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Purine Metabolism II

Osama Yousef

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Purine Metabolism II

Purine salvage pathway

Synthesis of Deoxyribonucleotides

Nucleotides degradationsIntestinal degradation of dietary nucleic acids

Degradation of purine nucleotides

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Purine salvage pathway

Salvage pathway: synthesize of purine neocletide from thepartial breakdown of previously synthesized nucleotides(turnover of cellular nucleic acids, or diet)

Two phosphoribosyl transferases are involved: APRT (adenine phosphoribosyl transferase) for adenine

HGPRT (hypoxanthine guanine phosphoribosyltransferase) for guanine or hypoxanthine

Both APRT and HGPRT use PRPP to form thenucleotides.

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Adenine phosphoribosyl transferase (APRT)

O

HH

OHOH

CH2

HO

OP

O

OH

OH

H

P

O

O-

O P

O

O-

O-

O

HH

OHOH

CH2

HH

OP

O

OH

OH

H2

adenine

PPi

PRPP

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Hypoxanthine guanine phosphoribosyl transferase

(HGPRT)

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Lesch-Nyhan syndrome

X-linked recessive disorder

Associated with complete deficiency ofHGPRTenzyme in ability to salvage hypoxanthine orguanine

Increase PRPP level, and decrease IMP and GMPlevels

Increase rate ofde novo pathway for purinesynthesis is about 200X

Increase uric acid level (Hyperuricemia) Gout

patients show mental aberrations, motordysfunction, behavioral disturbance (self-mutilation

by biting lips and fingers)

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Synthesis of Deoxyribonucleotides

Amount of RN

A(mRN

As, rRN

As andtRNAs) in the typical cell is 5-10 times of

DNA amount.

During the S-phase of cell cycle,ribonucleoside diphosphates (ADP, GDP,

CDP, UDP) deoxyribonucleoside

diphosphates (dADP, dGDP, dCDP, dUDP)by ribonucleotide reductase.

dNDP dNTP (by nucleoside diphosphate

kinases

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Synthesis of Deoxyribonucleotides

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Regeneration of Ribonucleotide Reductase

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Regulation ofRibonucleotide Reductase

The enzyme is a tetramer (dimer of dimers): R1 a dimer ofidentical a subunits, and R2 a dimer of identical b subunits(45 kD each)

3 binding sites

1-A

ctive site for binding of RNDP (A

DP, GDP, CDP, UDP)2-Activity sites for binding ofATP and dATP

ATPactivate the enzyme

dATP inhibits the enzyme

3- Substrate specificity site for binding ofATP, dATP, dTTP,dGTP

Binding of one regulate reduction of specific ribonucleotide.

(eg. Binding of dTTP stimulate reduction of GDP to dGDP)

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Regulation ofRibonucleotide Reductase

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Nucleotides degradations

Intestinal degradation ofdietary nucleic acids

1

) Cleavage begins at the pho

spho

diester bonds withendonucleases (pancreatic ribonuclease or DNase) in thesmall intestine.

2) The oligonucleotides resulting from previous action arethen cleaved by phosphodiesterases, producing 5' or 3'

monophosphates.

3) Nucleotidases cleave phosphates from the nucleotides,yielding nucleosides. (could be absorbed)

4) Nucleoside phosphorylases act to yield a base and ribose-

1-phosphate.

5) If bases or nucleosides are not reused for nucleic acidsynthesis via salvage pathways, the bases are furtherdegraded to uric acid (purines) oralanine and NH3

(pyrimidines).

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Degradation ofpurine nucleotides

1) Deaminases: AMP IMP

Adenosine Inosine

2) 5-nucleotidases: GMP Guanosine

IMP Inosine

3) Purine nucleoside phosphorelase:

InosineHypoxanthin

Guanosine Guanine

4) Guanase: Guanine Xanthine (deamination)

5) Xanthine oxidase: Hypoxanthine Xanthine

Xanthine

Uric Acid

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Degradation ofpurine nucleotides