 In this video I will describe the digestion and absorption of proteins, list the major digestive enzymes, which digestive or accessory organ produces that enzyme, the substrate that enzyme works on, and the products produced in that chemical reaction. So the enzymes we'll cover are pepsin, trypsin, chymotrypsin, carboxypeptidase, and the brush border enzymes. The chemical digestion of protein is initiated in the stomach. Pepsin is the enzyme that initiates the hydrolysis of peptide bonds within polypeptides, producing shorter peptides. Pepsin is first secreted as an inactive precursor. The chief cells at the base of gastric pits secrete pepsinogen, an inactive precursor that then becomes activated by the low pH of gastric juice. The parietal cells of the gastric pits secrete hydrochloric acid, which creates a low pH environment, activating pepsinogen to become pepsin. Then pepsin initiates chemical digestion of proteins by catalyzing hydrolysis of the peptide bonds within polypeptides, producing shorter peptides, but this is just the beginning of the chemical digestion for proteins. There's a large number of enzymes that catalyze the hydrolysis of peptide bonds between specific amino acids, and most of these are proteases secreted by the pancreas, and the pancreatic proteases are secreted first as inactive precursors that then become activated within the duodenum. Trypsin is secreted by acenar cells of the pancreas as the inactive precursor trypsinogen. Trypsinogen is activated by a brush border enzyme called enteropeptidase found in the duodenum, so enteropeptidase converts trypsinogen into the active enzyme trypsin, and so trypsin is a protease that can contribute to hydrolysis of polypeptides, but trypsin will also activate other precursors to proteases, other inactive precursors such as chymotrypsinogen, proelastase, and procarboxypeptidase, so trypsin will activate chymotrypsinogen to form the active enzyme chymotrypsin, trypsin will activate proelastase to form the active enzyme elastase, and trypsin, chymotrypsin, and elastase all catalyze hydrolysis of peptide bonds between specific amino acids, enabling polypeptides to be broken down into shorter peptides, carboxypeptidase, which is formed from the precursor procarboxypeptidase, and is activated by trypsin forming carboxypeptidase. Well, this enzyme carboxypeptidase catalyzes the hydrolysis of peptide bonds at the C terminus of a peptide, so the carboxyl end, the C terminus as a carboxyl group, and the enzyme carboxypeptidase removes one amino acid at a time from the C terminal end of a polypeptide. Protein digestion begins within the stomach, the low pH of gastric juice will denature proteins, then pepsin is the first protease that contributes to chemical digestion of polypeptides, catalyzing the hydrolysis of peptide bonds, producing fragments from the polypeptides that are still relatively large polypeptides. Then within the duodenum, the pancreatic enzymes trypsin, chymotrypsin, and elastase will catalyze hydrolysis of peptide bonds within polypeptides producing shorter peptides, then the pancreatic enzyme carboxypeptidase and the brush border enzyme amino peptidase catalyze hydrolysis from opposite ends of a peptide, the amino terminus or the end terminus is the opposite end from the carboxy terminus or C terminus of a peptide, and these enzymes will remove one amino acid at a time from the free end of a peptide. The carboxypeptidase is one of the pancreatic enzymes and amino peptidase is a brush border enzyme that's produced by the enterocytes of the small intestine. Another brush border enzyme produced by these enterocytes and embedded in the microvilli at the apical surface of an enterocyte is dipeptidase, and dipeptidase catalyzes hydrolysis of the peptide bond between the two amino acids in a dipeptide producing two amino acids, and so following the chemical digestion of protein amino acids will be absorbed. The absorption of amino acids involves a secondary active transport mechanism. There's a wide variety of amino acid transport proteins that some of them use a sodium co-transport mechanism similar to the transporters for monosaccharides like glucose that are transported in through secondary active transport using the concentration gradient of sodium to drive the transport of amino acids across the apical surface of the enterocyte. Some amino acids are absorbed as short peptides, dipeptides, and tripeptides are just two or three amino acids long, so here we can see a secondary active transport mechanism where a concentration gradient of hydrogen ions drives the transport of short peptides across the apical surface of the enterocyte. Then within the enterocyte, amino acids can exit the basolateral surface by facilitated diffusion, and then as the concentration of amino acids in the extracellular fluid rises, amino acids will diffuse through the capillaries into the blood, and then amino acids will be able to be distributed throughout the body in the blood. So to summarize protein digestion begins in the stomach, hydrochloric acid will denature proteins and activate the enzyme pepsinogen to form pepsin that pepsin is a protease that catalyzes hydrolysis of long polypeptides producing fragments just initiating the digestion of protein. Then as proteins, which are now fragments of those proteins enter into the duodenum, there are protein-digesting enzymes, pancreatic proteases that are secreted in pancreatic juice, and mixed with the chyne within the duodenum and continue catalyzing hydrolysis until we have short peptides, the tripeptides, dipeptides, as well as individual amino acids that can be absorbed. So the small intestine is the location where the majority of chemical digestion of protein occurs, although it's initiated by pepsin within the stomach, within the duodenum, the majority of chemical digestion occurs with the pancreatic proteases like trypsin, chymotrypsin, elastase, carboxypeptidase, all catalyzing hydrolysis of polypeptides producing shorter and shorter peptides and eventually amino acids, the brush border enzymes in the small intestine finish, chemical digestion as the dipeptidase breaks down, the shortest peptides producing amino acids, and then amino acids are absorbed across the enterocytes lining the mucous membrane of the small intestine and move into the bloodstream. So amino acids are transported throughout the body in the blood and amino acids can be taken up by cells throughout the body and used for the synthesis of proteins. The liver will function to regulate the distribution of amino acids throughout the body as the liver can have the first opportunity to remove amino acids from the blood as that blood is traveling from the digestive system through the hepatic portal vein into the liver. The liver will filter this blood and be able to remove amino acids and use those amino acids to produce proteins and also regulate the amount of amino acids that can then be distributed throughout the circulation to the rest of the body.