Carbamyl phosphate, biosynthesis

Precursors for the biosynthesis of biodn include pimelic acid, cysieinc, and carbamyl phosphate. Desthiobiotin acts as an intermediate. In plants, the production sites are seedlings and leaves In most animals, production is in the intestine. Storage site is the liver... 

CPSase catalyzes the formation of carbamyl phosphate from glutamine, bicarbonate, and two equivalents of ATP. The biosynthesis involves four partial reactions. GLNase catalyzes the formation of ammonia from glutamine. The remaining three partial reactions are catalyzed by SYNase. Bicarbonate is activated by ATP to form carboxyphosphate, which reacts with ammonia to form carbamate. The ATP-dependent phosphorylation of carbamate results in the production of carbamyl phosphate. 

Biosynthesis and Utilization of Acetyl Phosphate, Formyl Phosphate, and Carbamyl Phosphate and their Relations to the Urea Cycle... 

The condensation of carbamyl phosphate and L-aspartate, catalyzed by aspartate trans-carbamoylase (ATCase), produces iV-carba-myl-L-aspartate (Equation 17.38). This is one of the early steps in de novo pyrimidine biosynthesis, also a requirement for cell division. 

FIGURE 9.7 Pyrimidine biosynthetic pathway. The pathway of pyrimidine biosynthesis involves six steps and results in the production of uridine 58-monophosphate. Folate is not used in this pathway. The pathway commences with the transfer of the amide nitrogen of glutamine to bicarbonate to produce carbamyl phosphate. This molecule then reacts with aspartate to form the beginnings of the six-membered pyrimidine ring. 

Tl. Tatibana, M., and Ito, K., Carbamyl phosphate synthetase of the hematopoietic mouse spleen and the control of pyrimidine biosynthesis. Biochem. Biophys. Res. Commun. 26, 221-227 (1967). 

Aspartate transcarbamoylase (ATCase, EC 2.1.3.2) is an allosteric enzyme which controls the first step of p5oimidine de novo biosynthesis. It catalyzes the condensation of L-aspartic acid with carbamyl phosphate to produce carbamoyl-L-aspartate... 

Pyrimidine biosynthesis commences with a reaction between carbamyl phosphate and aspartic acid to give carbamyl aspartic acid which then nndergoes ring closure and oxidation to orotic acid. A reaction then occurs between orotic acid and 5-phosphoribosyl pyrophosphate to give orotidine-5-phosphate which on decarboxylation yields uridine-5-phosphate (UMP). By means of two successive reactions with ATP, UMP can then be converted into UTP and this by reaction with ammonia can give rise to cytidine triphosphate, CTP (11.126). 

Another form of spatial organization of metabolism that is often seen in eukaryotes but is less common in bacteria involves enzyme aggregates or multifunctional enzymes. An example is seen in S. cerevisiae where the first two reactions in pyrimidine nucleotide biosynthesis, the synthesis of carbamyl phosphate and the carbamylation of aspartate, are catalyzed by a single bifunctional protein (31). Both reactions are subject to feedback inhibition by UTP, in contrast to the situation inB. subtilis where aspartate transcarbamylase activity is not controlled. It is possible that an evolutionary advantage of the fusion of the genes... 

The intramitochondrial location of the arginine-specific carbamyl phosphate synthetase in N. crassa has the additional advantage of assuring separate pools of carbamyl phosphate for arginine and pyrimidine biosynthesis (41). Since this precludes the utilization of carbamyl phosphate produced in the course of arginine biosynthesis by aspartate transcarbamylase and of the pyrimidine-specific carbamyl phosphate by ornithine transcarbamylase, the control of these reac-... 

The Ornithine Urea Cycle Biosynthesis and Regulation of Carbamyl Phosphate Synthetase I and Ornithine Transcar-... 

The biosynthesis of carbamyl aspartate from ammonia, carbon dioxide, and aspartate is a two-step process, involving the intermediate formation of carbamyl phosphate. 

As early as 1949, it was demonstrated that injected or " C-labeled orotic acid was readily incorporated into DNA and RNA of mammalian tissue, indicating that orotic acid is a precursor of nucleic acid pyrimidine. The next step in pyrimidine biosynthesis is the formation of the first nucleotide in the sequence. It involves the reaction between ribosyl pyrophosphate and orotic acid to yield 5 -orotidylic acid the reaction is catalyzed by orotidylic pyrophosphorylase. Thus, the first steps of pyrimidine biosynthesis differ from the early steps of purine biosynthesis in at least two ways. Orotic acid, instead of being synthesized atom by atom as is the case for the purine ring, is made from the condensation of rather large molecules, namely, carbamyl phosphate and aspartic acid. Furthermore, all the steps of purine biosynthesis occur at the level of the nucleotide, but the the pyrimidine ring is closed at the level of the base. 

The first step in pyrimidine biosynthesis is the reaction catalyzed by aspartic transcarbamylase, a reaction in which the carbamyl group of carbamyl phosphate is transferred to aspartic acid to yield ureidosuccinic acid. In E. coli, the end products of the pyrimidine pathways, UTP and especially CTP, inhibit the transcarbamylase. Pardee and his associate, who discovered this important event, established that the site of action in the transcarbamylase molecule of the inhibitor is different from the site of action of the substrate. 

The cycle starts with carbamyl phosphate formation (this reaction was discussed in the section on pyrimidine biosynthesis). Carbamyl phosphate synthetase catalyzes the condensation of active CO2 with NH4 to yield carbamyl phosphate, a precursor of pyrimidines and urea. 

Humans cannot synthesize biotin and depend on dietary biotin originating in microbial and plant biosynthetic pathways. The route of biosynthesis of biotin was largely elaborated by Rolfe and Eisenberg (1968) working with Escherichia coll. In this pathway, dethiobiotin is formed from pimelyl-CoA (which can be synthesized from oleic acid) and carbamyl phosphate. Sulfur is incorporated into dethiobiotin in a synthase-dependent step, generating biotin (reviewed by Cam-poreale and Zempleni 2006). Figure 10.1 shows the chemical structure of biotin. 

The carbamyl phosphate is condensed with a molecule of aspartate giving ureidosuccinic acid, from which orotic acid is formed by cyclization and oxidation. In the presence of PRPP and a pyrophosphorylase this acid forms a ribotide and decarboxylation yields uridine monophosphate. The decarboxylation of the product of amination of orotidine phosphate gives cytidine monophosphate (Fig. 75). It can be seen that the pentose intermediate in pyrimidine nucleotide biosynthesis is PRPP, the same as for purine nucleotide biosynthesis. 

While the studies of Boyland and Roller and Elion and co-workers, which were conducted in vivo, do suggest that urethane has a specificity for pyrimidine biosynthesis, Kaye could not demonstrate in vitro any significant inhibition by urethane of several enzymes involved in nucleic acid metabolism. Both urethane and its A -hydroxy metabolite bear a structural resemblance to carbamyl phosphate and carbamyl-L-aspartate. The enzyme aspartate transcarbamylase begins pyrimidine biosynthesis by catalyzing the formation of carbamyl-L-aspartate from carbamyl phosphate and l-aspartate. Giri and Bhide have reported that in vivo administration of urethane decreased aspartate transcarbamylase activity of lung tissue of adult male and (to a lesser extent) female mice no in vitro inhibition could be demonstrated. 

The nature of the end product formed as the result of the fixation of COx and ammonia with liver preparations was established with the demonstration that synthetic carbamyl phosphate behaved as an intermediate in the biosynthesis of citrulline (389, 390). The compound isolated from liver systems, referred to as compound X (391), was shown to be identical with carbamyl phosphate (398S94). With this advance, the opportunity was provided for a more detmled study of the enzymes concerned with the sjmthesis of this compound. Two enzyme systems are now known which are capable of synthesizing carbamyl phosphate from COj, ammonia, and adenosine triphosphate (ATP). 

The enzymes discussed in the previous sections (carbamyl phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthetase, cleavage enzyme, and arginase) constitute the known enzymic steps in the sequence of reactions leading to the biosynthesis of urea in ureotelic animals in accordance with the cycle originally proposed by Krebs and Hen-seleit (458). A summary scheme showing the steps in this cycle and the relationship of some of the intermediates to other systems is shown in Fig. 2. 

A study of the mechanism of CO2 fixation in the biosynthesis of carbamyl phosphate has been reported and a hypothetical scheme proposed by Jones and Spector (511). 

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