Question

Which one is a component of the ornithine cycle?
(a)Arginine and ornithine
(b)Glycine and methionine
(c)Aspartic and glutamic acids
(d)Valine and cysteine

Answer 1

The liver contains a system of carrier molecules and enzymes, which quickly converts the ammonia (and carbon dioxide) into urea. This is called the urea cycle or ornithine cycle. Urea is the chief nitrogenous waste of mammals and comes from the breakdown of proteins (amino acids).

Complete answer:
Ornithine is the end product of the urea cycle. It again combines with carbamoyl phosphate to form citrulline. Arginine is formed at the fourth step of the urea cycle with the help of enzyme argininosuccinase, which acts reversibly to cleave arginino­succinate into arginine and fumarate.
One complete urea cycle:
-Consumes 2 molecules of ammonia.
-Consumes 1 molecule of carbon dioxide.
-Produces 1 molecule of urea.
-Regenerates one molecule of ornithine for another cycle.
The carbamoyl phosphate used in this cycle is synthesized in the mitochondria from bicarbonate and ammonia. This synthesis, in turn, is dependent on the presence of N-acetylglutamate, which activates the carbamoyl synthetase I enzyme. The formation of N-acetylglutamate is stimulated by high levels of arginine. The synthesis of urea is an energy-dependent process. Carbamoyl phosphate synthetase 1 requires 2 ATP molecules, which are hydrolyzed to ADP, while the argininosuccinate synthetase only requires a single ATP molecule.
Increased levels of free amino acids, indicated by elevated arginine levels, stimulate the synthesis of urea. Two enzymes are involved in the hydrolysis of arginine to yield ornithine and urea. Cytosolic ARG1 is the urea cycle enzyme and mitochondrial ARG2 catalyzes urea production from arginine.
So, The correct answer is Option ‘A’.

Note:
-The urea cycle is extremely important because the ammonia is neurotoxic and must be removed from the body. Hepatocytes are the liver cells where the urea cycle occurs.
-The first and second step of the urea cycle occurs within the mitochondrion and the subsequent steps in the cytosol.