638
chapter 27
Nucleotide Metabolism
(Chapter 19). Biosynthesis of pyrimidine nucleotides can
occur by a
de novo
pathway or by the reutilization of
preformed pyrimidine bases or ribonucleosides (salvage
pathway).
Salvage Pathways
Pyrimidines derived from dietary or endogenous sources
are salvaged efficiently in mammalian systems. They are
converted to nucleosides by nucleoside phosphorylases
and then to nucleotides by appropriate kinases.
P yrim idine
Ribose 5- phosphate «*
Phosphoryiase
Phosphate*
P yrim id in e-rib ose (n u c le o sid e )
ATP-^
Kinase
AOP*^
P yrim id in e-rib ose-p h osp h ate (nucleotide)
UMP also can by synthesized from uracil and PRPP by
uracil phosphoribosyltransferase.
De Novo
Synthesis
The biosynthesis of pyrimidine nucleotides may be con-
veniently considered in two stages: the formation of uri-
dine monophosphate (UMP) and the conversion of UMP
to other pyrimidine nucleotides.
Formation of UMP
The synthesis of UMP starts from glutamine, bicarbonate,
and ATP, and requires six enzyme activities. The sources of
the atoms of the pyrimidine ring are shown in Figure 27-25,
and the pathway is shown in Figure 27-26.
1. The pyrimidine base is formed first and then the
nucleotide by the addition of ribose 5-phosphate from
From a m id e
nitrogen of
glu tam in e
From CO,
From a sp a rta te
F IG U R E 2 7 -2 5
Sources of the pyrimidine ring atoms in
de novo
biosynthesis.
PRPP. In contrast, in
de novo
purine nucleotide
biosynthesis, ribose 5-phosphate is an integral part of
the earliest precursor molecule.
2. In the biosynthesis of both pyrimidine and urea (or
arginine) (Chapter 17), carbamoyl phosphate is the
source of carbon and nitrogen atoms. In pyrimidine
biosynthesis, carbamoyl phosphate serves as donor
of the carbamoyl group to aspartate with the
formation of carbamoyl aspartate. In urea synthesis,
the carbamoyl moiety of carbamoyl phosphate is
transferred to ornithine, giving rise to citrulline.
In eukaryotic cells, two separate pools of
carbamoyl phosphate are synthesized by different
enzymes located at different sites. Carbamoyl
phosphate synthetase I (CPS I) is located in the inner
membrane of mitochondria in the liver and, to lesser
extent, in the kidneys and small intestine. It supplies
carbamoyl phosphate for the urea cycle. CPS I is
specific for ammonia as nitrogen donor and requires
N-acetylglutamate as activator. Carbamoyl phosphate
synthetase II (CPS II) is present in the cytosol. It
supplies carbamoyl phosphate for pyrimidine
nucleotide biosynthesis and uses the amido group of
glutamine as nitrogen donor. The presence of
physically separated CPSs in eukaryotes probably
reflects the need for independent regulation of
pyrimidine biosynthesis and urea formation, despite
the fact that both pathways require carbamoyl
phosphate. In prokaryotes, one CPS serves both
pathways.
3. In mammalian tissue, the six enzymes are encoded by
three genes. One gene codes for a multifunctional
polypeptide (Pyr 1-3) that is located in the cytosol
and has carbamoyl phosphate synthetase II
(Figure 27-27), aspartate transcarbamoylase, and
dihydroorotase activity. Each subunit of Pyr 1-3 has a
molecular weight of
2 0 0
,
0 0 0
-
2 2 0
,
0 0 0
, and the native
enzyme exists as multiples of three subunits. The
second gene codes for dihydroorotate dehydrogenase
which is located on the outer side of the inner
mitochondrial membrane. Dihydroorotate, the
product of Pyr 1-3, passes freely through the outer
mitochondrial membrane and converted to orotate.
Orotate readily diffuses to the cytosol for conversion
to UMP. The third gene codes for another
multifunctional polypeptide known as UMP
synthase (Pyr 5,6). Pyr 5,6 (M.W. 55,000) contains
orotate phosphoribosyltransferase and orotidylate
(orotidine-5'-monophosphate) decarboxylase activity.
Use of multifunctional polypeptides is very efficient,
since the intermediates neither accumulate nor
become consumed in side reactions. They are
