In molecular biology, protein catabolism is the breakdown of proteins into smaller peptides and ultimately into amino acids.  Protein catabolism is a key function of digestion process.  Protein catabolism often begins with pepsin, which converts proteins into  polypeptides. These polypeptides are then further degraded.  In humans, the pancreatic proteases include trypsin, chymotrypsin, and other enzymes. In the intestine, the small peptides are broken down into amino acids that can be absorbed into the bloodstream. These absorbed amino acids can then undergo amino acid catabolism, where they are utilized as an energy source or as precursors to new proteins.
The amino acids produced by catabolism may be directly recycled to form new proteins, converted into different amino acids, or can undergo amino acid catabolism to be converted to other compounds via the Krebs cycle.