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)

Pyrimidine SynthesisUrea Synthesis

Miles et al. J. Biol. Chem. 274, 12193-12199 (1999)

Carbamoyl Phosphate Synthase and Carbamoyl Phosphate Synthase and Channeling of IntermediatesChanneling of Intermediates

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 UTPUridine monophosphate kinase Uridine diphosphate kinase

Nitrogen Donor(again!)

Uridine

Cytidine

5-fluorouracil

Methotrexate alsoinhibits this reaction

Thymidylate synthase is a major target for anti-cancer therapy.

See Figs 26.25-26.27 in reading for more details

Regulation of Pyrimidine Synthesis

Pyrimidine Breakdown

De novo purine synthesis

IMP

AMP

GMP

ADP ATP

GDP GTP

Adenosinemonophosphate

kinase

Adenosinediphosphate

kinase

Guanosinemonophosphate

kinase

Guanosinediphosphate

kinase

De novo pyrimidine synthesis

UMP UDPUridine monophosphate kinase

UTPUridine

diphosphatekinase

CTPdUDPdUMP

dTMP

Thymidylate synthase

dTDP dTTPThymidine monophosphate kinase

Thymidinediphosphate

kinase

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)

dUMP dTMPThymidylateSynthase

Cancer Therapy with 5-fluorouracil

•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

•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

O

FHN

O N

O

HO

2-O3POH2C dFUMP

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.