17.5:
Protein Digestion
Protein digestion primarily begins in the stomach, the strong acidic environment that can disrupt the protein structure, exposing peptide bonds within a polypeptide chain.
For instance, the stomach enzyme pepsin breaks the bonds to form smaller peptide fragments, shorter chains of amino acids. As the peptides move to the duodenum, the entry of chime activates several other enzymes like trypsin, that continue to break apart the peptide bonds, creating a variety of even shorter peptides.
The epithelial surface of the small intestine secretes several peptidases which are enzymes that break short peptide chains into individual amino acids. These amino acids are then absorbed into the bloodstream where they can be carried throughout the body to be used for protein synthesis or transported to the liver where they are used to produce energy.
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
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.
Suggested Reading
Goodman, Barbara E. “Insights into Digestion and Absorption of Major Nutrients in Humans.” Advances in Physiology Education 34, no. 2 (June 1, 2010): 44–53. [Source]