what is the difference between carbamoyl phosphate synthase I and carbamoyl phosphate II

Answer 1

Carbamoyl phosphate synthase I (CPS I) and carbamoyl phosphate synthase II (CPS II) are two different enzymes involved in the biosynthesis of carbamoyl phosphate, an important intermediate in various metabolic pathways.

1. Structure and localization:
- CPS I is a cytosolic enzyme primarily located in the liver mitochondria but can also be found in other tissues, such as the small intestine.
- CPS II is a cytosolic enzyme found in most cells, including liver, pancreas, and brain.

2. Function:
- CPS I is the first and rate-limiting enzyme in the urea cycle, a process that occurs in the liver mitochondria and converts toxic ammonia to urea for excretion.
- CPS II participates in the de novo pyrimidine synthesis pathway, which occurs in the cytoplasm and synthesizes the precursor molecule carbamoyl phosphate for the production of nucleotides.

3. Substrates:
- CPS I utilizes ammonia, bicarbonate, and ATP as substrates to produce carbamoyl phosphate.
- CPS II uses glutamine, bicarbonate, and ATP as substrates to generate carbamoyl phosphate.

4. Regulation:
- CPS I is regulated by N-acetylglutamate (NAG), which activates the enzyme. NAG is formed by the enzyme N-acetylglutamate synthase (NAGS) and acts as an allosteric activator of CPS I.
- CPS II is not regulated by NAG, and its synthesis and activity are regulated by the overall demand for pyrimidines in the cell.

In summary, CPS I is primarily involved in the urea cycle and is located in liver mitochondria, while CPS II is involved in de novo pyrimidine synthesis and is present in the cytoplasm of most cells. They differ in their substrates, regulation, and cellular localization.