Carbamoyl phosphate synthetase

Carbamoyl phosphate synthetase (CPS) is an essential enzyme involved in the biosynthesis of both pyrimidines and arginine. It plays a crucial role in the urea cycle, which is responsible for the detoxification of ammonia in mammals. CPS catalyzes the formation of carbamoyl phosphate, a key intermediate in these metabolic pathways.

Structure

CPS is a large, multi-domain enzyme that exists in two isoforms: CPS I and CPS II. CPS I is primarily found in the liver, while CPS II is present in other tissues. Both isoforms share a similar overall structure, consisting of multiple subunits that come together to form a functional enzyme.

CPS I is composed of three subunits: a small regulatory subunit, a large catalytic subunit, and an additional protein called N-acetylglutamate synthase (NAGS). NAGS is responsible for the synthesis of N-acetylglutamate (NAG), an allosteric activator of CPS I. The catalytic subunit contains the active site where the carbamoyl phosphate synthesis takes place.

CPS II, on the other hand, is a multifunctional enzyme that consists of a single polypeptide chain with three distinct domains: the glutamine amidotransferase (GAT), the carbamoyl phosphate synthetase (CPS), and the aspartate transcarbamoylase (ATC) domains. These domains work together to catalyze the sequential reactions leading to the synthesis of carbamoyl phosphate.

Mechanism of Action

The synthesis of carbamoyl phosphate by CPS involves the utilization of two substrates: bicarbonate and glutamine. The reaction occurs in multiple steps, with each step being catalyzed by a specific domain of CPS II.

First, the GAT domain of CPS II hydrolyzes glutamine to produce glutamate and ammonia. The ammonia released in this step is then utilized in the subsequent reaction. The CPS domain of CPS II combines the ammonia with bicarbonate and ATP to form carbamate, which is subsequently phosphorylated to generate carbamoyl phosphate. Finally, the ATC domain of CPS II transfers the carbamoyl group from carbamoyl phosphate to aspartate, leading to the formation of N-carbamoyl-L-aspartate.

Regulation

The activity of CPS is tightly regulated to maintain the balance of pyrimidine and arginine synthesis, as well as the detoxification of ammonia. CPS I is allosterically activated by N-acetylglutamate (NAG), which is synthesized by NAGS in response to the levels of arginine. NAG binds to CPS I and enhances its catalytic activity.

CPS II, on the other hand, is regulated by feedback inhibition. The end product of the pyrimidine biosynthesis pathway, cytidine triphosphate (CTP), acts as an allosteric inhibitor of CPS II. When the cellular levels of CTP are high, CPS II activity is reduced, preventing the overproduction of pyrimidines.

Clinical Significance

Mutations in the genes encoding CPS I and CPS II can lead to various metabolic disorders. Deficiencies in CPS I result in hyperammonemia, a condition characterized by the accumulation of ammonia in the blood. This can lead to severe neurological symptoms and can be life-threatening if not managed properly.

CPS II deficiencies are rare but can cause orotic aciduria, a disorder characterized by the excessive excretion of orotic acid in the urine. This condition is associated with impaired pyrimidine synthesis and can lead to growth retardation and anemia.

See Also

• Urea cycle
• Pyrimidine biosynthesis
• Arginine biosynthesis