Lecture 8: Nucleotide Metabolism

Question 1

What is the difference between a nucleoside and a nucleotide?

Answer 1

Nucleoside: Sugar group + Base
Nucleotide: Sugar group + Base + Phosphate Group

Question 2

What is the difference between purine and pyrimidine?

Answer 2

Purine has two rings (Adenine and Guanine)

Pyrimidine has one ring (Thymine, Uracil, Cytosine)

Question 3

What is the difference between a ribose and a deoxyribose sugar?

Answer 3

deoxyribose does not have -OH group at the 2’ sugar

Question 4

What are steps for de novo synthesis of purines?

Answer 4

Step 1: Ribose 5-P —> PRPP
*PRPP synthase

Step 2: PRPP –> PRA
*RATE LIMITING: Glutamine: PRPP Aminotransferase

Step 3: PRA –> IMP
*Many steps, uses folic acid and several AA

Step 4: IMP –> AMP or GMP

Question 5

What regulates de novo synthesis of purines?

Answer 5

End products inhibits synthesis of own products

AMP synthesis is stimulated by GTP
GMP synthesis is stimulated by ATP

Question 6

What are the steps for de novo synthesis of pyridines?

Answer 6

Step 1: Glutamine –> Carbamoyl Phosphate
*RATE LIMITING: Carbamoyl Phosphate Synthetase II

Step 2: Carbamoyl Phosphate –> Carbamoyl Aspartate
*Aspartate Transcaramoylase

Step 3: Carbamoyl Aspartate –> Dihydro Oratate

Step 4: Dihydro Oratate –> Orotate

Step 5: Orotate + PRPP –> OMP

Step 6: OMP –> UMP
*UMP synthase

Step 7: UMP –> UDP

To get Thyidine:
UDP –> dUDP –> dUMP –> dTMP –> dTDP –> dTTP

To get Cytosine:
UDP –> UTP –> dUTP –> dCTP

Question 7

What is Methotrexate?

Answer 7

Blocks formation of PRA –> IMP and dUMP –> dTMP

• prevents oxidation of NADPH
• disrupts DNA replication in cancer cells

Question 8

What are sulfa drugs?

Answer 8

Inhibit bacterial enzymes to incorporate PABA into folate

-disrupts DNA replication

Question 9

What happens when you deprive cells of GMP and dGTP?

Answer 9

Cannot make Guanine properly

-therapeutic agent that disrupts DNA replication

Question 10

How does purine catabolism work?

Answer 10

GUANINE
Remove ribose sugar from guanosine to make guanine and eventually xanthine
-Converted to uric acid via xanthine oxidase

ADENINE
Adenosine –> Inosine
*Adenosine deaminase

Inosine –> Hypoxanthine

Hypoxanthine —> Xanthine
*Xanthine oxidase

Xanthine –> Uric Acids
*Xanthine Oxidase

Question 11

What happens if there is an excess of Adenosine deaminase? What if there is an underproduction?

Answer 11

Overproduction of erthyrocyte isoform cause hemolytic anemia.

Underproduction associated with SCID

Question 12

What drug inhibits xanthine oxidase?

Answer 12

Allopurinol

Question 13

What disease is characterized by too much uric acid?

Answer 13

Gout
-Risk: diet rich in purines
Serum levels are greater than 9 mg/dL

Question 14

What is happening to the molecules as they undergo catabolic processes?

Answer 14

They are getting more oxidized.

Adenine: 0 oxygen
Guanine: 1 oxygen

Uric acid: 3 oxygen and is most oxidized
*acidic hydrogen and limited solubility in water

Question 15

How does SCID play a role with purine catabolism?

Answer 15

In SCID patients, they lack Adenosine Deaminase (ADA).
*Therefore, they have high levels of adenosine.

Adenosine is converted to AMP and ADP –> dAMP and dADP, which prevents production of other nucleotides.

DNA synthesis is impaired and contributes to compromised immune system.

Question 16

Describe pyrimidine catabolism.

Answer 16

Pyrimidines are catabolized into ketogenic, glucogenic, water-soluble compounds

*not too important

Question 17

What are salvage pathways for purines?

Answer 17

Adenine –> AMP
*APRT and PRPP

Guanine/Hypoxanthine –> GMP/IMP
*HGPRT and PRPP

Question 18

What is Lesch-Nyhan Syndrome?

Answer 18

Defects in HGPRT in purine salvage pathway
-Primary hyperuricemia, which leads to gout, urate kidney stones, and tendency for self mutilation

Problem: De novo synthesis proceeds at levels 200x normal

Question 19

What is acyclovir?

Answer 19

Antiviral agent that treats herpes family

-generates dTMP