Amino Acid Oxidation - II

Flashcards on amino acid oxidation and the urea cycle.

What are ammonotelic organisms?

Organisms that excrete nitrogen as NH3 and are mostly aquatic species.

What are ureotelic organisms?

Organisms that excrete nitrogen as urea and are mostly terrestrial animals.

What are uricotelic organisms?

Organisms that excrete nitrogen as uric acid, such as birds and reptiles.

Why is ammonia toxic to the brain?

It impairs the citric acid cycle, depletes neurotransmitters like glutamate and GABA, and causes excessive glutamine production leading to astrocyte swelling.

What is the fate of ammonia in the body?

Some is recycled for biosynthesis, and the excess is either excreted directly or converted to urea (or uric acid).

Who discovered the Urea Cycle?

Hans Krebs in 1932.

Where does the urea cycle occur?

In the liver cells, with reactions and enzymes distributed between the mitochondrial matrix and the cytosol.

How do amino groups enter the urea cycle?

As carbamoyl phosphate (formed in the matrix) and as aspartate (formed in the matrix by transamination of oxaloacetate and glutamate).

What are the five steps of the urea cycle?

1. Formation of carbamoyl phosphate. 2. Formation of citrulline. 3. Formation of argininosuccinate. 4. Formation of arginine. 5. Formation of urea.

What enzyme catalyzes the formation of carbamoyl phosphate in the urea cycle?

Carbamoyl phosphate synthetase -I.

What enzyme catalyzes the formation of citrulline in the urea cycle?

Ornithine transcarbamylase.

What enzyme catalyzes the formation of argininosuccinate in the urea cycle?

Argininosuccinate synthetase.

What enzyme catalyzes the formation of arginine from argininosuccinate in the urea cycle?

Argininosuccinate lyase.

What enzyme catalyzes the formation of urea in the urea cycle?

Arginase.

What activates Carbamoyl phosphate synthetase I allosterically?

N-acetylglutamate.

What determines N-acetylglutamate levels?

Concentrations of glutamate, acetyl-CoA, and arginine.

What is the overall stoichiometry of the urea cycle?

Aspartate + NH3 + CO2 + 3ATP → Urea + Fumarate + 2ADP + AMP + 2Pi + PPi + 3H2O

What is the aspartate-argininosuccinate shunt?

A connection between the urea cycle and citric acid cycle, linking amino groups and carbon skeletons of amino acids.

What is hyperammonemia?

Increased serum ammonia levels, often resulting from liver dysfunction, causing coma and neurological damage.

What are urea cycle disorders (UCD)?

Deficiencies in enzymes or transporters involved in the urea cycle, leading to hyperammonemia.

Name some enzymes that when deficient cause urea cycle disorders?

Carbamyl phosphate synthetase 1, Ornithine transcarbamylase, Argininosuccinate synthetase, Argininosuccinate lyase, Arginase.

How are UCDs (urea cycle disorders) treated?

Dietary protein restriction, hypercaloric non-protein solutions, nitrogen scavenger drugs, L-Citrulline supplements, L-Arginine supplements.