Protein Digestion and Amino Acid Absorption

• Proteases (proteolytic enzymes) break down dietary proteins into peptides and then their constituent amino acids in the stomach and intestine.
• Pepsin initiates protein breakdown in the stomach.
• Upon entering the small intestine, inactive zymogens secreted from the pancreas are activated to continue protein digestion.
• Enzymes produced by the intestinal epithelial cells are also required to fully degrade proteins.
• The amino acids generated by proteolysis in the intestinal lumen are transported into the intestinal epithelial cells, from which they enter the circulation for use by the tissues.
• Transport systems for amino acids are similar to transport systems for monosaccharides; both facilitative and active transport systems exist.
• There are a large number of overlapping transport systems for amino acids in cells.
• Protein degradation (turnover) occurs continuously in all cells.
• Proteins can be degraded by lysosomal enzymes (cathepsins).
• Proteins are also targeted for destruction by being covalently linked to the small protein ubiquitin.
• The ubiquitin-tagged proteins interact with the proteosome, a large complex designed to degrade proteins to small peptides in an adenosine triphosphate–dependent process.
• Amino acids released from proteins during turnover can be used for the synthesis of new proteins, for energy generation, or for gluconeogenesis.