Nucleotide Anabolism

Question 1

In addition to the salvage pathways, how else can purine and pyrimidines be synthesized? Where are the enzymes located in purine and pyrimidine synthesis?

Answer 1

de novo - from starting materials like CO2, NH3, and some of the AA Enzymes are in the cytosol

Question 2

What are the sources of C and N atoms in pyrimidines?

Answer 2

Aspartate (donates 3C and 1N) Bicarb (donates 1C) Glutamine amide (donates 1N)

Question 3

What are the steps in pyrimidine synthesis?

Answer 3

Amine from Glutamine Add CO2 Carbomyl Phosphate Add Aspartate Close the ring Orotic Acid Use PRPP to attach ribose-5-phosphate Remove CO2 to make Uridine (UMP) Cytosine or Thymidine From UMP, you can make Cytosine or Thymidine: UMP -> UDP -> UTP -> Cytosine UMP -> dUDP -> dUTP -> dUMP -> dTMP -> Thymidine

Question 4

What vitamin do you need to make Thymidine, and why?

Answer 4

Folate, because it is the methyl source (N5, N10-methylene H4 folate is the source of CH3)

Question 5

What does the drug 5-fluorouracil do to pyrimidine synthesis?

Answer 5

Prevents the cells from replicating. Used as a chemotherapy drug

Question 6

How do pyrimidine and purine synthesis differ?

Answer 6

Pyrimidine: first we make the single ring, then add sugar, then add phosphate Purine: start with PRPP, add N, add and build up double rings from multiple sources

Question 7

What are the sources of C and N atoms in purines?

Answer 7

Glutamine amide (2N) Glycine (1N, 2C) Aspartate amide (1N) Bicarb (1C) H4 Folate (2C)

Question 8

What is the generic purine ring called?

Answer 8

IMP

Question 9

How much ATP is required to synthesize a double ring purine?

Answer 9

4ATP (not including cost of PRPP)

Question 10

What is the rate limiting step in pyrimdine synthesis?

Answer 10

CPS-2 is the rate-limiting step -glutamine + CO2 + 2ATP -> carbomyl phosphate + 2ADP + PPi

Question 11

What happens if there is a defect in nucelotide anabolism?

Answer 11

Death (in utero)

Question 12

What does GMP (Guanosine) require?

Answer 12

NAD, ATP, and NH2 from Gln

Question 13

What does AMP (Adenosine) require?

Answer 13

GTP and NH2 from Asp

Question 14

What comes from IMP?

Answer 14

GMP or AMP

Question 15

What is the difference between Adenosine and Guanosine?

Answer 15

The placement of the amino group

Question 16

How are the concentrations of AMP and GMP regulated?

Answer 16

The concentration of one affects synthesis of the other because you have to have A to make G and have to have G to make A. Keeps concentrations balanced

Question 17

What does the antivrial drug Acyloguanosine (Acyclovir) do?

Answer 17

It impairs purine synthesis

Question 18

How do mono-, di-, and triphosphate nucleotides relate?

Answer 18

They are all interconvertible: CMP + ATP CDP + ADP GDP + ATP GTP + ADP getting to triphosphate form traces back to oxidative phosphorylation because all ADP go back to ETS to get rephosphorylated

Question 19

What is the only version of Thymidine created?

Answer 19

It only exists in the oxidative form, ie. DEOXYribonucelotide

Question 20

HMPS generates NADPH. How is NADPH used?

Answer 20

In anabolic/synthetic pathways: fat synthesis as well as to make deoxy forms of nucleoides

Question 21

How is PRPP formed?

Answer 21

Ribose-5-P + ATP –> AMP + PRPP

Question 22

Where does the Ribose-5-P come from?

Answer 22

HMPS

Question 23

What enzyme catalyzes the synthesis of PRPP?

Answer 23

ATP phosphoribosyl transferase

Question 24

What inhibits ATP phosphoribosyl transferase?

Answer 24

The accumulation of purines and pyrimidines (product feedback inhibition)

Question 25

What happens if there is a deficiency in HGPRTase (as in Lesch Nyhan)?

Answer 25

Hypoxanthine and Guanine will accumulate because breakdown of nucleotides is occurring instead of recycling The accumulation blocks more PRPP synthesis, so all nucleotide synthesis is shutdown

Question 26

What is PRPP used for?

Answer 26

Purine and Pyrimidine de novo synthesis, NAD, and salvage pathways

Question 27

If ATP phosphoribosyl transferase is inhibited, what happens?

Answer 27

Accumulation of purines and pyrimidines --\> Increase in purines causes an Increase in uric acid --\> Gout

Question 28

What is the relationship between Glucose 6 Phosphatase deficiency (Type 1, von Gierke's) and Gout?

Answer 28

G-6-phosphatase deficiency (Type I, von Gierke’s) --\> increase G6P --\> Glucose can't be released from the liver cells --\> Glucose gets pushed down HMPS pathway --\> increase ribose-5-P --\> increase PRPP --\> increase nucleotide synthesis --\> increase purines --\> Gout (secondary)

Question 29

If you make more purines than you need, what happens?

Answer 29

Purines can't be broken down for energy so you break them down. The waste products (uric acid) accumulate, and cause Gout

Question 30

Answer 30

Folate (vit B9)

Question 31

What is Folate's involvement in Purine (Thymidine in particular) synthesis?

Answer 31

n5,10-methyleneH4folate is methyl donor in purine metabolism (in dUMP --\> dTMP) Folate is also required for Glycine \<--\> Serine and Homocysteine --\> Methionine

Question 32

What clinical signs would you see in a folate deficicency?

Answer 32

Megaloblastic (pernicious) anemia

Question 33

What can interfere with folate absorption?

Answer 33

Antiseizure drugs

Question 34

Bacteria make their own folate. How can this be blocked?

Answer 34

Sulfonamide antibiotics target PABA

Question 35

What is needed to convert N5-methyl-H4 to H4-folate in mammals?

Answer 35

Cobalamin (Vit B12)

Question 36

What is B12 required for?

Answer 36

Folate absorption and Methionine synthesis

Question 37

What is needed to absorb B12?

Answer 37

Intrinsic Factor and Cobalt

Question 38

What drugs that inhibit dihydrofolate reductase (DHFR) and folate?

Answer 38

Certain Chemotherapy drugs, Antiprotozoal drugs, and Trimethorpim (an antibiotic used in combo with a Sulfonamide) -they all mimic DHFR

Question 39

What clinical symptoms would appear in a patient with folate deficiency?

Answer 39

Low blood cell count (—Megaloblastic anemia, —Leukopenia, —Thrombocytopenia, —Pancytopenia) —Atrophic glossitis —Inappetance and weight loss —Muscular weakness/fatigue —Neurological/behavioral signs Elevated levels of homocysteine in blood and urine (because can't metabolize/breakdown/recycle homocysteine back to methionine)

Question 40

What diseases are correlated with folate deficiency?

Answer 40

Cardiovascular and Neural

Question 41

If Carbamyl phosphate could not be be synthesized into UMP, what clinical signs would arise?

Answer 41

Orotic acid would accumulate causing Orotic aciduria, decrease pyrimidine, not making DNA, low CTP and TTP, megaloblastic anemia from not making purines

Question 42

How do you treat Orotic aciduria?

Answer 42

Feed Uridine to produce pyrimidines which will push UMP --\> UTP, which inhibits CPSII, decreasing orotate production

Question 43

A deficiency in Orotate phosphoribosyltransferase and/or orotidine-5’-P decarboxylase will cause what?

Answer 43

Orotic aciduria