Urea Cycle Overview

Study Notes

Urea Cycle Overview

The urea cycle (also known as the ornithine cycle) is a metabolic pathway that occurs in the liver and kidneys. It is the primary mechanism by which the body disposes of excess ammonia, a toxic waste product.

Location and Importance

Takes place in the liver and kidneys
Essential for removing ammonia from the body, which is toxic to the brain and other organs

Reactions and Enzymes

The urea cycle consists of five reactions, catalyzed by five enzymes:

CPSI (Carbamoyl Phosphate Synthase I): converts ammonia and CO2 into carbamoyl phosphate
OTC (Ornithine Transcarbamylase): converts carbamoyl phosphate and ornithine into citrulline
ASS (Argininosuccinate Synthase): converts citrulline and aspartate into argininosuccinate
ASL (Argininosuccinate Lyase): converts argininosuccinate into arginine and fumarate
ARG (Arginase): converts arginine into ornithine and urea

Regulation

The urea cycle is regulated by N-acetylglutamate, which is an allosteric activator of CPSI
The cycle is also regulated by the availability of substrates, such as ammonia and aspartate

Defects and Disorders

Defects in the urea cycle can lead to hyperammonemia, a condition characterized by elevated ammonia levels in the blood
Symptoms of hyperammonemia include vomiting, lethargy, and seizures
Inherited disorders of the urea cycle include CPSI deficiency, OTC deficiency, and ARG deficiency

Urea Cycle Overview

The urea cycle is a metabolic pathway that occurs in the liver and kidneys to dispose of excess ammonia, a toxic waste product.

Location and Importance

The urea cycle takes place in the liver and kidneys.
It is essential for removing ammonia from the body, which is toxic to the brain and other organs.

Reactions and Enzymes

The urea cycle consists of five reactions catalyzed by five enzymes.
CPSI converts ammonia and CO2 into carbamoyl phosphate.
OTC converts carbamoyl phosphate and ornithine into citrulline.
ASS converts citrulline and aspartate into argininosuccinate.
ASL converts argininosuccinate into arginine and fumarate.
ARG converts arginine into ornithine and urea.

Regulation

N-acetylglutamate is an allosteric activator of CPSI.
The urea cycle is regulated by the availability of substrates, such as ammonia and aspartate.

Defects and Disorders

Defects in the urea cycle can lead to hyperammonemia, a condition characterized by elevated ammonia levels in the blood.
Symptoms of hyperammonemia include vomiting, lethargy, and seizures.
Inherited disorders of the urea cycle include CPSI deficiency, OTC deficiency, and ARG deficiency.