Proteins are broken down into amino acids. Many of these amino acids are essential, which means that the human body can't manufacture them and needs to acquire them from foods.
Protein digestion begins in the stomach, where the large protein molecules are broken down into smaller polypeptides. This is caused by the action of the stomach's hydrochloric acid and enzymes called proteases. Many proteases are made in the pancreas, including trypsin, pancreatin and chymotrypsin. The breakdown of proteins during digestion is called proteolysis. This allows the body to utilize the amino acids that it can't manufacture.
The essential amino acids that result from proteolysis are leucine, histidine, lysine, valine, phenylalanine, threonine, tryptophan, methionine and isoleucine. Amino acids such as glutamic acid and alanine can be produced by the body, though others such as glutamine, glycine and cysteine might not be able to be produced if the person is under stress. These amino acids are necessary for the proper functioning of the body. Glutamine, for example, is used in the brain as a neurotransmitter, while glycine supports the health of red blood cells.
Amino acids are made of amine, carboxylic acid and a side chain that differentiates one amino acid from another.