Carbamoyl phosphate synthetase I CPS

Carbamoyl Phosphate Synthetase I (CPS I) provides the substrate, carbamoyl phosphate, for the urea cycle. 

In eukaryotic cells, two separate pools of carbamoyl phosphate are synthesized by different enzymes located at different sites. Carbamoyl phosphate synthetase I (CPS I) is located in the inner membrane of mitochondria in the liver and, to lesser extent, in the kidneys and small intestine. It supplies carbamoyl phosphate for the urea cycle. CPS 1 is specific for ammonia as nitrogen donor and requires N-acetylglutamate as activator. Carbamoyl phosphate synthetase II (CPS II) is present in the cytosol. It supplies carbamoyl phosphate for pyrimidine nucleotide biosynthesis and uses the amido group of glutamine as nitrogen donor. The presence of physically separated CPSs in eukaryotes probably reflects the need for independent regulation of pyrimidine biosynthesis and urea formation, despite the fact that both pathways require carbamoyl phosphate. In prokaryotes, one CPS serves both pathways. 

Carbamoyl Phosphate Synthetase 11 (CPS-11) differs in several ways from its isoform (CPS-I), the enzyme which provides carbamoyl phosphate for the Urea cycle (see "Protein Turnover / Ammonia Metabolism"). 

The carbamoyl phosphate synthetase (abbreviated to CPS-I) that is involved in the ornithine cycle differs from the enzyme that is involved in pyrimidine synthesis (carbamoyl phosphate synthetase-ll). The latter enzyme is cytosolic, requires glutamine for provision of nitrogen, rather than ammonia, and is regulated by different factors (Chapter 20). 

CPS-I carbamoyl phosphate synthetase IRS insulin-receptor substrate... 

Summary of the differences between carbamoyl phosphate synthetase (CPS) I and II. 

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). 

---