Synthesis of pyramidene

1. Uracil is the precursor of all pyrimidine nucleotides.
N1 and carbons 4, 5 and 6 are derived from ASP.
C2 and N3 are derived from carbamoyl phosphate.
Pyrimidine biosynthesis
HN
C
N
CH
CH
C
O
O
H
1
2
3
4
5
6

2. O
C
CH2
C
H2N
HO
COOH
H
NH2
C
O
O P
O
C
CH2
C
N
H
HO
COOH
H
C
O
H2N
+ Pi
The carbamate group of carbamoyl phosphate
is activated for group transfer (cf. ARG biosynthesis).
Aspartate transcarbamoylase transfers it to the amino
group of ASP to form N-carbamoyl aspartate.

3. Dihydro-orotase forms the pyrimidine ring of
5,6-dihydro-orotate.
Oxidation then gives orotate.
O
C
CH2
C
N
H
HN
COOH
H
C
O
O
C
CH
C
N
H
HN
C
O COOH
NAD+ NADH + H+
N. B. The pyrimidine ring is assembled and then attached to
ribose. The purine ring is constructed on a ribose sugar.

4. Orotate condenses with PRPP to form orotidine
monophosphate (OMP).
+ PRPP
Decarboxylation of OMP gives UMP.
- CO2
O
C
CH
C
N
HN
C
O COOH
O
OH
OH
CH2
O
P
P
O
C
CH
CH
N
HN
C
O
O
OH
OH
CH2
O

5. O
C
CH
CH
N
HN
C
O
Ribose -PPP
ATP ADP
NH2
C
CH
CH
N
N
C
O
Ribose -PPP
GLN GLU + Pi
UMP is converted to UTP.
Subsequent amination generates CTP.
Formation of deoxyribonucleotides.
The ribonucleotides ADP, GDP, CDP and UDP are converted
to deoxyribonucleotides (dNDPs) by ribonucleotide reductase.

6. Synthesis of dTMP
dUDP is hydrolysed to dUMP.
dTMP is synthesised by methylation of dUMP.
O
C
CH
CH
N
HN
C
O
Deoxyribose -P
O
C
C
CH
N
HN
C
O
CH3
Deoxyribose -P
+ Methylene-tetrahydrofolate + Dihydrofolate
This reaction is catalysed by thymidylate synthase
with N5-N10 methylene THF as methyl group donor.

7. N
C
C
N
CH2
N
H
H
H2C
H
H
Methylene - THF
N
C
C
N
CH2
HN
H
H
H
The methylene group is reduced to a methyl group at the
expense of oxidation of the cofactor to dihydrofolate.
+ dUMP + dTMP
DHF is converted back to THF by dihydrofolate reductase
(DHFR).

8. Rapidly proliferating cells like cancer cells require a steady
supply of dTMP.
5-Fluorodeoxyuridylate (F-dUMP) is an irreversible inhibitor
of thymidylate synthase. O
C
C
CH
N
HN
C
O
F
Deoxyribose -P
It is an important anti-cancer drug.
Most mammalian cells grow slowly and require less dTMP
(exceptions: cells of the bone marrow, intestinal mucosa
and hair follicles).
Thymidylate synthase is a good target for anti-cancer drugs.

9. N
C
C
N
CH2
HN
H
H
H
H
H
N
C
C
N
CH2
HN
H
H
H
Tetrahydrofolate
Dihydrofolate
N
C
C
N
CH2
HN
H
Folate
Higher organisms cannot synthesise THF and must obtain
folate in their diet.
Folate undergoes 2 reductions to form the active coenzyme
THF.
Both reductions require NADPH.
Both are catalysed by dihydrofolate reductase (DHFR).

10. N
N
N
H
H
N
H2N
OH
CH2
H
C
O
NH N
H
C
COOH
H
CH2 CH2 COOH
N
N
N
H
H
N
H2N
NH2
CH2
H
C
O
N
R
N
H
C
COOH
H
CH2 CH2 COOH
DHFR is also a good target for anti-cancer drugs.
If DHFR is inhibited, THF runs out
and all THF-dependent reactions are blocked.
Folate
R = H, aminopterin
R = CH3, methotrexate
Aminopterin and methotrexate inhibit DHFR and are used
to treat childhood leukaemias.