Here synthesis of deoxyribonucleotides is discussed.

1. Biosynthesis of
deoxyribonucleotides
Kavita Singhal
I.D-52618
Ph.D 3rd year
(Chemistry)

2. DNA vs. RNA review
 DNA composed of deoxyribonucleotides.
 Ribose sugar in DNA lacks hydroxyl group at 2’
Carbon.
 Uracil doesn’t (normally) appear in DNA
Thymine (5-methyluracil) appears instead.

3.  Purine/Pyrimidine degradation are the same for
ribonucleotides and deoxyribonucleotides
Biosynthetic pathways are only for ribonucleotide
production
 Deoxyribonucleotides are synthesized from
corresponding ribonucleotides
Deoxyribonucleotide synthesis

4.  Deoxyribonucleotides are produced from either
ribonucleotide di- or triphophosphates.
 The 2'-OH on the ribose sugar is reduced to a
hydrogen.
 NADPH + H+is the reducing agent.
 The enzyme is called ribonucleotide reductase.
 Thioredoxin is a physiologic reducing agent of
RNR (ribonucleotide reductase)
Conversion of ribonucleotides
to deoxyribonucleotides

5. Ribonucleotide reductase-:
 A Heterotetramer: (R1)2 and (R2)2 .
 Free Radical
 Involvement of multiple –SH groups.
 ribonucleotide reductase is left with a
disulfide group that must be reduced to
return to the original enzyme.

6. i. R1 (87 kD dimer)
active site
(binds the substrate
(NDPs):ADP, GDP ,UDP, CDP)
allosteric sites
(ATP-positive allosteric
effector, dATP negative
allosteric effector)
Substrate specificity site
When dTTP binds this site, will
stimulate dGDP synthesis)
ii. R2 (43 kD dimer)
Tyr 122 radical
Binuclear Fe(III) complex

7. R1
R2

9. Anticancer drug
e.g. Hydroxyurea

10. Aminopterin
Thymidylate synthase
dUDP dTMP

11. Supply of reducing equivalents:
 Theenzyme ribonucleotide reductase itself provides
the hydrogen atoms needed for reduction from its
sulfhydryl groups.
 Thereducing equivalents, in turn, are supplied
by thioredoxin, a monomeric protein with two
cysteine residues.
 Cys pair can swap H atoms with disulfide
formed regenerate original enzyme

12.  NADPH-dependent thioredoxin reductase
converts the oxidized thioredoxin to reduced
form which can be recycled again & again.
 Thioredoxin thus serves as a protein cofactor in an
enzymatic reaction.

13. Regulation of deoxyribonucleotide synthesis:
 Deoxyribonucleotides are mostly required for the
synthesis of DNA.
 The enzyme ribonucleotide reductase maintains the
adequate supply of deoxyribonucleotides.
 Ribonucleotide reductase isa complex enzyme with
multiple sites (active site &allosteric sites,substrate
specificity site) that control the formation of
deoxyribonucleotides.