Biosynthesis of Pyrimidines

The pyrimidine ring is synthesized first and is then attached to ribose 5- ph os-phate to eventually produce the nucleotide uridine 5 -monophosphate (UMP). 

PRPP H Orotate derived from glutamine. CPS-II, carbamoyl phosphate synthetase Asp, 

Closure of the ring followed by a reduction step leads to formation of the pyrimidine base orotate. 

Orotate is then connected to ribose 5-phosphate and decarboxylated to produce UMP. 

Synthesis of UTP and CTP occurs via phosphoiylation of UMP and intercon-version of the bases. 

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Consider carbamoyl phosphate, a precursor in the biosynthesis of pyrimidines ... 

The pyrimidine antagonists inhibit the biosynthesis of pyrimidine nucleotides or interfere with vital cellular functions, such as the synthesis or function of nucleic acids. The analogues of deoxycytidine and thymidine that are used are inhibitors of DNA synthesis while 5-fluorouracil (5-FU) an analogue of uracil, is an inhibitor of both RNA function and of the synthesis of thymidylate (see Fig. 2). PALA (N-phosphonoacetyl-L-aspartate), an inhibitor of as-... 

Allosteric enzymes are generally larger and more complex than nonallosteric enzymes. Most have two or more subunits. Aspartate transcarbamoylase, which catalyzes an early reaction in the biosynthesis of pyrimidine nucleotides (see Fig. 22-36), has 12 polypeptide chains organized into catalytic and regulatory subunits. Figure 6-27 shows the quaternary structure of this enzyme, deduced from x-ray analysis. 

Scheme 4 Biosynthesis of pyrimidine nucleotides (P = P03H2, PP = P206H3) (11a) ribosyl-5- ...

A small number of other biosynthetic pathways, which are used by both photosynthetic and nonphotosynthetic organisms, are indicated in Fig. 10-1. For example, pyruvate is converted readily to the amino acid t-alanine and oxaloacetate to L-aspartic acid the latter, in turn, may be utilized in the biosynthesis of pyrimidines. Other amino acids, purines, and additional compounds needed for construction of cells are formed in pathways, most of which branch from some compound shown in Fig. 10-1 or from a point on one of the pathways shown in the figure. In virtually every instance biosynthesis is dependent upon a supply of energy furnished by the cleavage to ATP. In many cases it also requires one of the hydrogen carriers in a reduced form. While Fig. 10-1 outlines in briefest form a minute fraction of the metabolic pathways known, the ones shown are of central importance. 

Carbamoyl phosphate synthase contributes to two processes (a) the initial enzyme in the biosynthesis of pyrimidines and (b) a component in the synthesis of arginine biosynthesis or the urea cycle. In bacteria both of these processes occur within the same compartment. In human beings the carbamoyl phosphate synthase involved in the urea cycle is contained in... 

Carbamyl-L-aspartate is the key precursor in the biosynthesis of pyrimidines. The enzyme aspartate transcarbamylase is inhibited by several pyrimidine nucleotides, notably cytidine triphosphate, and is activated by ATP, a purine nucleotide. Thus the enzyme is under feedback regulation, and controls the relative concentration of pyrimidine and purine nucleotides. 

Figure 7.6 (a) The first step in the biosynthesis of pyrimidines, (b) The proposed transition state for the carbamoyl phosphate/aspartic acid stage in pyrimidine synthesis, (c) The structure of sodium N-phosphonoacetyl-L-aspartate (PALA)... 

Treatment of HCN-polymer with 6.0NHC1 afforded adenine 1, AICN, 3,4-dihydroxypyrimidine 9 and 5-hydroxyuracil 10, while 1, AICN and orotic acid 11 were recovered after reaction with sodium hydroxide (Scheme 5). A reaction mechanism involving the formation of different aminopyridines as intermediates and reduction steps was proposed to explain the distribution of the obtained products. In accordance with the chemomimetic concept, orotic acid is a key intermediate in the current biosynthesis of pyrimidine nucleotides [62],... 

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

There are two multifunctional proteins in the pathway for de novo biosynthesis of pyrimidine nucleotides. A trifunctional protein, called dihydroorotate synthetase (or CAD, where the letters are the initials of the three enzymatic activities), catalyzes reactions 1, 2 and 3 of the pathway (HCC>5"- CAP— CA-asp—> DHO Fig. 15-15). The enzymatic activities of carbamoyl phosphate synthetase, aspartate transcarbamoylase and dihydroorotase, are contained in discrete globular domains of a single polypeptide chain of 243 kDa, where they are covalently connected by segments of polypeptide chain whch are susceptible to digestion by proteases such as trypsin. A bifunctional enzyme, UMP synthase, catalyzes reactions 5 and 6 of the pyrimidine pathway (orotate— OMP—> UMP Fig. 15-15). Two enzymatic activities, those of orotate phosphoribosyltransferase and OMP decarboxylase, are contained in a single protein of 51.5 kDa which associates as a dimer. 

In this experiment we will examine some of the properties of the aspartate transcarbamylase of Escherichia coli, which is typical of many enzymes subject to feedback inhibition and which has been studied extensively. Aspartate transcarbamylase (ATCase) catalyzes the first reaction unique to the biosynthesis of pyrimidine nucleotides. ATCase is subject to specific inhibition by quite low concentrations of one of its end products, cytidine 5 -triphosphate (CTP). This relationship and two other regulatory interactions important to the control of pyrimidine biosynthesis are summarized in Figure 9-1. 

Minic Z, Simon V, Penverne B, Gaill F, Herve G. Contribution 64. of the bacterial endosymbiont to the biosynthesis of pyrimidine nucleotides in the deep-sea tube worm Riftia pachyptila. J. Biol. 

Aspartate transcarbamoylase catalyzes the first step in the biosynthesis of pyrimidines, bases that are components of nucleic acids. The reaction catalyzed by this enzyme is the condensation of aspartate and carbamoyl phosphate to form A-carbamoylaspartate and orthophosphate (Figure 10.1). ATCase catalyzes the committed step in the pathway that will ultimately yield pyrimidine nucleotides such as cytidine triphosphate (CTP). How is this enzyme regulated to generate precisely the amount of CTP needed by the cell ... 

Carriers of OCT deficiency (estimated to be several thousand women in the U.S. A.) can be identified by administration of a single oral dose of allopurinol, a purine analogue, followed by measurement of urinary orotidine excretion. The underlying principle of this assay is that when the intramitochondrial carbamoyl phosphate accumulates in OCT heterozygotes, it diffuses into the cytoplasm stimulating the biosynthesis of pyrimidines. One of the intermediates in this pathway—orotidine—accumulates, leading to orotidinuria (Figure 17-8). 

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). 

The biosynthesis of pyrimidine nucleotides may be conveniently considered in two stages the formation of uridine monophosphate (UMP) and the conversion of UMP to other pyrimidine nucleotides. 

It has been indicated above that for purine requirements the protozoans depend on salvage of preformed purines. Contrary to this, the protozoal parasites have capability for the de novo biosynthesis of pyrimidines, which are equally essential for nucleic acid and protein synthesis. The pyrimidine biosynthesis in protozoans is very similar to the pathway mapped for eukaryotes [1,20,21]. 

In mammals, carbamoyl phosphate synthetase II is the key regulatory enzyme in the biosynthesis of pyrimidine nucleotides. The enzyme is inhibited by UTP, the product of the pathway, and stimulated by purine nucleotides. In many bacteria, aspartate carbamoyl transferase is the key regulatory enzyme. It is inhibited by CTP and stimulated by ATP. 

Figure 4-16 outlines the biosynthesis of pyrimidines and their conversion to the required deoxyribose triphosphates of uridine and cytidine, the necessary building blocks of RNA. The first step involving the condensation of carbamoyl phosphate with aspartic acid is catalyzed by aspartate transcarbamylase. This enzyme is strongly inhibited by the transition-state inhibitor PALA (Chapter 2). Other steps where drug intervention in the scheme can interfere to inhibit DNA synthesis are indicated. 

C] aspartie and orotic acids for the biosynthesis of pyrimidine nucleotides. 

Besides the siderophores, the menaquinone pathway also involves isochorismate synthase (review [100]). Menaquinone (vitamin K2) is an electron carrier involved in anaerobic ATP-generating redox reactions. It also plays a role in the anaerobic biosynthesis of pyrimidines, porphyrins and succinyl CoA [101-104]. Menaquinone is detectable in E. call. The accumulation of menaquinone (MK8) is significantly stimulated in the absence of oxygen. 

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