Transcript Key words are highlighted in yellow Pyrimidine Synthesis

Pyrimidine Metabolism and Cancer
Therapy
LEARNING OBJECTIVES
1.
Nomenclature of pyrimidines*
2.
Key features of biosynthetic pathway and origin of atoms in
pyrimidine ring
3.
Basis of chemotherapy using nucleotide analogs
*Key words are highlighted in yellow
Pyrimidine Synthesis
• The pyrimidine ring is completely synthesized, then
attached to a ribose-5-phosphate donated by PRPP
• Source of carbons and nitrogens less diverse than with
purines.
(Carbamoyl-P)
Urea Synthesis
Pyrimidine Synthesis
Carbamoyl Phosphate Synthase and
Channeling of Intermediates
Miles et al. J. Biol. Chem. 274, 12193-12199 (1999)
Enzymatic functions from one large protein (215,000 Mr)
Enzymatic functions from one large protein
What to
Know
1. Compare and contrast CPS I and CPS II
2. CPSII, aspartate transcarbamoylase, and dihydroorotase are three
enzymatic functions in one protein.
3. Oratate phosphoribosyltransferase and OMP decarboxylase are two
enzymatic functions in one protein. Deficiency leads to Orotic Aciduria.
Know symptoms and how to treat.
4. Orotate is made and then attached to a PRPP.
UMP
UDP
Uridine monophosphate kinase
UTP
Uridine diphosphate kinase
Uridine
Nitrogen Donor
(again!)
Cytidine
Thymidylate synthase is a major target for anti-cancer
therapy.
5-fluorouracil
See Figs 26.25-26.27 in reading
for more details
Methotrexate also
inhibits this reaction
Regulation of Pyrimidine Synthesis
Pyrimidine Breakdown
De novo purine synthesis
AMP
ADP
Adenosine
monophosphate
kinase
IMP
GMP
ATP
Adenosine
diphosphate
kinase
GDP
Guanosine
monophosphate
kinase
Guanosine
diphosphate
kinase
GTP
De novo pyrimidine synthesis
UMP
UDP
Uridine monophosphate kinase
dUMP
UTP
Uridine
diphosphate
kinase
dUDP
CTP
Thymidylate synthase
dTMP
dTDP
Thymidine monophosphate kinase
Thymidine
diphosphate
kinase
dTTP
Ribonucleotides to
Deoxyribonucleotides
Very Important!
Ribonucleotides to
Deoxyribonucleotides
Logic For Deoxynucleotide Synthesis
(Fig 26.24) in reading
•High [ATP], plenty of energy, ok to make DNA
•High [ATP] means the ribonucleotide reductase is active (ON)
•ATP in specificity site S favors CDP or UDP in catalytic site C  [dCDP] and
[dUDP] ↑
•dCDP and dUDP become metabolized to dTTP (thus justifying the synthesis of
dUMP even though it does not get incorporated into DNA)
•As [dTTP]↑, it will begin to occupy specificity site favoring GDP in catalytic site,
thus leading to [dGP]↑  [dGTP]↑
•As [dGTP]↑ it begins to occupy specificity site and thus favor ADP in catalytic
site, leading to [dADP]↑  which will replace ATP in activity site and turn
enzyme off
Uracil Analogues in Cancer
Therapy
Uracil
5-fluorouracil
β-D-arabinofuranosylcytosine (Ara-C)
Cancer Therapy with 5-fluorouracil
O
•5-FU is typically given with thymidine to
boost its effectiveness
•5-FU is converted to dFUMP
•dFUMP inhibits Thymidylate synthase
•In cancer cells, 5-FU is also incorporated
into RNA
F
HN
O
2-O
3POH2C
N
O
dFUMP
HO
•5-FU in RNA is detrimental to cancer cells
•Inhibition of Thymidylate synthase is
detrimental to both cancer and normal cells
•So, administration of thymidine protects
both normal and cancer cells, but 5-FU in
cancer cells kill them
Thymidylate
dUMP
Synthase
dTMP
Thymine Analogues in Cancer Therapy
Thymidine
Bromodeoxyuridine
Trifluorothymidine
Purine Analogues in Cancer Therapy
Hypoxanthine
6-mercaptopurine
6-thioguanine
Purines are used as free bases or nucleosides, as nucleotides are poorly
transported across the membrane.