Lecture 8-Nucleotide Metabolism II (Nakai)

Question 1

What converts dUTP –> dUMP?

Answer 1

dUTPase

Question 2

What converts dUMP –> dTMP?

Answer 2

thymidilate synthase

Question 3

What are the 2 causes of gout?

Answer 3

• difficulties in excretion of uric acid

- purines being overproduced

Question 4

ADA deficiency results in _______

Answer 4

SCID

Question 5

HGRTase deficiency results in _____

Answer 5

Lesch-Nyhan syndrome

Question 6

allopurinol

Answer 6

• analog of hypoxanthene that inhibits xanthine oxidase
• used to treat gout
• hypoxanthine and xanthine build up from deficient xanthine oxidase activity and can be excreted by the kidneys

Question 7

azaserine/acivicin

Answer 7

• gln analogs used as chemotherapeutic agents–they would disrupt glutamine amidotransferases ( remember! gln PRPP amidotransferase!)
• not used as much due to toxic side effects since
• Gln is involved in: purine biosynthesis, N metablism and Gln (?) is used as a CNS NT

Question 8

Hydroxyurea

Answer 8

ribonucleotide reductase inhibitor

- used as a chemotherapeutic agent and it inhibits DNA synthesis specifically with little effect on RNA synthesis

Question 9

fluorouracil; what kind of inhibition does it use?

Answer 9

• analog of thymine that’s readily converted to FdUMP which inhibits thymidylate synthase (dUMP –> dTMP)
• its a SUICIDE INHIBITOR

Question 10

Methotrexate and trimethorpim

Answer 10

• folate analogs that competitively and directly inhibit dihydrofolate reductase
• prevent regeneration of THF and therefore the methylating agent (N5,N10 methylene THF)
• this is used as a cofactor in the reaction with thymidylate synthase

Question 11

What will ultimately reoxidize ribonucleotide reductase so it itself can oxidize another NTP?

Answer 11

NADPH

Question 12

When dCTP binds the secondary site of ribonucleotide reductase what does it promote to bind?

Answer 12

nothing!

Question 13

When dTTP binds to the secondary site of ribonucleotide reductase what does it promote to bind? What does it inhibit?

Answer 13

• GDP

- dCTP/dCDP; dUDP/dTTP

Question 14

What is ATP’s effect when it binds the primary site of ribnoucleotide reductase? dATP?

Answer 14

• turns RNR on

- turns RNR off

Question 15

Why does DNA Pol need equal levels of all nucleotides in the cell?

Answer 15

• it has a low Km and high Vmax
• increasing one dNTP will cause it to have a higher Km for that nucleotide and the chance that it will be incorrectly incorporated is very high

Question 16

Why are there differing affinities for each of the nucleotides at the secondary site of RNR?

Answer 16

The affinities are based on the levels naturally occurring within the cell and this aims to balance them out.

Question 17

Thymidilate synthase is the target of _______.

Answer 17

Many chemotherapies

–REMEMBER: dUMP–>dTMP

Question 18

Thymidilate synthase mediates what kind of reaction? Using what?

Answer 18

• 1 C transfer

- N5, N10 methylene THF

Question 19

What converts dihydrofolate from thymidilate synthase reaction back to N5, N10 methylene THF

Answer 19

• dihydrofolate reductase + NADPH

- Ser hydroxymethyltransferase + PLP + Ser

Question 20

Dihydrofolate reductase is targeted for _____

Answer 20

chemotherapeutic agents

Question 21

Catabolism of purines is especially high where?

Answer 21

• liver

Question 22

What is usually the 1st step of nucleotide catabolism?

Answer 22

• dephosphorylation

Question 23

adenosine deaminase

Answer 23

deaminates adenosine to inosine

Question 24

Inosine has a _____ base

Answer 24

hypoxanthine

Question 25

How do you get uric acid from hypoxanthine?

Answer 25

- hypoxanthine --> xanthine --> uric acid | - each of these reactions use xanthine oxidase

Question 26

What are the only cofactors used in purine catabolism?

Answer 26

H2O

Question 27

What does guanine get converted too (i.e., where does it meet up with the AMP degradation)?

Answer 27

- xanthine

Question 28

Why can uric acid build up in joints?

Answer 28

- pKa 5.7 and therefore low solubility in aqueous solution

Question 29

What happens when uric acid builds up in joints?

Answer 29

- WBCs (PMNLs) eat up the crystals - release hydrolytic enzymes - causes inflammation, pain

Question 30

What is the basic salvaging reaction?

Answer 30

adenine + PRPP --> AMP + PPi | - adenosine phosphoribosyl transferase

Question 31

HRGTase mediates what 2 reactions?

Answer 31

- hypoxanthine + PRPP --> IMP + PPi | - guanine + PRPP --> GMP + PPi

Question 32

Where is HRGTase esp important in why?

Answer 32

- neural cells | - dont have the energy to do purine synthesis

Question 33

Why does HRGTase deficiency result in LN syndrome? What is LN characterized by?

Answer 33

- brain is esp dependent on HRGTase because it doesn't want to use its energy stores on nucleotide synthesis - brain forced to use de novo pathway and gets an ATP deficiency - Characterized by: - you get an accumulation of PRPP because of the enzyme deficiency - high PRPP signals liver to pump out purines at a very high rate - high purines = high uric acid