Pyrimidine metabolism Carbamyl phosphate

Figure 9-1 Sites of feedback inhibition in carbamyl phosphate metabolism of E. coli. Note that aspartate trascarbamylase is the first enzyme on the unique pathway to pyrimidine compounds.

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... 

Figure 13-1 shows that reduction of the 5 6 double bond is the first step in the breakdown of the pyrimidine ring. This is followed by oxidative cleavage of the 3 4 bond to form ureidopropionic or ureidoisobutyric acid, from which /3-amino acids are formed with the release of NHj and CO. Whether carbamate or carbamyl phosphate is the primary product is not known. The /3-amino acids are then oxidatively deaminated to /3 dehydes, which are converted to acids which are variously metabolized. 

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. 

In preceding sections of this chapter, the important metabolic reactions which yield ammonia have been discussed. Certain of these systems are capable of fixing ammonia (glutamic dehydrogenase, alanine dehydrogenase, L-amino acid oxidase, etc.). The fixation of ammonia in the glutamine synthetase system will be discussed in Chapter 17. The present section will deal with (a) enzymes which fix ammonia to form carbamyl phosphate and (b) enzymes which utilize carbamyl phosphate for the synthesis of arginine (and urea) and pyrimidines. 

It should be noted that the synthesis of carbamyl phosphate provides an important precursor for two metabolic pathways, one leading to the synthesis of the amino acid arginine and the other leading to the synthesis of pyrimidines via orotic acid. 

Carbamoyl phosphate HjN-COO PO3H2, an energy-rich phosphoiylated carbamate and an important metabolic intermediate. Carbamic acid, NHjCOOH, is unstable in fi ee form. Carbamate removed hydrolytically fi om carbamyl compounds, e.g. ureidopropionic acid (see Pyrimidine degradation), decomposes immediately into COj and NHj. Cp. is a specific precursor of arginine and urea (see Urea cycle), and of pyrimidines via orotic acid (see Pyrimidine biosynthesis). 

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