Purine Metabolism

Question 1

Describe the structure of purines and pyrimidines

Answer 1

1) Purines (two rings one 6 and one 5):
- Adenine: addition of NH2 on C6 Instead of H
- Guanine: addition of double bond O on C6 Instead of H

-Other purines include Xanthine, Hypoxanthine, Inosine & Uric acid

2) Pyrimidine (one ring of 6): (Thymine, Cytosine & uracil)

Question 2

What is the basic structural composition of nucleotides?

Answer 2

1) Nitrogenous base
2) Pentose sugar
3) Phosphate

Question 3

What is the source of nitrogenous bases?

Answer 3

1) De novo synthesis (in liver)
2) Salvage pathway (Diet, Cell death, RNA turnover (mainly in the brain & bone marrow)

Question 4

What is the structural difference between RNA & DNA?

Answer 4

RNA = D-Ribose
DNA = 2-Doxy-D-ribose
The difference is that in the DNA C2 has H instead of OH, which affects the secondary structure and stability

Question 5

What is meant by nucleoside formation?

Answer 5

It is the formation of a bond between a nitrogenous base and a ribose

• Cytosine + ribose = cytidine (Nucleoside)
• Uracil + ribose = Uridine (Nucleoside)
• Adenine + ribose = Adenosine (Nucleoside)
• Guanine + ribose = Guanosine (Nucleoside)
• Thymine + ribose = Thymidine/ deoxythymidine
• Nucleoside + phosphate = nucleotide

Question 6

What is a nucleotide?

Answer 6

it is the addition of a phosphate to a nucleoside

Question 7

What are the functions of nucleotides?

Answer 7

1) DNA & RNA building blocks
2) Nucleoside 5’-triphosphates are carriers of energy
3) Bases serve as a recognition site
4) Cyclic structures are signal molecules and regulators of cellular metabolism and reproduction (Like Cyclic-AMP)
5) Structural component of some coenzymes (CoA, FAD, NADH, NADPH)
6) GTP drives protein synthesis
7) CTP drives lipid synthesis
8) UTP drives carbohydrate metabolism

Question 8

What is an inosine?

Answer 8

it is when a hypoxanthine binds to a ribose sugar

Question 9

In bullet points describe the process of De novo synthesis of purine rings

Answer 9

• Amino acids are the nitrogen donors
• CO2 and N-formyltetrahydrofolate: carbon donor
• These atoms are added to preformed ribose sugar
• Occurs in the liver

1) Formation of 5-phosphoribosyl-1-pyrophosphate (PRPP) from ribose-5-phosphate, catalyzed by (PRPP synthetase) and it is inhibited via (IMP, AMP, & GMP) which are the end product of purine synthesis

• We start building a purine ring on the PRPP

2) The amide group (from glutamine “AA nitrogen source”) replaces the pyrophosphate group via aminotransferase (inhibited by the end product) this is the committed step in purine synthesis

3) Atoms are added from their precursors to the PRPP structure which forms IMP (Inosine monophosphate “hypoxanthine + ribose”), which will later be converted into adenosine (adenylate) or guanosine (guanylate)

4) To produce GMP you need ATP + nitrogen from glutamine, but for AMP you need GTP + nitrogen from aspartate (if they are available in excess de novo synthesis stops at the aminotransferase step)

5) Monophosphate is di/triphosphate by (kinase enzyme) using ATP as a phosphate source

Question 10

From where do we get the ribose-5-phosphate?

Answer 10

• Produced in the (PPP) pentose phosphate pathway “Hexo monophosphate pathway”, converts glucose-6-phosphate into ribulose-5-phosphate and NADPH
• The most important enzyme is glucose-6-phosphate dehydrogenase, it is the rate-limiting step
• NADPH is used as an antioxidant in the CYP450

Question 11

What is the disorder that arises with the deficiency of the enzyme glucose-6-phosphate dehydrogenase enzyme?

Answer 11

Favasim (hemolytic anemia)

Question 12

What is the parent purine?

Answer 12

• Inosine monophosphate

Question 13

How do we synthesize deoxyribonucleotides?

Answer 13

By the reduction of purine nucleoside (ribose + nitrogen) diphosphate (ADP, GDP) to their deoxy forms (dADP, dGDP) by ribonucleotide reductase (allosterically inhibited by dATP & dADP)

Question 14

Describe the salvage pathway of purine synthesis.

Answer 14

• Purines synthesized by the normal turnover of cellular nucleic acids, or obtained from the diet can be converted into nucleoside triphosphates and used by the body
• Reduces the energy expenditure
• Important in the brain

Two enzymes are involved:
1) Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) (Hypoxanthine + PRPP to IMP) this enzyme also mediates (Guanine+ PRPP= GMP)
- Attaches the ring to PRPP producing IMP or GMP
2) Adenine phosphoribosyl transferase (APRT) combined with PRPP = AMP
- Where they utilize PRPP as the source of ribose-5-phosphate

Question 15

What are the intermediates in purine catabolism?

Answer 15

1) Removing the phosphate group (via nucleotidase), AMP-ADENOSINE, GMP=GUANOSINE (guanosine - guanine to xanthine via deaminase then uric acid)

2) Adenosine is converted into Inosine via adenosine deaminase

3) Inosine will undergo hydrolysis of the ribose sugar leaving the hypoxanthine ring back

4) Hypoxanthine is oxidized into xanthine (via xanthine oxidase) which is further oxidized into uric acid

5) The end product is CO2, urea & water

Question 16

What will happen in case of a deficiency in the enzyme adenosine deaminase (ADA)?

Answer 16

accumulation of adenosine & dATP, inhibiting ribonucleotide reductase

Question 17

What causes SCID?

Answer 17

• It is a deficiency in ADA (Adenosine deaminase)
• Autosomal recessive
• ADA enzyme has the highest activity in lymphocytes
• ADA deficiency - Accumulation of adenosine - converted to ribonucleotide/deoxyribonucleotides (via kinases)- increase in dATP = inhibit ribonucleotide reductase enzyme (prevents the production of all deoxyribose-containing nucleotides) - inhibits DNA synthesis in lymphocytes - no stem cell proliferation

Question 18

A deficiency in adenosine deaminase (ADA) will result in?

Answer 18

SCID (Severe Combined Immunodeficiency), because lymphocytes are highly active and proliferative

Question 19

What is the clinical hallmark of SCID?

Answer 19

Repetitive and frequent bacterial, viral, and fungal infections that persist despite standard medical treatment.

Question 20

What is GOUT?

Answer 20

A disorder characterized by hyperuricemia with recurrent attacks of acute arthritic joint inflammation due to the deposition of uric acid crystals

Question 21

What are the types of GOUT?

Answer 21

1) Primary gout: caused by inborn error of metabolism, (like uric acid overproduction)

2) Secondary gout: caused by other diseases like cancer, chronic renal insufficiency, etc

Question 22

What is the treatment of GOUT?

Answer 22

we give ALLOPURINOL

as it inhibits xanthine oxidase leading to the accumulation of hypoxanthine and xanthine which are more soluble than uric acid

Question 23

Define Lesch-Nyhan syndrome

Answer 23

• X-linked recessive disorder
• Deficiency in HGPRT “part of salvage pathway” (hypoxanthine phosphoribosyl transferase), thus activating the de novo pathway
• This deficiency will cause a rise in PRPP (& guanine and hypoxanthine), which in turn will increase the de novo purine biosynthesis which will increase catabolism thus uric acid in the blood which will damage the CNS, causing renal stones and gouty arthritis

Question 24

What is a special symptom of LNS?

Answer 24

self-harming behaviour

Question 25

Can megaloblastic anemia develop in people with LNS?

Answer 25

YES, as they poorly metabolize vitamin B12