41.5: Nitrogenous Wastes - Nitrogenous Waste in Terrestrial Animals- The Urea Cycle
- Discuss the urea cycle
Mammals, including humans, are the primary producers of urea. Because they secrete urea as the primary nitrogenous waste product, they are called ureotelic animals. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals. It is the main nitrogen-containing substance in the urine of mammals. Urea is a colorless, odorless solid, highly soluble in water, and practically non-toxic. Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, the most notable one being nitrogen excretion. Urea is widely used in fertilizers as a convenient source of nitrogen. It is also an important raw material for the chemical industry.
Apart from mammals, urea is also found in the urine of amphibians, as well as some fish. Interestingly, tadpoles excrete ammonia, but shift to urea production during metamorphosis. In humans, apart from being a carrier of waste nitrogen, urea also plays a role in the countercurrent exchange system of the nephrons, which allows for re-absorption of water and critical ions from the excreted urine. This mechanism, controlled by an anti-diuretic hormone, allows the body to create hyperosmotic urine, which has a higher concentration of dissolved substances than the blood plasma. This mechanism is important to prevent the loss of water, to maintain blood pressure, and to maintain a suitable concentration of sodium ions in the blood plasmas.
The urea cycle is the primary mechanism by which mammals convert ammonia to urea. Urea is made in the liver and excreted in urine. The overall chemical reaction by which ammonia is converted to urea is
2 NH 3 (ammonia) + CO 2 + 3 ATP + H 2 O → H 2 N-CO-NH 2 (urea) + 2 ADP + 4 P i + AMP.
The urea cycle utilizes five intermediate steps, catalyzed by five different enzymes, to convert ammonia to urea. The amino acid L-ornithine is converted into different intermediates before being regenerated at the end of the urea cycle. Hence, the urea cycle is also referred to as the ornithine cycle. The enzyme ornithine transcarbamylase catalyzes a key step in the urea cycle. Its deficiency can lead to accumulation of toxic levels of ammonia in the body. The first two reactions occur in the mitochondria, while the last three reactions occur in the cytosol.
Key Points
- Ureotelic animals, which includes mammals, produce urea as the main nitrogenous waste material.
- 2 NH 3 + CO 2 + 3 ATP + H 2 O → H 2 N-CO-NH 2 + 2 ADP + 4 Pi + AMP is the chemical reaction by which toxic ammonia is converted to urea.
- The urea cycle involves the multi-step conversion (carried out by five different enzymes ) of the amino acid L- ornithine into different intermediates before being regenerated.
Key Terms
- ureotelic : animals that secrete urea as the primary nitrogenous waste material
- ornithine : an amino acid, which acts as an intermediate in the biosynthesis of urea
- urea : a water-soluble organic compound, CO(NH2)2, formed by the metabolism of proteins and excreted in the urine