Biosynthesis of Purine Ribonucleotide. Salvage pathway, Hyperuricemia, Gout Lesch-Nyhan Syndrome

2. Purine nucleotide can be synthesized by two
pathways….
De novo pathway (new synthesis from amphibolic
intermediates)
Salvage pathway
By phoshoribosylation of free purine bases and
By phosphorylation of purine nucleosides
In de novo synthesis, Purine nucleotides are
synthesized not the free purine bases.

3. Precursors for the de novo synthesis of purine
C2
C
6
N1
N
9
N
3
7
N
C5
C4
8C
Glutamine
Aspartate
Glycine
CO2
N10 Formyl
THF
N5 N10 Formyl
THF
In de novo synthesis, Purine ring is assembled on
ribose-5-phosphate, from a variety of precursors.

4. Reactions of de
novo synthesis
of purine
α-D-Ribose-5-phosphate
PRPP synthase
ATP
AMP
Phosphoribosylpyrophosphate (PRPP)
PRPP Glutamyl
amidotransferase
Glutamine
Glutamate
5-phospho-β-D-Ribosylamine
Phosphoribosyl
glycinamide synthase
Glycine + ATP
ADP+Pi
Glycinamide ribosyl-5-phosphate
Formyl transferase
N5, N10 – methenyl
THF
THF
H2O
PPi
Mg++
Mg++
Formylglycinamide ribosyl-5-phosphate
Synthase
Glutamate
ATP, Mg++
Glutamine
Formylglycinamidine ribosyl-5-phosphate
Synthase
ATP Mg++
Ring closure

5. 5- Aminoimidazole ribosyl-5-phosphate
Carboxylase
CO2
5- Aminoimidazole carboxylate ribosyl-5-phosphate
Synthase
Aspartate + ATP
ADP+Pi
5- Aminoimidazole 4- succinyl
carboxamide ribosyl-5-phosphate
Formyl transferase
N10 – methenyl
THF
THF
5- Formylaminoimidazole
4- carboxamide ribosyl-5-phosphate
Cyclohydrolase
H2O
Inosine monophosphate
5- Aminoimidazole 4- carboxamide
ribosyl-5-phosphate
Adenosuccinate
lyaseFumarate

6. Conversion of IMP to AMP and GMP
Inosine monophosphate (IMP)
Adenylsuccinate
synthase
Aspartate
H2O
Adenylsuccinate
Adenosine monophosphate (AMP)
Fumarate
Adenylsuccinase
GTP, Mg++
IMP
dehydrogenaseNAD+
NADH + H+
H2O
Xanthosine monophosphate (XMP)
Glutamine
Glutamate
Guanosine phosphate
synthase
ATP
Guanosine monophosphate (GMP)

7. Formation of Purine Nucleoside Diphosphate
and Nucleoside Triphosphate
Nucleoside monophosphate
(AMP, GMP)
Nucleoside monophosphate
kinase
ATP
ADP
Nucleoside diphosphate
(ADP, GDP)
Nucleoside diphosphate
kinase
ATP
ADP
Nucleoside triphosphate
(ATP, GTP)

8. It is controlled by…
Concentration of PRPP
Feedback regulation
Concentration of PRPP
Increased conc. PRPP stimulates purine
nucleotide synthesis. Which in turn depends on…
Availability of ribose-5-phosphate and
The activity of PRPP synthase

9. α-D-Ribose-5-phosphate
PRPP
synthase
PRPP
phosphoRibosylamine
Glutamyl
amidotransferase
IMP
AMP GMP
ADP GDP
ATP GTP
-
-
--
-
-
++
Committed step

10. Definition-
The conversion of free purines to the
corresponding nucleotides directly.
Sources of Purine
The free purines are formed in the cells
during the metabolic degradation of nucleic
acids & nucleotides.
Also obtained from the dietary sources.

11. -
This pathway provides purine nucleotide for
the tissues which are incapable of their
biosynthesis by de novo pathway.
e.g. human brain has low levels of PRPP
amidotransferase, & also RBCs and
polynuclear leukocyte cannot synthesize 5-
phoshoribosylamine.
Therefore, these tissues depends on this
pathway for purine nucleotide synthesis.
This is much simpler & required less energy.

12. C
C
N
NN
N
C
C
C
NH2
C
C
N
N
H
N
N
C
C
C
NH2
P-O-H2C
O
Adenine phosphoribosyl
transferase
PRPP PPi
Adenine
AMP
Hypoxanthine
C
C
N
N
H
N
N
C
C
C
O
Hypoxanthine-Guanine
phosphoribosyl transferase
PRPP PPi
C
C
N
NN
N
C
C
C
O
P-O-H2C
O
IMP
Guanine
C
C
N
N
H
N
N
C
C
C
O
H2N
Hypoxanthine-Guanine
phosphoribosyl transferase
PRPP PPi
C
C
N
NN
N
C
C
C
O
H2N
P-O-H2C
O
GMP

13. Deoxyribonucleotides are synthesized by reduction of
ribonucleoside diphosphates (both purine & pyrimidine).
The –OH group on C2 of ribose is replaced by a H atom.
It is catalyzed by ribonucleotide reductase.
This enzyme is active only when cells have actively
synthesizing DNA (preparative to cell division).
The reaction also requires thioredoxin, thioredoxin
reductase and NADPH.
Ribonucleoside diphosphate
(ADP, GDP, CDP, UDP)
Ribonucleotide
reductase
Reduced
Thioredoxin
Oxidized
Thioredoxin
Deoxyribonucleoside diphosphate
(dADP, dGDP, dCDP, dUDP)
NADPH+H+NADP+
Thioredoxin
reductase

14. Purines (Adenine & guanine) are catabolised to
uric acid.
Purine Nucleotides are degraded by the pathway,
in which first phosphate group is removed by the
action of nucleotidase to give nucleoside.
Then it is further degraded to uric acid as
follows……

15. AMP
Nucleotidase
H2O
Adenosine
Adenosine
deaminase
H2O
NH3
Inosine
Xanthine oxidase
H2O H2O2
Xanthine
Xanthine oxidase
H2O
Uric acid
Hypoxanthine
Nucleotide
phosphorylase
Pi
GMP
Pi
Nucleotidase
H2O
Pi
Guanosine
Ribose-1-P
Nucleotide
phosphorylase
Pi
Ribose-1-P
Guanine
Guanase
H2O2
➢Xanthine oxidase contains FAD,
molybdenum and iron.
➢It is found in liver and small
intestine
➢X.O librates H2O2 which is
harmfull to the tissues.
➢Catalase convert H2O2 to H2O

17. Uric acid is the end product of purine catabolism.
At physiological PH uric acid is mostly present in
plasma as sodium urate which is in soluble form.
The normal blood level of uric acid is 3-7 mg/dl.
The daily excretion varies from 500-700mg.
It excreted in to ways…
Via Kidney: majority of uric acid is excreted via kidney
(uricosuria).
Via gut: Smaller amount of urate is excreted via gut,
where it broken down by bacteria.
Significance of Uric acid
Uric acid plays an important role as antioxidant.
It is very effective scavenger of free radicals.
Its antioxidant property can be compared with vitamin C

18. Due to ↓ U.A ExcretionDue to ↑ U.A synthesis
Secondary due toPrimary
idiopathic
✓Renal insufficiency
✓Metabolic acidosis
✓Lactic acidosis
✓Diabetes Mellitus
✓Starvation
✓ Unknown reason✓ PRPP synthetase
✓ PRPP amidotransferase
✓ HGPRTase
Secondary
due to
Primary
(genetic)
✓↑ Dietary intake
✓↑ Nucleic acid turnover
✓Various cancers Such as..
(Leukemia, polycythemia.)
✓Von Gierkes disease

19. Gout is a metabolic disease associated with overproduction
of uric acid.
An elevated serum uric acid conc. is known as hyperuricemia
In sever hyperuricemia, crystals of sodium urate get
deposited in the soft tissues, particularly in the joints.
Such deposits are commonly known as tophi.
This causes inflammation in the joints resulting in the
painful gouty arthritis.
Sodium urate or uric acid may precipitate in kidneys and
ureters that results in renal damage and stone formation.
Prevalence of gout is about 3 per 1,000 persons.
Mostly affecting males and post-menopausal womens are
also equally susceptible.

20. Tophi

21. Gout is of two type…
Primary gout
Secondary gout
Primary gout:-
It is an inborn error of metabolism due to defect in
enzymes of purine biosynthesis. Such as…
PRPP synthetase
PRPP amidotransferase
HGPRTase
Secondary gout:-
It is due various diseases causing increased synthesis or
decreased excretion of uric acid.
↑ Dietary intake
↑ Nucleic acid turnover
Various cancers Such as..
(Leukemia, polycythemia.)
starvation
Renal insufficiency
Lactic acidosis
Diabetes Mellitus

22. SecondaryPrimary (Genetic)
Due to enzyme defect Due to decreased
excretion
Due to increased
synthesis
✓Renal
insufficiency
✓Lactic acidosis
✓Diabetes Mellitus
✓↑ Dietary intake
✓↑ Nucleic acid turnover
✓Various cancers Such as..
(Leukemia, polycythemia.)
✓Starvation
✓Von Gierkes disease
✓ PRPP synthetase
✓ PRPP amidotransferase
✓ HGPRTase

23. Gout
(Hyperuricemia)
Secondary
Due to decreased
excretion
Renal insufficiency
Lactic acidosis
Diabetes Mellitus
Due to increased
synthesis
↑ Dietary intake
↑ Nucleic acid turnover
Various cancers
Starvation
Primary (Genetic)
Due to enzyme
defect in …
- PRPP synthetase
- PRPP
amidotransferase
- HGPRTase

24. Gout can be treated combination of nutritional
therapy and drug therapy.
Foods rich in nucleotides and nucleic acids such as
liver, coffeine and tea should restrict in the diet. And
also the alcohol.
Drug for the treatment of primary gout is allopurinol.
Allopurinol is structural analog of hypoxanthine that
competitively inhibits the enzyme xanthine oxidase.
Allopurinol acts as suicide inhibitor in this reaction.
It get oxidized to alloxanthine by XO.
Alloxanthine is more effective inhibitor of XO.
This leads to the accumulation of hypoxanthine and
xanthine. These two compounds are more soluble than
uric acid, hence easily excreaed in the urine.

25. It is due to the deficiency of enzyme HGPRTase.
It was first described in 1964 by Michael Lesch (a
medical student) and Wiliam L. Nyhan (his teacher).
It is an X-linked inherited disorder, that affect only
males.
In absence of HGPRTase, the salvage pathway is inoperative
and purine cannot reconverted to nucleotides; instead they
are degraded to uric acid.
And also cause the accumulation of PRPP, which stimulate
synthesis and degradation of purine.
This results in increased conc of uric acid in plasma and urine

26. Symptoms:-
Hyperuricemia
Neurological abnormalities such as…
Mental retadation
Aggressive behavior
Learning disability
Biting of fingers and lips (self-mutilation)
Gout
Urinary tract stones
Treatment:-
Allopurinol helps to reduces uric acid formation, has
no affect on the neurological manifestation in these
patients.

27. Purine Nucleoside phosphorylase deficiency:-
Purine Nucleoside phosphorylase deficiency is
associated with impaired T-cell function with normal
B-cell function.
In the cells of patients with this deficiency, dGTP
accumulates.
Like dATP, dGTP allosteric inhibits ribonucleotide
reductase.
Both are autosomal recessive disorders with
symptoms of recurrent and chronic infection.