Protein catabolism

Proteins are hydrolyzed into their constituent amino acids by a variety of enzymes within the cell. Most of the time, the amino acids are recycled during the synthesis of new proteins. If there are excess amino acids, or if the body is in a state of starvation, some amino acids will be shunted into the pathways of glucose catabolism. Each amino acid must have its amino group removed prior to entry into these pathways. The amino group is converted into ammonia. In mammals, the liver synthesizes urea from two ammonia molecules and a carbon dioxide molecule. Thus, urea is the principal waste product in mammals, produced from the nitrogen atom in amino acids, and is excreted in the urine. The carbon backbones of some of the amino acids used in urea synthesis can become a source of energy by being converted to acetyl-CoA and entering the Krebs cycle.