Purine metabolism

1. Kwame Nkrumah University of
Science & Technology, Kumasi, Ghana
Purine Metabolism
(Biosynthesis and Catabolism)

2. LEARNING OUTCOMES
At the end of this lecture students should:
 Understand nucleosides*, nucleotides, and their function in
DNA and RNA.
 Understand the structure and function of purines.
 Understand the origin of atoms in the purine ring.
 Understand the essential features of purine metabolism and
catabolism.
 Understand clinical aspects of purine metabolism and
deficiencies.

3. Nucleotides
Chemical compound composed of three components: (1)
heterocyclic base; (2) sugar (usually a pentose); and (3) one
or more phosphate groups
Adenosine monophosphate (AMP)
Base
Pentose sugar
Phosphate
Glycosidic bond

4. Nucleotides actively participate in many biochemical
reactions
ATP and GTP (nucleotides) are energy sources.
Uridine derivatives of sugars participate in carbohydrate
metabolism
Coenzymes (NAD, FAD, Co-A) are nucleotide
derivatives
[ATP], [ADP], [AMP] act as allosteric (Activation of
protein from other than active site) regulators of key
enzymes in nucleotide synthesis.

5. Building blocks for DNA and RNA

6. Building blocks for DNA and RNA

7. Important Notes about Nucleotides
 Diet Nucleoprotein, purine and pyrimidine bases do not directly
incorporate in Nucleic acids synthesis. The human body synthesize
the nitrogenous bases for nucleic acid, ATP, NAD, Co-A from
Amphibolic intermediate.
 Nucleoprotein from diet are digested in intestinal tract.
 Nucleic acids are degraded with help of ribonucleases,
deoxyribonucleases, polynucleotidases to nucleotides. Furthermore,
it produces uric acid with the help of phosphorylase enzymes.

8. The Nitrogenous Bases
In DNA:
Adenine
Guanine
*Thymine*
Cytosine
In RNA:
Adenine
Guanine
*Uracil*
Cytosine

9. Base Ribonucleoside Ribonucleotide Deoxyribonucleotide
Adenine Adenosine Adenylate Deoxyadenylate
Guanine Guanosine Guanylate Deoxyguanylate
Cytosine Cytidine Cytidylate Deoxycytidylate
Thymine Thymidine Ribothymidylate Thymidylate
Uracil Uridine Uridylate Deoxyuridylate
Hypoxanthine Inosine Inosinate Deoxyinosinate
Xanthine Xanthosine Xanthylate Deoxanthylate
RNA is sensitive to alkaline degradation
The Nitrogenous Bases

10. Two Important Points
1. The phosphate groups are
responsible for the net
negative charge associated
with DNA and RNA.
2. The hydroxyl group at the 2’-
position accounts for the
greater ease with which RNA is
degraded by alkali.

11. Mechanism of RNA Hydrolysis
Hydrolysis occurs by nucleophilic attack of the 2’-hydroxyl group on the
polarized phosphate to yield a 2’-3’ cyclic phosphodiester intermediate
(circled) that subsequently spontaneously hydrolyzes to a mix of 2’- and 3’-
phosphomonoesters.

12. Biosynthesis of Purine Nucleotide
All form of life synthesize the Purine even
Pyrimidine nucleotide except parasitic protozoa
Nucleotides can synthesize by
• De Novo pathway: Nucleotides are constructed from
simple precursors from metabolic process.
• Salvage pathways: Recovery and recycling of
nucleotides obtained in the diet

13. De novo purine synthesis

14. The Purine Ring
The purine ring is synthesized by a series of reactions that add
the carbon and nitrogen atoms to a pre-formed ribose-5-
phosphate.
The ribose-5-phosphate is synthesized as part of the Hexose
Monophosphate pathway.
In humans, all necessary enzymes are found in the cytoplasm of
the cell.

15. Source For Ribose-5-Phosphate

16.  The pentose sugar is always a ribose, which may be reduced
to deoxyribose after nucleotide synthesis is complete.
 5-Phosphoribosyl-1-pyrophosphate (PRPP) is also
involved in synthesis of pyrimidine nucleotides, NAD+, and
histidine biosynthesis.
Conversion of Ribose-5-phosphate to
PRPP

17. 1. First step of purine
synthesis is committed
step and rate limiting
step.
2. Intracellular concentrations
of glutamine and PRPP
control the reaction rate.
3. Inhibited by AMP, GMP,
and IMP.
4. Requires 4 ATP molecules.
De novo purine synthesis

18. Synthesis of Adenine and Guanine
Ribonucleotides

19. Synthesis of Adenine and Guanine
Ribonucleotides
 IMP does not accumulate in the cell but is rapidly converted to AMP
and GMP. In the first reaction, aspartate’s amino group is linked to
IMP in a reaction driven by the hydrolysis of GTP to GDP Pi to yield
adenylosuccinate.
 In the second reaction, adenylosuccinate lyase eliminates
fumarate from adenylosuccinate to form AMP.This enzyme also
catalyzes Reaction 9 of the IMP pathway.
 IMP dehydrogenase catalyzes the NAD-dependent oxidation of
IMP to form xanthosine monophosphate (XMP). XMP is then
converted to GMP by the replacement of its 2-keto group with
glutamine’s amide nitrogen in a reaction driven by the hydrolysis of
ATP to AMP Ppi.

20. Synthesis of ADP, ATP, GDP and GTP
 Corresponding nucleoside triphosphates are required for the synthesis of Nucleic
acid.
 In phosphorylation reactions respective nucleoside monophosphate are
synthesized in presence of base specific nucleoside monophosphate kinases.
 These enzymes do not discriminate between ribose and deoxyribose in the
substrate.
 Adenylate kinase is particularly active in the liver and in muscle, where the
turnover of energy from ATP is high.

21. Dug effects on Folates and Tetrahydrofolic acid
synthesis

22.  Sulfonamides inhibit purine synthesis in bacteria by interfering with folate
synthesis.
 That methotrexate inhibits purine synthesis by inhibiting dihydrofolate
reductase.
 AMP, GMP, and IMP inhibit the reaction in the rate limiting step while
PRPP is an activator.
 Para-aminobenzoic acid (PABA) is not directly involved in the de novo
synthesis of purines but plays a crucial role in the biosynthesis of another
important group of compounds called folates or folic acid.
 Folates are essential cofactors in various cellular processes, including
the de novo synthesis of purines and pyrimidines, as well as in various
one-carbon transfer reactions crucial for nucleotide synthesis,
methylation reactions, and DNA synthesis.
Other Effects on Purine metabolism

23. Regulation of Purine Nucleotide Biosynthesis

24. Salvage pathway
 In the salvage pathway, purine bases produced by degradation of RNA or
DNA and intermediate of purine synthesis can be directly converted to the
corresponding nucleotides.
 The salvage pathway mainly involves the conversion of free purine bases
into their corresponding nucleotides using phosphoribosyl pyrophosphate
(PRPP) as a co-substrate.
 The key steps in the salvage pathway of purine biosynthesis are as follows:
 Uptake of Purine Bases: Cells can obtain free purine bases from various
sources, including the breakdown of nucleic acids (e.g., DNA and RNA)
during cell turnover and the diet. Purine bases, such as adenine, guanine,
and hypoxanthine, can enter the cell through specific transporters.
 Phosphoribosylation: Once inside the cell, the free purine bases are
converted into nucleotides through the action of specific enzymes called
phosphoribosyltransferases. These enzymes transfer the 5-phosphoribosyl
group from PRPP to the purine base, resulting in the formation of the
corresponding nucleotide.

25. Two phosphoribosyl transferases are involved
in salvage pathway
 APRT (adenine phosphoribosyl transferase) for adenine to form
AMP
 HGPRT (hypoxanthine guanine phosphoribosyl transferase) for
guanine or hypoxanthine
The significance of salvage pathway
 Save the fuel.
 Some tissues and organs such as brain and bone marrow are only
capable of synthesizing nucleotides by salvage pathway
Salvage pathway

27. Lesch-Nyhan syndrome
 First described in 1964 by Michael Lesch and William L. Nyhan.
 There is a defect or lack in the Hypoxanthine-Guanine
phosphoribosyl transferase (HGPRT) enzyme encoded by HPRT1
gene
 A defect may be in the production or activity of HGPRT
 Causes increased level of Hypoxanthine and Guanine (in
degradation to uric acid)
 Sex-linked (X-linked) metabolic disorder: only males
 The prevalence is approximately 1 in 380,000
 Mutations in the HPRT1 gene

28.  The rate of purine synthesis is increased about 200-fold
Loss of HGPRT leads to elevated PRPP levels and
stimulation of de novo purine synthesis.
 Uric acid level rises and there is gout disease
 In addition there are mental aberrations or changes
 Patients will self-mutilate by biting lips and fingers off
Lesch-Nyhan syndrome

29.  Build up of hypoxanthine and guanine
 Degradation of hypoxanthine and guanine results in increased uric acid
 Excess uric acid in urine often results in orange crystals in the diaper of affected
children.
 Severe mental retardation
 Self-mutilation
 Involuntary movements
 Gout
Lesch-Nyhan Syndrome

30. Autism
 About 25% of autistic persons may have purines at high
levels.
 So purine test is a diagnostic test for autistic patients
o Biochemical findings from this test disappear
adolescence ( between 13-19 age).
o Must obtain urine specimen in infancy, but it’s may
difficult to do!
oPink urine due to uric acid crystals may be seen in
diapers

31. Degradation of Purine Nucleotide

32. Degradation of Purines

33. Purine Catabolism and Salvage
All purine degradation leads to uric acid
Ribonucleases, Deoxyribonucleases, Polynucleotidases
degrade the nucleic acids.
Ingested nucleic acids are degraded to nucleotides in
intestine
Pancreatic nucleases
Intestinal phosphodiesterases
Group-specific nucleotidases and non-specific
phosphatases degrade nucleotides into nucleoside
Direct absorption of nucleosides
Further degradation

34. Nucleosidase (Hydrolysis reactions)
The is intracellular reaction
Nucleoside + H2O base + ribose
n. Phosphorylase
Nucleoside + Pi base + r-1-phosphate
Importantly, Most of the ingested nucleic acids are
degraded and excreted

35. Intracellular Purine Catabolism
Nucleotides broken into nucleosides by action of
nucleotidase (Hydrolysis reactions)
Purine nucleoside phosphorylase (PNP)
Inosine to Hypoxanthine
Xanthosine to Xanthine
Guanosine to Guanine

36. Intracellular Purine Catabolism
Xanthine is the point of convergence for the
metabolism of the purine bases
Xanthine converted to Uric acid
Xanthine oxidase catalyzes two reactions
Purine ribonucleotide (In case of RNA) degradation
pathway is same for purine deoxyribonucleotides (In
case of DNA)

37. Adenosine Degradation

38. N
HC
N
C
C
C
N
CH
N
NH2
Ribose-P
AMP
HN
HC
N
C
C
C
N
CH
N
O
Ribose-P
IMP
HN
HC
N
C
C
C
N
H
CH
N
O
HN
C
N
H
C
C
C
N
H
CH
N
O
O
HN
C
N
H
C
C
C
N
H
C
N
O
O
O
GMP
Hypoxanthine
Uric Acid Xanthine
Xanthine Oxidase
(2,6,8-trioxypurine)
Adenosine
Deaminase
The end product of purine metabolism

39.  Uric acid is the excreted end product of purine catabolism in
primates, birds, and some other animals.
 The rate of uric acid excretion by the normal adult human is
about 0.6 g/24 h, arising in part from ingested purines and in
part from the turnover of the purine nucleotides of nucleic
acids.
 The normal concentration of uric acid in the serum of adults
is in the range of 3-7 mg/dl.
Uric acid

40.  Gout is a disease of the joints, usually in males, caused by
an elevated concentration of uric acid in the blood and
tissues.
 The joints become inflamed, painful, and arthritic, owing to
the abnormal deposition of crystals of sodium urate.
 The kidneys are also affected, because excess uric acid is
deposited in the kidney tubules
GOUT

41. dATP builds up and inhibits
ribonucleotide reductase.
Inhibition of ribonucleotide reductase results in no dNDP being produced.
No synthesis of DNA in lymphocytes results in Severe Combined
Immunodeficiency Syndrome (SCIDS).
XMP
Xanthosine

42. Gout
 Characterized by hyperuricemia and acute arthritic joint
inflammation by deposition of uric acid crystals
 Primary gout is genetic and mainly affects men over 30
 Secondary gout is associated with leukemia, polycythemia,
HGPRT deficiency, renal insufficiency, lifestyle (rich foods).

43. 1
2
1
3

44. HN
HC
N
C
C
C
N
H
CH
N
O
Hypoxanthine
HN
HC
N
C
C
C
N
H
N
H
C
O
Allopurinol
Allopurinol – a suicide inhibitor used to treat Gout
Xanthine oxidase
Xanthine oxidase

45. Drugs That Promote Gout
 Certain drugs can cause secondary gout due to actions
on the kidney that affect uric acid reabsorption,
secretion, and excretion.
– Diuretics, due to their actions on the kidney, can
increase uric acid reabsorption and lead to
hyperuricemia and resultant gout. Since recent data
has linked hypertension and hyperuricemia, initial
serum urate levels are an important consideration
when prescribing diuretics (especially thiazides) to
control blood pressure in order to avoid elevating urate
even further.

46. Drugs That Promote Gout
Caspi et al. Arth Rheum. 43, 103-108 (2000)
 High doses of aspirin (>3 g/day) are uricosuric. Low doses, however, cause
uric acid retention. Caspi et al studied the effects of mini-dose aspirin,
utilized as a platelet inhibitor, on uric acid. The study found that doses as
low as 75 mg/day increased serum urate.
 This indicates that elevated serum urate may be a problem in the large
population of patients that take mini-dose aspirin for this purpose. In
addition, these patients are often elderly with compromised renal
function, and they obtain this therapy over-the-counter.
 Pyrazinamide, ethambutol, and niacin are other agents associated with
gout because they suppress uric acid secretion.

47. What did you learn?
Summary