Nucleotide synthesis and metabolism

Question 1

Where does purine synthesis occur

Answer 1

Liver

Question 2

Are purines assembled and then ribosylated, or synthesized onto an existing ribose?

Answer 2

Purines are synthesized onto an already phophorylated ribose-5-phosphate.

Question 3

What substrates are needed for purine synthesis

Answer 3

Ribose-5-phosphate, 
glycine
glutamine
CO2
Aspartate
N10-Formyl-THF
6ATP

Question 4

What is the initial purine synthesized

Answer 4

Inosine-monophosphate IMP.

Question 5

What is the salvage pathway for purines?

Answer 5

Purine BASES with no attached ribose are salvaged.

APRT, Adenine PhosphoRibosyl Transferase
and
HPRT, Hypoxanthine-guanine PhosphoRibosyl Transferase

HPRT deficiency results in Lesch-Nyan syndrome. Elevated PRPP, Uricemia,
Decreased IMP and GMP,

Question 6

What is PRPP

pyro-

Answer 6

Phospho-Ribosyl-PyroPhosphate

This is the substrate used to add Ribose-phosphate to a Purine base.

Question 7

What enzyme and co-factor is used to reduce Ribonucleotides to Deoxyribonucleotides

Answer 7

Ribonucleotide diphosphate reductase, and Thioredoxin cofactor, plus an ATP allosteric activator.

Thioredoxin reductase plus NADPH is then needed to reduce the oxidized thioredoxin back to useable reduced state.

Question 8

What are the products of purine metabolism (degradation)

Answer 8

Adenosine –> Inosine –> Hypoxanthine –> Uric acid

Guanosine –> Xanthine –> Uric acid.

Question 9

What is the first enzyme of Pyrimidine synthesis

Answer 9

Carbamoyl Phosphate Synthetase 2. CPT II.

CO2 + Glutamine + 2ATP –> Carbamoyl phosphate + 2 ADP + Pi

Question 10

What are the differences between CPT 1 and CPT2

Answer 10

Both are in the kidney.

Substrates
CPT1: HCO3- NH3 and ATP. Bicarbonate, Amonia and ATP

CPT2: CO2 + Glutamine + 2 ATP –> Carbamoyl Phos + 2ADP + Pi

Question 11

What is the first enzyme of Purine synthesis (de novo)

Answer 11

1) PRPP synthetase
Ribose-5-phosphate + ATP => PRPP + AMP

2) Glutamine:PRPP amidotransferase
PRPP + Glutamine => 5-phsophoribosylamine + glutamate

Question 12

What are the substrates needed for Pyrimidine de novo synthesis

Answer 12

Asparate, Glutamine, CO2, 4 ATP, and PRPP.

Question 13

What is the first pyrimidine synthesized in the de novo pathway

Answer 13

Orotidine 5 monophosphate, OMP

Question 14

What enzyme contains the CPT 2 catalytic domain?

Answer 14

CAD.
It contains:
CPT2
Aspartate transcarbamoylase
Dihydroorotase

Catalyzes:
CO + Glutamine + 2ATP + Aspartate -> -> -> Dihydro Orotate

Question 15

What enzyme follows CAD?

Answer 15

UMP Synthetase:
Contains 2 domains:
Orotate phosphoribosyl transferase
and 
OMP decarboxylase

Question 16

Are pyrimidines synthesized onto and existing Ribosyl-phosphate or are they synthesized first and then phospho-ribosylated?

Answer 16

The pyrimidines are synthesized and the then ribose-phosphate is added.

Question 17

What cofactor is used to methylate dUMP to generate dTMP

Answer 17

The thymidylate synthetase enzyme uses the THF tetrahydrofolate cofactor.

Question 18

Pyrimidine salvage

Answer 18

Does not occur to a large degree, because during degradation, the pyrimidine rings are cleaved to beta-alanine (from CMP and UMP) and beta-aminoisobutyrate (from TMP).

Pyrimidine bases can be salvaged by uracil phosphorylase or pyrimidine nucleoside phosphorylase, which adds ribose-1-phosphate. Then uridine/cytidine/tyhmidine kinase removes the 1 phos and adds 5 phos.

Question 19

Deoxynucleoside salvage

Answer 19

deoxycytidine kinase. Salvages base-riboses, by phosphorylating them.
Salvages deoxy- Adeonsine, deoxy-Guanosine, AND deoxy-Cytidine

Targeted in leukemias because it is lymphoid specific.

Question 20

Thymidine kinase

Answer 20

Very specific to salvage Thymidine (deoxyribose-thymine

Expressed only during active S-phase of cells.

Gancyclovir/Acyclovir are antibiotics which target this enzyme to kill rapidly dividing cells.