The body cannot store excess amino acids. Dietary proteins are digested to amino acids, which are used to make new proteins. Any amino acids that are not needed are broken down in the liver, so that the energy in them can be used and their nitrogen safely removed.Deamination is the removal of the amine group (−NH2) from an amino acid. In liver cells the amine group is removed as ammonia (NH3), leaving a keto acid. The keto acid can be respired, via the Krebs cycle, or converted into glucose, glycogen or fat for storage.Ammonia is very soluble and highly toxic: it raises the pH of cells and interferes with enzymes and nerve function. So the liver immediately combines it with carbon dioxide in the ornithine cycle to make urea, which is much less toxic:2NH3 + CO2 → CO(NH2)2 + H2O Urea passes from liver cells into the blood plasma, is carried to the kidneys, filtered out of the blood and excreted in urine.
The liver can also make amino acids that are in short supply by transamination: an amine group is transferred from one amino acid to a keto acid, forming a different amino acid. This lets the body make the non-essential amino acids, keeping a balanced supply for protein synthesis. Essential amino acids cannot be made this way and must come from the diet.By deaminating excess amino acids and converting ammonia to urea, the liver keeps the blood concentrations of amino acids and nitrogenous waste within safe limits.
The liver can also make amino acids that are in short supply by transamination: an amine group is transferred from one amino acid to a keto acid, forming a different amino acid. This lets the body make the non-essential amino acids, keeping a balanced supply for protein synthesis. Essential amino acids cannot be made this way and must come from the diet.By deaminating excess amino acids and converting ammonia to urea, the liver keeps the blood concentrations of amino acids and nitrogenous waste within safe limits.