Purine Nucleotide Metabolism

Question 1

Initial reaction

Answer 1

Transfer of two phosphate groups from ATP to Ribose-5-phosphate to form PRPP
Catalyzed by PRPP synthetase
Regulated and rate limiting
Feedback inhibited by A and G

Question 2

Major site of purine synthesis

Answer 2

Liver

Question 3

Overall determinant of denovo purine nucleotide production

Answer 3

PRPP concentration

Question 4

Second step

Answer 4

• PRPP to 5-phosphoribosylamine using glutamine as NH donor
• rate limiting and committed step catalyzed by glutamine PRPP amidotransferase
• feedback inhibited allosterically by AMP binding to one inhibitor and GMP to another inhibiting amidotransferase

Question 5

Steps involving glutamine

Answer 5

2,5,15. Donates NH replacing O

Question 6

Steps involving folic acid

Answer 6

4,10. Donates carbon and O.

-can use these steps to modify reaction and stop nucleotide production

Question 7

End product

Answer 7

IMP. Base is hypoxanthine.

Attached to sugar is called inosine

Question 8

CO2

Answer 8

Contributes O

Question 9

Glycine

Answer 9

N,C. Middle part of rings

Question 10

Aspartate

Answer 10

Left ring top left NH

Question 11

Formate

Answer 11

Right ring right corner C

Question 12

Glutamine analogs

Answer 12

Look like glutamine so interfere with enzymes that use glutamine to build purines. Block pathway (steps 2,5,15)
-Azaserine, research only too toxic

Question 13

Inhibitors of folate metabolism

Answer 13

Blocks steps 4 and 10.

• Sulfonamines used in bacteria inhibit PABA BUT NOT HUMANS BC WE DONT MAKE FOLIC ACID
• methotrexate used in cancer, competitively inhibit dihydrofolate reductase, similar in structure to folic acid

Question 14

Making THF

Answer 14

Add 4 hydrogen’s to folic acid by dihydrofolate reductase

-folic acid made from PABA by bacteria, we can’t make folic acid

Question 15

THF

Answer 15

• single carbon donor in steps 4,10

- can accept single carbon from either side chain of serine or form formate.

Question 16

Folic acid deficiency

Answer 16

Neural tube defects, anencephaly, spina bifida.

-need folic acid to make nucleotides in rapidly dividing neural tubes, slowed if not present

Question 17

Key purine salvage transferase enzymes

Answer 17

APRT and HGPRT

Question 18

Purine salvage with adenine

Answer 18

• Phosphoribosylation.
• Need adenine phosphoribosyl transferase
• make AMP from adenine using PRPP, where the sugar phosphate comes from

Question 19

HGPRT

Answer 19

• most important salavage enzyme
• salavage free bases, hypoxanthine or guanine by phosphoriosylation reaction using PRPP to form IMP or GMP
• deficiency causes Lesch-Nyhan syndrome

Question 20

Lesch Nyhan Syndrome

Answer 20

• X linked recessive, gout, kidney stones, neurological problems, mental retardation and self mutilation
• overproduction of purines, feed forward of purine synthesis.
• they lack HGPRT and overaccumulate PRPP, degradation pathway turns into uric acid

Question 21

Purine degradation

Answer 21

• Starts with adenosine Deanimase removing amino group to yield inosine
• purine nucleoside phospholyase removes sugar to yield a free base
• guanase or xanthine oxidase converts to xanthine
• xanthine oxidase converts to uric acid

Question 22

Uric Acid

Answer 22

Insoluble in water, precipitates.

• kidney stones, gout
• free radical sync

Question 23

Gout

Answer 23

Precipitation of urate to crystals in synovial fluid of joints

• causes inflammation
• excess purine prod or deficiency in salvage enzyme HGPRT

Question 24

Allopurional

Answer 24

Prevents gout attacks by blocking xanthine oxidase, accumulating guanine and hypoxanthine which are more soluble.

Question 25

Purine nucleoside phosphorylase deficiency

Answer 25

Autosomal recessive, no T cells

Question 26

ADA deficiency

Answer 26

Autosomal recessive, SCID, no B or T cells bc are destructed. -ribonucleotide reductase inhibited by dATP, prevents production of other dNTPs, inhibiting DNA synthesis and lymphocytes can’t proliferate quickly