urea
Summary
Urea, (NH₂)₂C=O, is the main nitrogen waste from amino acid breakdown in humans. The liver makes it from toxic ammonia in the urea cycle, and the kidneys excrete it. Testable point: the liver makes urea, and it is not uric acid, which comes from purines.
Detail
Meaning: Urea, (NH₂)₂C=O, is the main nitrogen waste of amino acid breakdown in humans. Transamination and deamination release NH₄⁺, which is toxic. The liver converts it to urea in the urea cycle: carbamoyl phosphate forms in mitochondria from NH₄⁺ and CO₂, later steps occur in the cytosol, and arginase cleaves arginine into urea and ornithine. The second nitrogen comes from aspartate. Urea is neutral and soluble, so blood carries it to the kidneys for excretion. The cycle consumes ATP.
On the MCAT: Expect questions on where urea is made (liver) and excreted (kidney), and on what happens if the cycle fails (blood ammonia rises). Fumarate released in the cycle links it to the citric acid cycle. Passages may compare animals: many aquatic species excrete ammonia, mammals excrete urea, and birds and reptiles excrete uric acid. At high concentration, urea also denatures proteins by disrupting noncovalent interactions.
Common confusion: Urea is not uric acid. Uric acid is the end product of purine breakdown in humans, while urea carries nitrogen from amino acids. Also, the kidney excretes urea, but the liver makes it.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.