predominant dioxopyrimidine nucleotide incorporated into DNA. First, deoxythymidine 5-monophosphate
(dTMP) is biosynthesised from dUMP viathe enzyme thymidylate synthase. The methyl group is provided
by N^5 ,N^10 -methylenetetrahydrofolate, which also acts as an electron donor (Figure 3.81) and becomes oxidised
to dihydrofolate. Tetrahydrofolate is regenerated by dihydrofolate reductase (DHFR) using NADPH as the
reductant. These two enzymes are excellent targets for cancer chemotherapy because cancer cells have an
increased level of DNA synthesis and, thus, a heavy requirement for dTMP (see 5-fluorouracil and
methotrexate in Section 3.7.1).
dTMP is next converted into dTTP in two stages by means of a thymidylate kinase and then a nucleo-
side diphosphate kinase. In virus-infected cells, the viral thymidine kinase (Section 3.7.2) often also plays
the role of a thymidylate kinase.
3.5 Catabolism of Nucleotides
The degradation of nucleotides is of major importance as a target for drug design. RNA is metabolically
much more labile than DNA and is constantly being synthesised and degraded. Degradation occurs ini-
tially through the action of ribonucleases and deoxyribonucleases, which form oligonucleotides that are
further broken down to nucleotides by phosphodiesterases.
Nucleotides are hydrolysed to nucleosides by nucleotidases (and by phosphatases). Of great importance is
the final cleavage of nucleosides by inorganic phosphate to base and ribose 1--phosphate (or 2-deoxyribose
1--phosphate) catalysed by the widely distributed enzyme PNP (Figure 3.82). The ribose phosphate can
then be isomerized to ribose 5-phosphate and reused for the synthesis of PRPP. In mammalian tissues, adeno-
sine and deoxyadenosine are resistant to the phosphorylase. AMP is therefore first deaminated by adenylate
122 Chapter 3
O
ROHO OHBHa HbXHS SHO
ROHO OHBHbXHaHS SHO
ROHO OH 2BHbXHaHS SO
ROHO HBHa HbXS SO
ROHO HBHbXHaS SO
ROHO HbBXHaHS S-H 2 Oprotein
prot
ein protei
npro
tein
prot
ein proteinFigure 3.80 Postulated mechanism for reduction of nucleotides to deoxynucleotides by E. coli ribonucleotide
reductase (XTyr^122 O)