Question

List and explain the role of the enzymes needed for the following routing of nitrogen into the urea cycle: Start with a molecule of aspartate in peripheral tissue (e.g., muscle) and end with the nitrogen derived from that original aspartate molecule released as free ammonia in the liver. (Hint: routing should be by the major non-toxic carrier of nitrogen in the blood).

Answer 1

The urea cycle can be described in 5 different pathways involved in it .First of all two nitrogen atoms enter the urea cycle as NH4+ and aspartate. The first two steps in the cycle take place in the mitochondrial matrix and the rest of the steps take place in the cytosol. Thus the urea cycle spans two cellular compartments of the liver cell.

• In the first step of the Krebs-Henseleit cycle, ammonia produced in the mitochondria is converted to carbamoyl phosphate by an enzyme called carbamoyl phosphate synthetase I. The reaction can be given as follows:

NH3 + CO2 + 2ATP → carbamoyl phosphate + 2ADP + Pi

• The second step involves the transfer of a carbamoyl group from carbamoyl phosphate to ornithine to form citrulline. This step is catalyzed by the enzyme ornithine transcarbamoylase (OTC) . The reaction is given as follows:

Carbamoyl phosphate + ornithine → citrulline + Pi

Citrulline thus formed is released into the cytosol for use in the rest of the steps of the cycle.

• The third step is catalyzed by an enzyme called argininosuccinate synthetase, which uses citrulline and ATP to form a citrullyl-AMP intermediate, which reacts with an amino group from aspartate to produce argininosuccinate. This reaction can be given as follows:

Citrulline + ATP + aspartate → argininosuccinate + AMP + PPi

• The fourth step involves the cleavage of argininosuccinate to form fumarate and arginine. Argininosuccinate lyase is the enzyme catalyzing this reaction, which can be represented as follows:

Argininosuccinate → arginine + fumarate

• In the fifth and last step of the urea cycle, arginine is hydrolyzed to form urea and ornithine. This is catalyzed by arginase and can be given as follows:

Arginine → urea + ornithine

The overall reaction can be given as follows:

2NH3 + CO2 + 3ATP g urea + 2ADP + AMP + PPi + 2Pi

Ammonia which is a toxic product of nitrogen metabolism , should be removed from our body. The urea cycle or ornithine cycle converts excess ammonia into urea in the mitochondria of liver cells. The urea formed, then enters the blood stream, is filtered by the kidneys and is ultimately excreted in the urine.

The overall reaction for urea formation from ammonia is as follows:

2 Ammonia + CO2 + 3ATP ---> urea + water + 3 ADP.