WEEK 11 (Purine Metabolism)

Question 1

What are the different Nucleotide roles in the body?

Answer 1

• RNA and DNA monomers
• ATP (energy)
• Physiologic mediators
• GTP (second messenger pathways)
• cAMP levels (blood flow)
• cGMP (second messenger pathways)

Question 2

What are De novo pathways?

Answer 2

When organisms synthesise purine and pyrimidine nucleotides from low-molecular-weight precursors in amounts sufficient for their needs

Question 3

What are Salvage pathways?

Answer 3

The utilisation of pre-formed purine and pyrimidine compounds that would be otherwise lost to biodegradation

Question 4

What does a Nucleoside and a Nucleotide consist of?

Answer 4

Nucleoside = Base + Sugar
Nucleotide = Base + Sugar + Phosphate

Question 5

What are the two different Purines?

Answer 5

Adenine & Guanine

Question 6

What are the three different Pyrimidines?

Answer 6

Cytosine, Uracil & Thymine

Question 7

What does the salvage/reuse of purine and pyrimidine bases involve?

Answer 7

Molecules released by NUCLEIC ACID DEGRADATION

Degradation can occur INTRACELLULARLY (as a result of cell death) or through DIGESTION of nucleic acids ingested in diet

Question 8

Describe the route in animals by which bases and nucleosides become available

Answer 8

In animals, the EXTRACELLULAR HYDROLYSIS of ingested nucleic acids represents the major route by which bases and nucleosides become available. CATALYSIS occurs by ENDONUCLEASES which digest nucleic acids in the small intestine and produce MONONUCLEOTIDES

Question 9

What happens to bases and nucleosides not reused for nucleic acid synthesis via salvage pathways?

Answer 9

The purine and pyrimidine bases are further degraded to URIC ACID or β-ureidopropionate

Question 10

Describe the reutilisation of purine and pyrimidine bases

Answer 10

1) ENDONUCELASES cleave nucleic acids to form OLIGONUCLEOTIDES
2) PHOSPHODIESTERASES cleave oligonucleotides to form NUCLEOSIDE MONOPHOSPHATES
3) NUCLEOTIDASES hydrolyse mononucleotides forming NUCLEOSIDES
4) PHOSPHORYLASES hydrolyse nucleosides into NUCLEOBASES (uric acid & β-ureidopropionate)

Question 11

What is the aim of Purine synthesis?

Answer 11

To generate AMP and GMP

Question 12

What is needed to create AMP and GMP in Purine synthesis?

Answer 12

• Ribose phosphate (HMP Shunt)
• Amino acids
• Carbons (CO2 & Tetrahydrofolate)

Question 13

What is 5-Phospho-α-D-ribosyl-1-pyrophosphate (PRPP)?

Answer 13

5-Phospho-α-D-ribosyl-1-pyrophosphate (PRPP) is an activated ribose-5-phosphate derivative used in both SALVAGE and DE NOVO PATHWAYS

Question 14

Describe how Inosinic acid biosynthesis can be controlled from PRPP

Answer 14

PURINES are synthesised at the nucleotide level, starting with PRPP conversion to PHOSPHORIBOSYLAMINE and PURINE RING assembly on the amino group. Control over the biosynthesis of INOSINIC ACID is provided through feedback regulation of early steps in PURINE NUCLEOTIDE SYNTHESIS. PRPP SYNTHETASE is inhibited by various purine nucleotides (AMP, ADP & GDP) and PRPP AMIDOTRANSFERASE is inhibited by AMP, ADP, GMP & GDP.

Question 15

How is GMP formed from Inosine Monophosphate (IMP)?

Answer 15

INOSINE MONOPHOSPHATE (IMP) is converted into XANTHOSINE MONOPHOSPHATE (XMP) via IMP DEHYDROGENASE. XMP is then converted to GMP using XMP AMINASE.

Question 16

How is AMP formed from Inosine Monophosphate (IMP)?

Answer 16

INOSINE MONOPHOSPHATE (IMP) is converted into ADENYLOSUCCINATE via ADENYLOSUCCINATE SYNTHETASE. Adenylosuccinate is then converted to AMP using ADENYLOSUCCINATE.

Question 17

What is Ribavirin?

Answer 17

• An antiviral
• Inhibits IMP dehydrogenase
• Blocks conversion IMP to GMP
• Inhibits synthesis of guanine nucleotides (purines)

Question 18

What is Mycophenolate?

Answer 18

• An Immunosuppressant
• Inhibits IMP dehydrogenase

Question 19

Describe the summary pathway from ribose phosphate to AMP & GMP

Answer 19

Ribose phosphate from HMP shunt is converted to PRPP. PRPP is then converted to IMP using Aspartate, Glycine, Glutamine, THF & CO2. IMP is then converted to AMP & GMP.

Question 20

What are the properties of Purine Salvage?

Answer 20

• Salvages bases are ADENINE, GUANINE & HYPOXANTHINE
• Converts back into nucleotides (AMP, GMP & IMP)
• Requires PRPP

Question 21

Describe the formation of IMP from Hypoxanthine

Answer 21

Hypoxantine & PRPP are converted to Inosine Monophosphate (IMP) using HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE (HGPRT)

Question 22

Describe the formation of GMP from Guanine

Answer 22

Guanine & PRPP are converted to Guanine Monophosphate (GMP) using HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE (HGPRT)

Question 23

Describe the formation of AMP from Adenine

Answer 23

Adenine & PRPP are converted to Adenosine Monophosphate (AMP) using ADENINE PHOSPHORIBOSYLTRANSFERASE (APRT)

Question 24

What does Purine nucleotide catabolism form?

Answer 24

URIC ACID which is excreted

[some animals further oxidise the purine rings to ALLANTOIN and then to ALLANTOIC ACID which is either excreted or further catabolised to UREA/AMMONIA]

Question 25

What are the properties of 6-Mercaptopurine

Answer 25

• Chemotherapy agent
• Mimics hypoxanthine/guanine
• Added to PRPP by HGPRT forming Thioinosinic acid
• Inhibits multiple steps in de novo synthesis
• decrease in IMP/AMP/GMP

Question 26

What are the properties of Azathioprine

Answer 26

• Immunosuppressant
• Converted to 6-MP

Question 27

What are the effects of Uric acid?

Answer 27

• Uric acid and its Urate salts are very INSOLUBLE
• Insolubility provides a route for egg-laying animals for getting rid of excess nitrogen in a closed environment
• HYPERURICEMIA (GOUT) in humans which is a chronic elevation of blood uric acid levels

Question 28

What can prolonged or acute elevation of blood urate lead to?

Answer 28

Precipitation as crystals of SODIUM URATE in the synovial fluid of joints. These precipitates cause INFLAMMATION resulting in painful ARTHRITIS which can lead to SEVERE DEGENERATION of the joints

Question 29

What is Gout?

Answer 29

HYPERURICEMIA (GOUT) in humans which is a chronic elevation of blood uric acid levels

CAUSES:
- Overproduction of PURINE NUCLEOTIDES leading to excessive URIC ACID SYNTHESIS or from impaired URIC ACID EXCRETION through the kidney.
- Mutations in PRPP AMIDOTRANSFERASE that make it less sensitive to feedback inhibition by purine nucleotides
- Deficiency of HGPRT

TREATMENT:
- ALLOPURINOL which strongly inhibits XANTHINE OXIDASE
[inhibition causes accumulation of HYPOXANTHINE and XANTHIN which are more soluble and readily excreted than uric acid]

Question 30

Describe Lesch-Nyhan syndrome

Answer 30

Lesch-Nyhan syndrome is an X-linked trait caused by HGPRT deficiency (HGPRT gene is located on the X chromosome).

CAUSES:
- Excess uric acid production
- Excess de novo purine synthesis
- Increased PRPP & IMP

SYMPTOMS:
- Severe gouty arthritis
- Malfunction of the nervous system
- Behavioural disorders, learning disabilities, hostile/aggressive behaviour

NO TREATMENT (individuals rarely live beyond 20 years)

Question 31

Describe Severe Combine Immune Deficiency (SCID)

Answer 31

Patients are susceptible (often fatally) to infectious diseases because of an inability to mount an immune response; Both B and T lymphocytes are affected.

In many cases the conditions is caused by a lack of the degradative enzyme ADENOSINE DEAMINASE (ADA). Deficiency of ADA leads to accumulation of dATP which is known to be a potent inhibitor of DNA replication.

Question 32

Describe the immunodeficiency resulting from the lack of Purine nucleoside phosphorylase (PNP)

Answer 32

Decreased activity of this enzyme leads to accumulation of dGTP. This accumulation affects DNA replication but less severely than excessive dATP.

The phosphorylase deficiency destroys only the T class of lymphocytes and not the B cells.