Argininosuccinate synthase

Human Argininosuccinate Synthetase tetramer

Argininosuccinate synthase is an enzyme that plays a crucial role in the urea cycle, which is essential for the detoxification of ammonia in the liver. This enzyme catalyzes the condensation of citrulline and aspartate to form argininosuccinate, a key step in the conversion of ammonia to urea.

Structure

Argininosuccinate synthase is a homotetramer, meaning it is composed of four identical subunits. Each subunit contributes to the formation of the active site where the catalytic reaction occurs. The enzyme is encoded by the ASS1 gene in humans.

Active site of Thermus thermophilus argininosuccinate synthetase

Function

The primary function of argininosuccinate synthase is to catalyze the reaction between citrulline and aspartate, producing argininosuccinate. This reaction is ATP-dependent and is a critical step in the urea cycle, which converts toxic ammonia into urea for excretion from the body.

Mechanism

The enzyme binds ATP and citrulline to form a citrullyl-AMP intermediate. Aspartate then attacks this intermediate, resulting in the formation of argininosuccinate and the release of AMP. This reaction is crucial for the continuation of the urea cycle.

Clinical Significance

Deficiencies in argininosuccinate synthase can lead to a disorder known as citrullinemia, which is characterized by elevated levels of citrulline in the blood. This condition can result in severe neurological impairment if not treated promptly. Management typically involves dietary restrictions and supplementation with arginine.

Related Enzymes

Argininosuccinate synthase works in conjunction with other enzymes in the urea cycle, including ornithine transcarbamylase, argininosuccinate lyase, and arginase. These enzymes collectively facilitate the conversion of ammonia to urea.

Related Pages

• Urea cycle
• Citrullinemia
• Ornithine transcarbamylase
• Argininosuccinate lyase
• Arginase

Citrulline metabolism

External Links

• ASS1 gene - NCBI
• OMIM Entry on Argininosuccinate Synthase