Metabolism of purine nucleotidesA- De Novo synthesis: of AMP and GMP

Sources of the atoms in purine ring:

• N1: derived from NH2 group of aspartate• C2 and C8: from formate group• N3 and N9 : amide group of glutamine• C4, C5 , N7: glycine amino acid• C6: from CO2

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Notes on purine nucleotides biosynthesis:

1. Phosphoribosyl pyrophosphate (PRPP) is the source of ribose -5-phosphate. PRPP react with glutamine where pyrophosphate is removed and is replaced by NH2 of glutamine.

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2- The ring is then formed from their atoms sources (aspartic, glutamine, glycine, etc) with a specific order (how?).

3- PRPP is an activator to pathway i.e, increased PRPP leads to overproduction of purine nucleotides

4- The pathway ends with the formation of a purine nucleotide called : Inosine monophosphate (IMP) which is the precursor of AMP and GMP which then converted into ATP and GTP, respectively

←AMP and GMP

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Question:

Write the sources of atoms in purine ring explaining the

order of its addition to the ring.

NB: IMP is a nucleotide contain purine base which is hypoxanthine (6 –oxy purine). Hypoxanthine is a purine base not enter in DNA or RNA structure

hypoxanthine

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Catabolism ( breakdown) of purine nucleotides

Synthesis of uric acid:

Uric acid is the end product of purine metabolism in human.

AMP or GMP is metabolized to give hypoxanthine which is then

converted into xanthine and finally into uric acid as in the next slide.

Most of uric acid is excreted by the kidney. The remaining uric acid

travels through the intestines, where bacteria help break it down.

Normally these actions keep the level of uric acid in the blood plasma

at a healthy level, which is below 6.8 mg/dL. But under certain

circumstances, the body produces too much uric acid or removes too

little. In either case, concentrations of uric acid increase in the blood.

This condition is known as hyperuricemia.7

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Gout: is a disorder characterized by high levels of uric acid in blood

(hyperuricemia), with deposition of urate crystals in special

sites in the body like joints, and surrounding tissues and

sometimes in the kidney. Gout is a type of arthritis

Urate crystals are detected in synovial fluid of the joint

Causes:1- decreased excretion of uric acid by the kidney due to renal disease,

acidic urine that decrease uric acid excretion, some drugs inhibit uric acid excretion such as thiazide diuretics. Sometimes it is inherent.

2- Diet rich in purines such as red meat, duck, liver, xanthine beverages like tea, coffee, cola.

3-Overproduction of uric acid due to increased synthesis of purine nucleotides which may be idiopathic (with unknown cause) or due to increased levels of PRPP that stimulate synthetic pathway of purine nucleotides.

Symptoms: 1- Hyperuricemia: increased uric acid levels in blood 2- arthritis, inflammation especially in joints due to deposition of urate crystals leading to hot red and swollen joints with severe pain. 3- redness, swelling of big toe.

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4- it may also present as tophi (masses of urate crystals deposited under skin) appears after several years.

5- It may lead to kidney stones,

Tophi, in chronic cases, lumpy deposits of urate just under the skin 11

Treatment: Allopurinol, analogue of hypoxanthine (structurally similar). It competitively inhibits xanthine oxidase, so prevents the conversion of hypoxanthine to xanthine and xanthine to uric acid.

Uricosuric agents: drugs used to increase excretion of uric acid by the kidney such as probenecid.

Anti-inflammatory drugs is recommended also.12

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B- Salvage pathway of purines: Or resynthesis of purine nucleotides:

Purine bases (guanine, hypoxanthine and adenine) resulting during purine catabolism, may not complete the degradation to give uric acid, but react with PRPP again to resynthesize purine nucleotides. This occurs in some organs in which de no vo synthesis can not takes place e.g. in brain and RBCs. This pathway needs two enzymes:

1- Adenine phosphoribosyl transferase (APRTase)

2- Hypoxanthine-guanine phosphoribosyl transferase (HGPRTase)

Both enzymes use PRPP as the source of ribose-5-phosphate

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The first enzyme catalyze the transfer of ribose-5-P from PRPP to adenine to synthesize AMP.

The second salvage enzyme (HGPRTase) mediates the

transfer of ribose-5-P into hypoxanthine or guanine to

regenerate IMP or GMP.

Pyrimidine metabolism

Sources of carbon and nitrogen atoms in pyrimidine ring:

N1, C4, C5 and C6 → from aspartate

C2 from CO2

N3 → from amide group of glutamine16

De no vo biosynthesis of pyrimidine nucleotides (UTP, CTP):

• pyrimidine nucleotides are synthesized by a stepwise series of

reactions to form UMP.

• Pyrimidine ring is formed first then ribose-5- phosphate is added via

PRPP.

• NB. In purine synthesis, ribose-5-P is added from the first step, then,

the ring is formed.

• The rate limiting step in de novo synthesis of pyrimidine is the first

step which is the formation of cabamoyl phosphate from glutamine and

CO2 in the presence of 2ATP and carbamoyl phosphate synthetase II

which is the rate limiting enzyme. 17

Pyrimidine Catabolism

1- Ring Cleavage:

pyrimidine ring can be opened and degraded to highly soluble β-

amino acids such as β- alanine and β-aminoisobutyrate

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Some Functions of nucleotides:

1- Enter in the structure of DNA and RNA.

2- ATP and GTP are sources of energy

3- CTP is required for synthesis of phospholipids that enter in the

structure of cell membrane

4- enter in the synthesis of some coenzymes such as NAD, FAD,

FMN.

2- Salvage pathway of pyrimidine nucleotides:

Pyrimidine base are salvaged to form nucleotides by pyrimidine

phosphoribosyl transferase which transfer ribose-5-phosphate from

PRPP to pyrimidine base to form nucleotide.

Example: uracil base + PRPP → UMP + PPi.