Nucleotide Metabolism Flashcards
What is ATP ?
A nucleotide, or a nucleoside triphosphate - NOT a nucleotide triphosphate
What are the major purine bases ?
Adenine and Guanine
What are the major pyrimidine bases ?
Cytosine, Thymine and Uracil
What kind of rings are the bases ?
All the bases are heterocyclic rings (with both C and N)
What are the functions of nucleotides ?
- Energy transduction
- Substrates for RNA and DNA biosynthesis
- Coenzymes
- Metabolic regulators
Explain Energy Transduction ?
- 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
Explain Substrates for RNA and DNA biosynthesis ?
- 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)
Purines are synthesised as ?
- The ribonucleotides, (not as the free bases)
- A necessary prerequisite is the synthesis of the activated form of ribose 5-phosphate
What does Ribose 5-phosphate react with ?
ATP to form 5-Phosphoribosyl-1-pyrophosphate (PRPP)
Explain the reaction and regulation for PRPP ?
- 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
Explain the reaction and regulation for Glutamine-PRPP amidotransferase ?
- 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
Explain Gout causes ?
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
What is PRPP also used in?
It is also used in pyrimidine biosynthesis, in synthesis of histidine and tryptophan
What can PRPP also be used in ?
NAD+ synthesis, and is also used in the purine salvage pathway
What does activity depend on ?
The state of aggregation of the enzyme
What are the end products ?
ADP, GDP give mixed inhibition
The purine ring is created by ?
10 stepwise additions to PRPP, generating the primary purine inosine monophosphate (IMP)
What are the final steps in purine de novo biosynthesis ?
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
Feedback inhibition of PRPP synthesis slows ?
All nucleotide synthesis
High PRPP levels promote ?
Nucleotide synthesis
Feedback inhibition of phosphoribosylamine synthesis is ?
Specific for purines
“A” nucleotides promote?
“G” synthesis and vice-versa
What blocks purine synthesis ?
Azaserine
What does de novo purine synthesis begin with ?
The conversion of ribose-5-phosphate to 5-phosphoribosyl-1-pyrophosphate (PRPP), a reaction catalysed by PRPP synthetase (PRPS)
What is the first committed step in purine synthesis ?
The formation of 5-phosphoribosylamine via the enzyme Glutamine PRPP amidotransferase
What does a Glutamine analog inhibit ?
Glutamine PRPP amidotransferase and act as Anti-tumour
What is the starting point for the biosynthesis of pyrimidine nucleotides ?
The synthesis of carbamoyl phosphate from glutamine
Explain Methotrexate ?
Works as a competitive inhibitor of
folate to inhibit DFR
Explain 5 fluorouracil ?
Inhibits thymidylate
synthase
Pyrimidines are assembled as ?
The free base
Purines were made ?
Using ribotide intermediates
The pyrimidine pathway is?
Unbranched
IMP was the ?
Branch point compound converted to GMP & AMP by different pathways
Feedback inhibition operates against ?
Carbamoyl phosphate synthesis
What does High ATP levels promote ?
Pyrimidine biosynthesis
What is the substrate for ribonucleotide reductase?
Ribonucleoside diphosphate
