Carbamoyl phosphate synthetase glutamine-dependent

Dihydroorotate dehydrogenase, the enzyme catalyzing the dehydrogenation of dihydroorotate to orotate (reaction 4 of the pathway Fig. 15-15), is located on the outer side of the inner mitochondrial membrane. This enzyme has FAD as a prosthetic group and in mammals electrons are passed to ubiquinone. The de novo pyrimidine pathway is thus compartmentalized dihydroorotate synthesized by trifunctional DHO synthetase in the cytosol must pass across the outer mitochondrial membrane to be oxidized to orotate, which in turn passes back to the cytosol to be a substrate for bifunctional UMP synthase. Mammalian cells contain two carbamoyl phosphate synthetases the glutamine-dependent enzyme (CPSase II) which is part of CAD, and an ammonia-dependent enzyme (CPSase /) which is found in the mitochondrial matrix, and which is used for urea and arginine biosynthesis. Under certain conditions (e.g., hyperammonemia), carbamoyl phosphate synthesized in the matrix by CPSase I may enter pyrimidine biosynthesis in the cytosol. 

These substrates are different than those with the carbamoyl phosphate used for urea synthesis. The enzyme for the glutamine-dependent carbamoyl phosphate synthetase (CPS II) is in the cytosol, whereas that for urea synthesis (CPS I) is in the mitochondrion. The glutamine-dependent carbamoyl phosphate synthetase is present in most cells, whereas the mitochondrial carbamoyl phosphate synthetase is present primarily in the liver, kidney, and intestines (Fig. 20.6). 

In the nematodes the five soluble enzymes of pyrimidine de novo biosynthesis were found in crude extracts of Nippostrongylus brasiliensis and Trichuris muris (81). The first enzyme of de novo synthesis, glutamine-dependent carbamoyl-phosphate synthetase, was deteeted in Ascaris ovary extracts (115), and aspartate transcarbamoylase activity, the second enzyme of de novo synthesis, was found in A. suum (113). 

Aoki, T. and Oya, H. (1979) Glutamine-dependent carbamoyl-phosphate synthetase and control of pyrimidine biosynthesis in the parasitic helminth Schistosoma mansoni. Comp. Biochem. Physiol. 63-B 511-515. 

Mammalian cells contain two carbamoyl phosphate synthetases the glutamine-dependent enzyme (CPSase II) which is part of CAD and an ammonia-dependent enzyme (CPSase /) which is localized in the mitochondrial matrix and which is used in urea and arginine biosynthesis. Under certain conditions (e.g., hyperammonemia), carbamoyl phosphate synthesized in the matrix by CPSase I may enter pyrimidine biosynthesis in the cytoplasm. 

Glutamine-dependent carbamoyl phosphate synthetase, aspartate carbamoyltransferase and dihydro-orotase were co-purified as a high molecular weight complex from an extract of unfertilized eggs of Rana catesbeiana [144]. UTP was required to maintain the integrity of the complex during... 

Similar complexes were observed in rat livers and in other tissues [145-149]. The extensively purified complex of glutamine-dependent carbamoyl phosphate synthetase, aspartate carbamoyltransferase and dihydro-orotase from rat liver had a sedimentation coefficient of 27 S (approximately 900000 daltons). Treatment of the complex with pancreatic elastase caused a selective inactivation of carbamoyltransferase with concomitant dissociation of the complex [159]. 

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