For pharmacy students

1. NUCLEIC ACID
INTRODUCTION &
METABOLISM
By: Kirankumar Solanki

2. INTRODUCTION TO NUCLEIC
ACID
 Nucleic acids are biopolymers, or large
biomolecules (any molecule that is present
in living organisms which is mainly made up
of carbon and hydrogen) essential for all
known forms of life.
 The term nucleic acid is the overall name
for DNA and RNA.

3. INTRODUCTION TO NUCLEIC
ACID
 Elemental composition – carbon, hydrogen,
oxygen, nitrogen and phosphorus.
 There are two types of nucleic acids, namely
deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA).
 Primarily, nucleic acids serve as storage and
transmitters of genetic information.

4. Functions of nucleic acids
1. DNA is the chemical basis of heredity and may be
regarded as the reserve bank of genetic
information.
2. DNA is exclusively responsible for maintaining the
identity of different species of organisms over
millions of years.
3. Every aspect of cellular function is under the
control of DNA.
4. The genes control the protein synthesis through
the mediation of RNA.
5. An adenine nucleotide, ATP, is the universal

5. NUCLEIC ACIDS
 DNA was discovered in 1869 by Johann
Friedrich Miescher, a Swiss researcher.
 The demonstration that DNA contained genetic
information was first made in 1944, by Avery,
Macleod and Mac Cary.
 DNA (deoxyribonucleic acid) and RNA
(ribonucleic acid), are made from monomers
known as nucleotides (The basic component
of biological nucleic acids).

6. INTRODUCTION TO NUCLEIC
ACID
Nucleoti
de
Nucleoti
de
Nucleic acid

7. NUCLEOTIDES
 Nucleotides are composed of a nitrogenous
base, a pentose sugar and a phosphate.

8. Nitrogenous base
 The bases found in DNA and RNA are basic
because they are heterocyclic aromatic amines.
 Two of these bases-adenine (A) and guanine (G)-
are purines.
 The other three- cytosine (C), thymine (T), and
uracil (U) are pyrimidines.

9. Nitrogenous base
 Adenine (A), guanine (G) and Cytosine(C)
are found in both DNA and RNA.
 Uracil (U) is found only in RNA, and
thymine (T) is found only in DNA.
 Both DNA and RNA contain four bases:
two pyrimidines and two purines.
 For DNA, the bases are A, G, C, and T; for
RNA, the base are A, G, C,U.

11. Purines

12. Pyrimidines
 Thymine differs from uracil only in the methyl
group in the 5 position.

13. SUGARS
 The five carbon monosaccharides (pentoses)
are found in the nucleic acid structure.
 RNA contains D-ribose while DNA contains
D-deoxyribose.
 Ribose and deoxyribose differ in structure at
C2.
 Deoxyribose has one oxygen less at C2
compared to ribose.

14. Structures of sugars

15. SUGARS
 The combination of sugar and base is known as
nucleoside.
 The purine bases are linked to C-1 of the
monosaccharide through N-9 (the nitrogen at
position 9 of the five membered ring) by a N-
glycosidic bond.
 The nucleotide made of guanine and ribose is
called guanosine.
 The pyrimidine bases are linked to C-1 of the
monosaccharide through their N-1 by a N-
glycosidic bond.

16. Nucleoside

17. PHOSPHATE
 The third component of nucleic acids is
phosphoric acid.
 When this group forms a phosphate ester
bond with a nucleoside, this result is acid
compound known as a nucleotide.
 For example, adenosine combines with
phosphate to form the nucleotide 5'-
monophosphate (AMP)

18. PHOSPHATE

20. Biosynthesis of Nucleotides
 The pathways for the biosynthesis of
nucleotides fall into two classes:
De novo pathways
 Nucleotides are synthesized from new
simple precursor molecules.
Salvage pathways
 It uses recovered bases and nucleotides
formed during the degradation of RNA and
DNA.

21. De novo synthesis of purines
 Purine synthesis occurs in all tissues. The major
site of purine synthesis is in the liver and, to a
limited extent, in the brain.
 In de novo (from scratch) pathways, the
nucleotide bases are assembled from simpler
compounds.
 The framework for a purine base is synthesized
piece by piece directly onto a ribose-based
structure.
 Substrates: Ribose-5-phosphate; glycine;
glutamine; H2O; ATP; CO2; aspartate.

22. De novo synthesis of purines
 Following compounds contribute to the purine ring
of the nucleotides.
1. N1 of purine is derived from amino group of
aspartate.
2. C2 and C8 arise from formate of N10-formyl THF.
3. N3 and N9 are obtained from amide group of
glutamine.
4. C4, C5 and N7 are contributed by glycine.
5. C6 directly comes from CO2.

23. Sources of Different Atoms of Purine
Ring

24. Steps of the pathway
1. Ribose 5-phosphate,
produced in the hexose
monophosphate shunt of
carbohydrate metabolism is
the starting material for
purine nucleotide synthesis.
 It reacts with ATP to form
phosphoribosyl
pyrophosphate (PRPP).

25. Steps of the pathway
2. Glutamine transfers its
amide nitrogen to PRPP to
replace pyrophosphate and
produce 5-
phosphoribosylamine.
 The enzyme PRPP
glutamyl
amidotransferase is
controlled by feedback
inhibition of nucleotides
(IMP, AMP and GMP).
 This reaction is the
‘committed step’ in purine

26. Steps of the pathway
3. Phosphoribosylamine
reacts with glycine in the
presence of ATP to form
glycinamide ribosyl 5-
phosphate or
glycinamide ribotide
(GAR).

27. Steps of the pathway
4. N10-Formyl
tetrahydrofolate donates the
formyl group and the
product formed is
formylglycinamide ribosyl 5-
phosphate.

28. Steps of the pathway
5. Glutamine transfers the
second amido amino
group to produce
formylglycinamidine
ribosyl 5-phosphate.
6. The imidazole ring of the
purine is closed in an
ATP dependent reaction
to yield 5-aminoimidazole
ribosyl 5-phosphate.

29. Steps of the pathway
7. Incorporation of CO2
(carboxylation) occurs to
yield aminoimidazole
carboxylate ribosyl 5-
phosphate.
 This reaction does not
require the vitamin biotin
and/or ATP which is the
case with most of the
carboxylation reactions.

30. Steps of the pathway
8. Aspartate condenses with
the product in reaction 7
to form aminoimidazole 4-
succinyl carboxamide
ribosyl 5-phosphate.

31. Steps of the pathway
9. Adenosuccinate lyase
cleaves off fumarate and
only the amino group of
aspartate is retained to
yield aminoimidazole 4-
carboxamide ribosyl 5-
phosphate.

32. Steps of the pathway
10. N10-Formyl
tetrahydrofolate donates a
one-carbon moiety to
produce
formaminoimidazole 4-
carboxamide ribosyl 5-
phosphate.
 With this reaction, all the
carbon and nitrogen atoms
of purine ring are
contributed by the

33. Steps of the pathway
11. The final reaction
catalysed by
cyclohydrolase leads to
ring closure with an
elimination of water
molecule.
 The product obtained is
inosine monophosphate
(IMP), the parent purine
nucleotide from which
other purine nucleotides
can be synthesized.

34. Synthesis of AMP and GMP from
IMP

35. Inhibitors of pathway

36. Salvage pathway for synthesis
of purines
 The free purines (adenine, guanine and
hypoxanthine) are formed in the normal
turnover of nucleic acids (particularly RNA),
and also obtained from the dietary sources.
 The purines can be directly converted to
the corresponding nucleotides, and this
process is known as ‘salvage pathway’.

37. Steps of the Salvage pathway
1. Adenine phosphoribosyl transferase
catalyses the formation of AMP from
adenine.
2. Hypoxanthine-guanine phosphoribosyl
transferase (HGPRT) converts guanine to
GMP.
3. Hypoxanthine-guanine phosphoribosyl
transferase (HGPRT) converts and
hypoxanthine, to IMP.
 Phosphoribosyl pyrophosphate (PRPP) is
the donor of ribose 5-phosphate in the

38. Steps of the Salvage pathway

39. De novo synthesis of
pyrimidines
 The synthesis of pyrimidines is a much
simpler process compared to that of purines.
 Aspartate, glutamine (amide group) and CO2
contribute to atoms in the formation of
pyrimidine ring.
 Pyrimidine ring is first synthesized and then
attached to ribose 5-phosphate.

40. Sources of different atoms of
pyrimidine ring

41. Steps in synthesis of
pyrimidines
1. Glutamine transfers its
amido nitrogen to CO2
to produce carbamoyl
phosphate.
 This reaction is ATP-
dependent and is
catalysed by cytosomal
enzyme carbamoyl
phosphate synthetase
II (CPS II).

42. Steps in synthesis of
pyrimidines
2. Carbamoyl phosphate
condenses with aspartate
to form carbamoyl
aspartate.
 This reaction is catalysed
by aspartate
transcarbamoylase.

43. Steps in synthesis of
pyrimidines
3. Dihydroorotase catalyses
the pyrimidine ring closure
with a loss of H2O & form
Dihydroorotate.
 The three enzymes—CPS
II, aspartate
transcarbamoylase and
dihydroorotase are the
domains (functional units)
of the same protein.
 This is a good example of a
multifunctional enzyme.

44. Steps in synthesis of
pyrimidines
4. The next step in
pyrimidine synthesis is
an NAD+ dependent
dehydrogenation,
leading to the
formation of orotate.

45. Steps in synthesis of
pyrimidines
5. Ribose 5-phosphate is
now added to orotate to
produce orotidine
monophosphate (OMP).
 This reaction is catalysed
by orotate
phosphoribosyltransferase,

46. Steps in synthesis of
pyrimidines
6. OMP undergoes
decarboxylation to
uridine mono-
phosphate (UMP).

47. Steps in synthesis of pyrimidines

48. Steps in synthesis of
pyrimidines
8. By an ATP-dependent kinase reaction,
UMP is converted to UDP which serves as
a precursor for the synthesis of dUMP,
dTMP, UTP and CTP.
9. Ribonucleotide reductase converts UDP to
dUDP by a thioredoxin-dependent
reaction.
10. Thymidylate synthetase converts dUDP to
deoxythymidine monophosphate (dTMP).

49. Steps in synthesis of
pyrimidines
11. UDP undergoes an ATP-dependent kinase
reaction to produce UTP.
Cytidine triphosphate (CTP) is synthesized
from UTP by amination.
CTP synthetase is the enzyme and
glutamine provides the nitrogen.

50. Salvage pathway of
pyrimidines
 The pyrimidines (like purines) can also
serve as precursors in the salvage pathway
to be converted to the respective
nucleotides.
 This reaction is catalyzed by pyrimidine
phosphoribosyltransferase which utilizes
PRPP as the source of ribose 5-phosphate.

51. METABOLISM PURINE
NUCLEOTIDES

52. Steps involved in purine
metabolism
1. The nucleotide monophosphates (AMP, IMP and
GMP) are converted to their respective
nucleoside forms (adenosine, inosine and
guanosine) by the action of nucleotidase.

53. Steps involved in purine
metabolism
2. The amino group, either from AMP or
adenosine, can be removed to produce IMP
or inosine, respectively.

55. Steps involved in purine
metabolism
3. Inosine and guanosine are, respectively,
converted to hypoxanthine and guanine
(purine bases) by purine nucleoside
phosphorylase.
 Adenosine is not degraded by this enzyme,
hence it has to be converted to inosine.
4. Guanine undergoes deamination by
guanase to form xanthine.

56. Steps involved in purine
metabolism
5. Xanthine oxidase is an important enzyme
that converts hypoxanthine to xanthine, and
xanthine to uric acid.
 This enzyme contains FAD, molybdenum and
iron, and is exclusively found in liver and small
intestine.
 Xanthine oxidase liberates H2O2 which is
harmful to the tissues.
 Catalase cleaves H2O2 to H2O and O2.
 The end product of purine metabolism in
humans is uric acid.

57. Degradation of uric acid
 Uric acid (2,6,8-trioxypurine) is the final
excretory product of purine metabolism in
humans.
 Uric acid can serve as an important
antioxidant by getting itself converted
(non-enzymatically) to allantoin.

58. Degradation of uric acid
1.Most animals (other than primates),oxidize uric
acid by the enzyme uricase to allantoin, where
the purine ring is cleaved.
2.Allantoin is then converted to allantoic acid
with help of allantoinase enzyme and excreted
in some fishes.
3.Further degradation of allantoic acid may
occur to produce urea with help of allantoicase
enzyme(in amphibians, most fishes and some
molluscs).
4.Urea get converted to ammonia with help of
urease(in marine invertebrates).

59. Degradation of uric acid
(in animals other than humans)

60. Disorders associated with purine
metabolism
 Hyperuricemia
 Hyperuricemia refers to an elevation in the
serum uric acid concentration.
 This is sometimes associated with increased
uric acid excretion (uricosuria).

61. Disorders associated with
purine metabolism
 Gout
 Gout is a metabolic disease associated with
overproduction of uric acid.
 ln severe 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 a
painful gouty arthritis.
 Sodium urate and/or uric acid may also precipitate in
kidneys and ureters that results in renal damage and
stone formation.

62. Types of Gout
 Primary gout
 Primary gout is an inborn error of metabolism
due to overproduction of uric acid. This is
mostly related to increased synthesis of purine
nucleotide.
 Secondary gout
 Secondary gout is due to various diseases
causing increased synthesis or decreased
excretion of uric acid.

63. Urate deposits in Gout

64. Urate deposits in Gout

65. Reasons of gout
 Variant forms of PRPP synthetase.
 Lack of feedback control of PRPP
glutamylamidotransferase causes elevated
synthesis purine nucleotides.
 HGPRT deficiency(salvage pathway) causes
increased synthesis of purine nucleotides.
 Glucose 6-phosphatase deficiency.
 It increases levels of ribose 5-phosphate and PRPP
and, ultimately, purine overproduction.
 Elevation of glutathione reductase
 It also increases levels of ribose 5-phosphate and
PRPP and, ultimately, purine overproduction.