NH3
Summary
NH₃ is ammonia, the nitrogen waste released when amino acids are broken down. At physiological pH it exists mostly as ammonium, NH₄⁺. It is toxic, so the liver converts it to urea in the urea cycle for excretion.
Detail
Meaning: NH₃ is ammonia, the product of removing amino groups from amino acids. Transaminases move an amino group onto α-ketoglutarate to make glutamate. Glutamate dehydrogenase then performs oxidative deamination, releasing NH₄⁺ and regenerating α-ketoglutarate. Free ammonia is toxic, so it travels to the liver as glutamine or alanine. In liver mitochondria, carbamoyl phosphate synthetase I combines NH₄⁺, CO₂ and ATP to start the urea cycle. The two nitrogens in urea come from ammonia and from aspartate. In general and organic chemistry, NH₃ is also a weak base and nucleophile because of its lone pair.
Equation: pH = pKa + log([NH₃]/[NH₄⁺]). Concentrations are in mol/L; pH and pKa have no units. The pKa of NH₄⁺ is about 9.25.
On the MCAT: Passages trace where amino acid nitrogen goes. You may be asked which step releases free ammonia, which organ makes urea, or what accumulates if the urea cycle is impaired (ammonia and glutamine rise). You may also be asked to predict that at pH 7.4, well below the pKa, ammonia is almost entirely NH₄⁺.
Common confusion: Transamination only moves nitrogen between molecules and releases no free ammonia. Oxidative deamination is the step that releases NH₄⁺. Also, humans excrete nitrogen mainly as urea, not as ammonia.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.