Nucleotide Metabolism

Question 1

What is ATP ?

Answer 1

A nucleotide, or a nucleoside triphosphate - NOT a nucleotide triphosphate

Question 2

What are the major purine bases ?

Answer 2

Adenine and Guanine

Question 3

What are the major pyrimidine bases ?

Answer 3

Cytosine, Thymine and Uracil

Question 4

What kind of rings are the bases ?

Answer 4

All the bases are heterocyclic rings (with both C and N)

Question 5

What are the functions of nucleotides ?

Answer 5

• Energy transduction
• Substrates for RNA and DNA biosynthesis
• Coenzymes
• Metabolic regulators

Question 6

Explain Energy Transduction ?

Answer 6

• ATP is by far the commonest energy transducer
• Other nucleoside triphosphates also provide energy in some situations, e.g. GTP during protein synthesis and UTP during glycogen biosynthesis

Question 7

Explain Substrates for RNA and DNA biosynthesis ?

Answer 7

• The four ribonucleoside triphosphates are essential substrates for RNA synthesis
• The four 2’-deoxyribonucleoside triphosphates are essential substrates for DNA replication (and this is their only substantive role)

Question 8

Purines are synthesised as ?

Answer 8

• The ribonucleotides, (not as the free bases)

- A necessary prerequisite is the synthesis of the activated form of ribose 5-phosphate

Question 9

What does Ribose 5-phosphate react with ?

Answer 9

ATP to form 5-Phosphoribosyl-1-pyrophosphate (PRPP)

Question 10

Explain the reaction and regulation for PRPP ?

Answer 10

• Reaction: PRPP synthetase catalyses the rate-limiting step in both purine and pyrimidine biosynthesis and is found in nearly all tissues
• PRPP synthetase catalyses the transfer of a pyrophosphate group from ATP to ribose-5-P to form PRPP, which is the “activated sugar” required for nucleotide synthesis. The pyrophosphate group has a high energy phosphate bond, hence the term “activated sugar”
• Regulation: Because PrPP synthetase catalyses the rate-limiting step for all nucleotide biosynthesis, it is allosterically inhibited by both purine and pyrimidine (thymidine diphosphate) ribonucleotides

Question 11

Explain the reaction and regulation for Glutamine-PRPP amidotransferase ?

Answer 11

• Reaction: The purine base is constructed in a series of reactions that sequentially adds carbon and nitrogen atoms directly to the ribose moiety
• The first-committed step in purine biosynthesis is catalysed by the enzyme glutamine-PRPP amidotransferase which replaces the pyrophosphate group of PRPP with the amide nitrogen of glutamine
• Regulation: Purine nucleotides (AMP, GMP and IMP) are allosteric inhibitors of glutamine-PRPP amidotransferase.
• PRPP is a positive allosteric activator of glutamine-PRPP amidotransferase, as well as activator of pyrimidine biosynthesis

Question 12

Explain Gout causes ?

Answer 12

Mutations in PRPP amidotransferase makes the enzyme less sensitive to feedback inhibition by purine nucleotides

• loss of control also increases flux through the rate-limiting step
• increased synthesis of purines and uric acid

Question 13

What is PRPP also used in?

Answer 13

It is also used in pyrimidine biosynthesis, in synthesis of histidine and tryptophan

Question 14

What can PRPP also be used in ?

Answer 14

NAD+ synthesis, and is also used in the purine salvage pathway

Question 15

What does activity depend on ?

Answer 15

The state of aggregation of the enzyme

Question 16

What are the end products ?

Answer 16

ADP, GDP give mixed inhibition

Question 17

The purine ring is created by ?

Answer 17

10 stepwise additions to PRPP, generating the primary purine inosine monophosphate (IMP)

Question 18

What are the final steps in purine de novo biosynthesis ?

Answer 18

Two levels of regulation are present:

1) The first-committed step in each branch pathway is allosterically regulated via feedback inhibition by its respective end product
2) GTP supplies energy for AMP synthesis and ATP supplies energy for GMP synthesis

Question 19

Feedback inhibition of PRPP synthesis slows ?

Answer 19

All nucleotide synthesis

Question 20

High PRPP levels promote ?

Answer 20

Nucleotide synthesis

Question 21

Feedback inhibition of phosphoribosylamine synthesis is ?

Answer 21

Specific for purines

Question 22

“A” nucleotides promote?

Answer 22

“G” synthesis and vice-versa

Question 23

What blocks purine synthesis ?

Answer 23

Azaserine

Question 24

What does de novo purine synthesis begin with ?

Answer 24

The conversion of ribose-5-phosphate to 5-phosphoribosyl-1-pyrophosphate (PRPP), a reaction catalysed by PRPP synthetase (PRPS)

Question 25

What is the first committed step in purine synthesis ?

Answer 25

The formation of 5-phosphoribosylamine via the enzyme Glutamine PRPP amidotransferase

Question 26

What does a Glutamine analog inhibit ?

Answer 26

Glutamine PRPP amidotransferase and act as Anti-tumour

Question 27

What is the starting point for the biosynthesis of pyrimidine nucleotides ?

Answer 27

The synthesis of carbamoyl phosphate from glutamine

Question 28

Explain Methotrexate ?

Answer 28

Works as a competitive inhibitor of

folate to inhibit DFR

Question 29

Explain 5 fluorouracil ?

Answer 29

Inhibits thymidylate

synthase

Question 30

Pyrimidines are assembled as ?

Answer 30

The free base

Question 31

Purines were made ?

Answer 31

Using ribotide intermediates

Question 32

The pyrimidine pathway is?

Answer 32

Unbranched

Question 33

IMP was the ?

Answer 33

Branch point compound converted to GMP & AMP by different pathways

Question 34

Feedback inhibition operates against ?

Answer 34

Carbamoyl phosphate synthesis

Question 35

What does High ATP levels promote ?

Answer 35

Pyrimidine biosynthesis

Question 36

What is the substrate for ribonucleotide reductase?

Answer 36

Ribonucleoside diphosphate