Urea Cycle

Carbamide, or urea, is the diamide form of carbonic acid. Urea is widely used as a fertiliser, a feed additive, and a raw material in the production of pharmaceuticals and polymers. It is a colourless crystalline material that melts at 132.7°C (271° F) and decomposes even before reaching the boiling point of urea. Additionally, urea is the primary end product of the metabolic breakdown of proteins in all mammals and a few fishes. It is found not just in mammals’ urine, but also in their milk, bile, blood, and perspiration.

During the protein breakdown process, all of the amino groups of the amino acids that contain proteins are naturally eliminated. These groups are subsequently transformed to Ammonia, which is highly harmful to humans. To protect the body from it, the human liver converts it to urea. The urea is then expelled through the kidneys in the form of urine.

Structure of Urea

Urea is a metabolic waste product that serves no physiological purpose in the body. It disintegrates in the kidney and bloodstream before being eliminated in the urine. Urea is an organic molecule composed of two distinct groups linked by the carbonyl functional group. Because urea is non-toxic, it readily dissolves in water. Apart from its colorlessness, it is also odourless.

The carbonyl group in urea is covalently linked to two amide groups. This is how carbamide received its name, which is derived from carboxylic+amide. A carbonyl group is formed when two carbon atoms are covalently linked to an oxygen atom. A nitrogen atom is bonded to two distinct hydrogen atoms to form an amide group. When two of these amide groups are covalently linked to a carbonyl group, urea is produced. To formulate the formula, the atoms in urea are as follows:

• 1 carbon atom 
• 1 oxygen atom 
• 2 nitrogen atoms 
• 4 hydrogen atoms

Function

Catabolism of amino acids produces waste ammonia. All animals require a means of excreting this substance. The majority of aquatic creatures, or ammonotelic organisms, excrete ammonia in its natural state.  Organisms that are unable to eliminate nitrogen as ammonia safely convert it to a less hazardous chemical, such as urea, via the urea cycle, which is mostly found in the liver. The liver produces urea, which is subsequently released into the bloodstream and carried to the kidneys, where it is eventually expelled as urine. The urea cycle is critical for these organisms, as excess nitrogen or ammonia can be extremely damaging.  In most insects and birds, ammonia is metabolised to uric acid or its urate salt, which is expelled in solid form.

The initial reaction is to enter the urea cycle.

Ammonia is transformed to carbamoyl phosphate prior to the start of the urea cycle. Carbamoyl phosphate synthetase I catalyses the process, which involves the utilisation of two ATP molecules. Following that, the carbamoyl phosphate enters the urea cycle.

The urea cycle’s stages

1. Citrulline is formed from carbamoyl phosphate. The carbamoyl phosphate group is contributed to ornithine via catalysis by ornithine transcarbamoylase, resulting in the formation of a phosphate group.
2. The amino group of aspartate and the carbonyl group of citrulline combine to generate argininosuccinate. This reaction is catalysed by argininosuccinate synthetase and is ATP-dependent.
3. Argininosuccinate is cleaved into arginine and fumarate by argininosuccinase.
4. Arginase catalyses the cleavage of arginine to produce urea and ornithine. Ornithine is then returned to the mitochondria to restart the urea cycle.

The urea circle’s byproducts

As mentioned previously, many vertebrates use the urea cycle to convert ammonium to urea, which is non-toxic to the body. While this is beneficial, the urea cycle has additional impacts. For instance, the consumption of two ATP, the creation of urea, the generation of H+, the condensation of HCO3- and NH4+into regenerable forms, and lastly the consumption of NH4+.

Conclusion  

We conclude that the urea cycle’s primary goal is to eliminate harmful ammonia from the body. Every day, a healthy adult excretes 10 to 20 g of ammonia from his or her body. A faulty urea cycle results in an overabundance of ammonia in the body, which can cause hyperammonemia and other disorders.