Nucleic Acids in Chemistry and Biology

synthesis of carbamoyl phosphate is inhibited by UMP. In the next step, the pyrimidine ring is formed by
cyclization and loss of water and is followed by dehydrogenation to give orotic acid.
The enzymes involved in the last three steps form a multi-enzyme complex in eukaryotes (but not in
prokaryotes) and are located on a single 200 kDa polypeptide chain. A potent inhibitor of the first enzyme,
aspartate transcarbamoylase, is N-phosphonoacetyl-L-aspartate (PALA) (Figure 3.77). PALA is an example
of a transition state inhibitor, which mimics the transition state of a reaction. PALA binds tightly to aspar-
tate transcarbamoylase and has proved to be useful in the production and isolation of the enzyme complex.
Orotate then reacts with PRPP to give orotidylate (Figure 3.78). There is inversion of configuration at C-1
and the
-nucleotide is formed. The equilibrium of the reaction is once again driven forward by hydrolysis
of pyrophosphate. Finally, UMP is produced by decarboxylation. The other pyrimidine nucleotides are derived
from UMP after its conversion into UTP (Section 3.4.3).

3.4.2.2 Salvage Pathways. The enzyme, orotate phosphoribosyl transferase, which is involved in the

production of orotidylate from orotate, can also utilize a number of other pyrimidines that are produced as

120 Chapter 3

O

C O P

O

O

O

H 2 N

CH

H 2 N COO

H 2 C

COO

CH

NH COO

CH 2

O 2 C

C

H 2 N

O

HN

H

N

CH 2

O

O

COO

H

HN

H

N

O

O COO

NAD

H 2 O

O

C O P

O

O

O

H 2 N

glutamine + 2ATP + HCO 3 +2ADP+Pi +glutamate

carbamoyl phosphate

synthetase

carbamoyl

phosphate

+

aspartate

transcarbamoylase

Pi

aspartate carbamoylaspartate

dihydro-

orotase

orotic acid dihydroorotate

dihydroorotate

dehydrogenase

NADH + H

carbamoyl phosphate

Figure 3.76 De novo biosynthesis of pyrimidines; formation of orotate

ON

H

P

O

O CO 2 O OO

Figure 3.77 N-Phosphonoacetyl-L-aspartate (PALA)

HN

NH

O

O COO

PRPP PPi

O

HO OH

O P O

O

O

N

NH

O

O

OOC

H CO

O

HO OH

O P O

O

O

N

NH

O

O

orotate orotidylate UMP

phosphoribosylorotate

transferase

Figure 3.78 Formation of UMP from orotate

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