17.5: Protein Digestion
Protein digestion begins in the stomach, where the highly acidic environment can easily disrupt protein structure by exposing the peptide bonds of polypeptide chains. After polypeptide chains are broken into individual amino acids by a series of digestive enzymes, the amino acids are transported to the liver via the bloodstream to produce energy.
Pepsin is a protease, or protein-digesting enzyme, that is produced in the stomach and is one of the main digestive enzymes in the human digestive system. Working in conjunction with chymotrypsin and trypsin released in the small intestine, pepsin severs the links between specific types of amino acids to form shorter polypeptide chains. Other enzymes, called peptidases, then split off one amino acid at a time from the ends of these polypeptide chains. The small intestine can easily absorb the resulting amino acids.
The Liver and Protein Metabolism
The liver plays an essential role in the metabolism of proteins. Liver cells alter digested amino acids from the small intestine so that they can be used to produce energy or make carbohydrates and fats. A byproduct of this process is a toxic substance called ammonia, which the liver then converts into a much less toxic substance called urea. Urea is then released into the blood and transported to the kidneys, which excrete urea out of the body through urine.