Nucleic Acids in Chemistry and Biology

a result of hydrolysis of DNA or RNA. In a similar way as in the salvage of purines, phosphorylases catal-
yse nucleoside formation from a variety of pyrimidines with either ribose 1-phosphate or 2-deoxyribose
1-phosphate. A cellular kinase is also required to convert the nucleoside product into the corresponding
5 -nucleotide. Thus, uridine kinase will accept both uridine and cytidine as substrates while thymidine kinase
will accept deoxyuridine as well as deoxythymidine. The fact that many viral thymidine kinases have a
reduced specificity for their substrates enables a distinction to be made between normal and virally-
infected cells and has led to a strategy for viral interference (Section 3.7.2). Nucleoside transferases will
catalyse base exchange between nucleosides exclusively in the 2-deoxy series.

3.4.3 Nucleoside Di- and Triphosphates

The immediate biosynthetic precursors of the nucleic acids are normally the nucleoside triphosphates;
whereas, diphosphates can also be used in energy conversions. Diphosphates are obtained from the corres-
ponding monophosphates by means of a specific nucleoside, monophosphate kinase. Adenylate kinase
converts AMP into ADP while UMP kinase converts UMP into UDP. Both enzymes use ATP as the phos-
phoryl donor. Nucleoside triphosphates are interconvertible with diphosphates through nucleoside diphos-
phate kinase, an enzyme that has a broad specificity. Thus Y and Z (Figure 3.79) can be any of the several
purine or pyrimidine ribo- or deoxyribonucleosides.
Cytidine triphosphate is formed from UTP by replacement of the oxygen atom at C-4 by an amino group.
In E. coli the donor is ammonia, but in mammals the ammonia comes from the amide group of glutamine.
In both cases, ATP is required for the reaction (Figure 3.74).

3.4.4 Deoxyribonucleotides

Deoxyribonucleotides are formed by the reduction of the corresponding ribonucleotides. The 2-hydroxyl
group of the ribose is replaced by a hydrogen atom in a reaction that takes place at the level of the ribonu-
cleoside 5-diphosphate. The mechanism is rather complicated. The key enzyme is ribonucleotide reductase
(ribonucleoside diphosphate reductase) and the electrons required for the reduction of the ribose are trans-
ferred from NADPH to sulfhydryl groups at the catalytic site of the enzyme. The enzyme from E. coli is a
prototype for most eukaryotic reductases. A larger subunit (2 86 kDa) binds the NTP substrate and the
smaller subunit (2 43 kDa) contains a binuclear iron centre and a tyrosyl radical at residue-122. A mech-
anism based on all the available data is shown (Figure 3.80). The reduction of ribonucleoside diphosphates
is controlled by allosteric interactions (an allosteric enzyme is one in which the binding of another substance,
usually product, alters its kinetic behaviour) through the use of two allosteric sites that bind a number of
nucleoside 5-triphosphates and lead to a variety of conformations, each with different catalytic properties.
In the event that any dUTP is formed from dUDP, it is rapidly hydrolysed to dUMP by an active dUTPase,
which thereby limits the incorporation of dUTP into DNA. Nonetheless, some uracil residues do occur in
DNA, which may in part arise through deamination of cytosines. These premutagenic events are repaired
by uracil DNA glycosylase (Section 8.11.3). As a result, deoxythymidine 5-triphosphate (dTTP) is the

Nucleosides and Nucleotides 121

UMP + ATP UDP + ADP

UMP kinase

AMP + ATP

UMP kinase

YDP + ZTP YTP + ZDP

nucleoside diphosphate

kinase

2 ADP

Figure 3.79 Biosynthesis of nucleoside di- and triphosphates