Biochemistry II PHR 375Biochemistry II PHR 375

Nucleotides Metabolism

Hassan MokhamerHassan MokhamerHassan MokhamerHassan Mokhamer

bybybyby

Sources Of Atoms in purines

Synthesis Of Purines nucleotides

De Novo Pathway(Steps)

• 1. Synthesis of 5’_ phosphoribosyl-1-pyrophosphate(PRPP)

The key regulatory enzyme is phosphoribosyl pyrophosphate synthase

Ribose-5-Phosphate

PRPP Synthase

5-Phosphoribosyl -1-pyrophosphate

2. Synthesis of 5’_ phosphoribosylamine ATP AMP

5-Phosphoribosyl -1-pyrophosphate 5-Phosphoribosylamine

Glutamine Glutamate

H2O PPi

• 3. Synthesis of Inosine 5’- Monophosphate (IMP):

5-Phosphoribosylamine

Glycine

Folic Acid

Glutamine

Aspartate

CO2

Inosine Monophosphate (IMP)

HN

HC

N

C

C

C

N

CH

N

O

4

5

HH

CH2

OH OH

H HO

O2-O3P

• 4. Conversion of IMP to either AMP or GMP

IMP

Inosine Monophosphate (IMP)

Adenyl succinate

Xanthosine

Monophosphate

AMP

GMP

ADP ATP

GDP GTP

Inhibitors of purines synthesis

Methotrexate

That competitively inhibits the reduction of

dihdrofolate to tetrahydrofolate . Thus this drugdihdrofolate to tetrahydrofolate . Thus this drug

will limit the amount of tetrahydrofolate available

for use in purine synthesis

Conversion of ribonucleotides to deoxyribonucleotides

• The nucleotides synthesized by de novo and salvage

pathways are ribonucleotides and can be used as building

blocks in RNA synthesis.

• The cellular level of deoxyribonucleotides is usually very

low, they are increased only at the time of DNA

replication.replication.Ribonucleoside diphosphate Deoxyribonucleoside diphosphate

Thioredoxin ( Oxidized)Thioredoxin ( Reduced)

NADPH + H+NADP+

Ribonucleotide Reductase

Thioredoxin Reductase

Catabolism of purine nucleotides

• A. Uric acid is the end product of purinecatabolism in man which is excreted in urine.

• B. Net excretion of total uric acid in normalhuman is about 500 mg/day.

• C. Normal human plasma uric acid average 2-6mg/dl in female and 3 – 7 mg/dl.

• D. Uric acid is oxidized into Allantion by theaction of Uricase enzyme which is lacked inhuman.

Uric acid crystals and stones

• The solubility of uric acid depends upon pH .

• Uric acid becomes insoluble (Crystals) at the

site of urine acidification ( the distal tubules).site of urine acidification ( the distal tubules).

• The more acidity the more crystals formation

Uric acid stone

Disorders of purine catabolism

• A. Hyperuricemia :

1. Definition: it is a condition in which serum urate

level is increased above normal level and exceeds

its solubility limit.its solubility limit.

2. Causes of hyperuricemia

a) Primary hyperuricemia

1. Increased activity of PRPP synthase : this

leads to purine overproduction and excretion.

2. Decreased the activity of HGPRTase (hypoxanthine guanine

phosphoribosyl transferase)

( Lesch Nyhan syndrome)

• HGPRTase Block salvage pathway PRPP

Purine synthesis (de novo synthesis) Uric acidPurine synthesis (de novo synthesis) Uric acid

Hyperuricemia

3. Glucose -6- phosphatase (Von Gierke’s disease)

• b) Secondary hyperuricemia :

1. It is due to increase the rate of cell division

and tissue turnover as in cancer and

Leukemia.

2. Decreased renal excretion of uric acid: as

in renal failure or due to drugs as

diuretics or lead poisoning.

3.Effect of hyperuricemia (Gout)

• a) Tophi formation : increased insoluble urate leads

to crystallization of sodium urate in soft tissue and

joints, which results in formation of deposits called

Tophi.

• 1. the tophi cause an inflammatory reaction

called gouty arthritis.called gouty arthritis.

• 2. the joints that firstly affected are small joints

specially those of big toes

b) Renal stones: deposition of urate crystals in

renal tubules may leads to stone formation e.g

kidney stones.

c) Lesch Nyhan Syndrome:

this syndrome is characterized by :-

1. Hyperuricemia and gout1. Hyperuricemia and gout

2. Uric acid renal stone

3. Neurological disorders and mental

retardation.

4.Treatment of hyperuricemia

• a) Treatment of the cause.

• b) Allopurinol:

it is a structural analogue of hypoxanthine that

competitively inhibits xanthine oxidase enzyme

decreasing formation of uric acid

Metabolism of pyrimidine nucleotides

• 1. Sources of atoms in pyrimidine ring

Biosynthesis of pyrimidine

• + + CO2CO2 GlutamineGlutamine ATP Carbamoyl phosphate

CarbamoylCarbamoyl phosphate phosphate

synthase II synthase II

Orotic AcidOrotic AcidOrotic AcidOrotic Acid

UMPUMPUDPUDP

UTPUTP

dUDPdUDP

CTPCTP

dUMPdUMPdTMPdTMP

Regulation of Pyrimidine synthesis

• Carbamoyl phosphate synthase II is the key

enzyme in this pathway:

• 1. It is stimulated by PRPP

• 2. it is inhibited by uracil nucleotides

Mitochondrial

Carabamoyl phosphate

synthase I

Cytosolic

Carabamoyl phosphate synthase

II

Function Urea synthesis Pyrimidine synthesisFunction

Site Mitochondrial of liver cells Cytosol of most tissue cells

Substrate NH3, CO2 and ATP Glutamine, CO2 and ATP