TK genes

Question 48

Herpes TK can phosphorylate purine nucleoside analogues known as _________

Answer 48

acyclovirs

Question 49

How do acyclovir nucleotides work?

Answer 49

kill cells by inhibiting all DNA synthesis

Question 50

In gout, what is the gene defect, metabolite that accumulates and clinical symptoms?

Answer 50

multiple causes uric acid painful joints

Question 51

In SCID, what is the gene defect, metabolite that accumulates and clinical symptoms?

Answer 51

adenosine deaminase (purine salvage pathway) deoxyadenosine and derivatives thereof loss of immune system, including no T or B cells

Question 52

In immunodeficiency disease, what is the gene defect, metabolite that accumulates and clinical symptoms?

Answer 52

purine nucleoside phosphorylase (purine salvage pathway) purine nucleosides Partial loss of immune system

Question 53

In Lesch-Nyhan syndrome, what is the gene defect, metabolite that accumulates and clinical symptoms?

Answer 53

hypoxanthine-guanine phosphoribosyltransferase (purine salvage pathway) purines, uric acid mental retardation, self mutilation

Question 54

In hereditary orotic aciduria, what is the gene defect, metabolite that accumulates and clinical symptoms?

Answer 54

UMP synthase (de novo pyrimidine synthesis) Orotic acid Growth retardation