1.3 Digestion of Proteins

• What are dietary proteins made of?

  Long chains of amino acids
• How are the characteristics of proteins determined?

  By types and arrangements of amino acids
• What is the optimal pH range for pepsin activity?

  2.0 to 3.0
• Why must stomach juices be acidic for protein digestion?

  To activate pepsin for protein digestion
• What cells secrete hydrochloric acid in the stomach?

  Parietal (oxyntic) cells
• What is the average pH of stomach contents after mixing?

  Around 2.0 to 3.0
• What is a significant feature of pepsin digestion?

  It digests collagen, a tough protein
• Why is collagen digestion important for meat digestion?

  It allows other enzymes to penetrate meats
• What percentage of total protein digestion does pepsin provide?

  10 to 20 percent
• What process does pepsin use to split proteins?

  Hydrolysis at peptide linkages
• Where does most protein digestion occur?

  In the upper small intestine
• What are the major pancreatic proteolytic enzymes?

  Trypsin, chymotrypsin, carboxypolypeptidase, elastase
• What do trypsin and chymotrypsin do to protein molecules?

  They split them into small polypeptides
• What does carboxypolypeptidase do?

  Cleaves individual amino acids from polypeptides
• What is proelastase converted into?

  Elastase
• What does elastase digest?

  Elastin fibers in meats
• What percentage of proteins are digested to amino acids by pancreatic juices?

  A small percentage
• What are the final products of protein digestion in the intestinal lumen?

  Amino acids, dipeptides, and tripeptides
• What are enterocytes?

  Cells lining the villi of the small intestine
• What is the brush border in enterocytes?

  Microvilli projecting from the cell surface
• What types of peptidases are important in enterocytes?

  Aminopolypeptidase and dipeptidases
• What do peptidases do in the small intestine?

  Split polypeptides into tripeptides and dipeptides
• How are amino acids transported in enterocytes?

  Through the microvillar membrane
• What happens to dipeptides and tripeptides inside enterocytes?

  They are digested to single amino acids
• What percentage of absorbed protein digestive products are individual amino acids?

  More than 99 percent
• What can rare absorption of whole protein molecules cause?

  Allergic or immunologic disturbances
• What is the role of hydrochloric acid in the stomach?

  To create an acidic environment for pepsin
• What is the function of the gastric glands?

  To secrete digestive juices including hydrochloric acid
• How does the digestion of proteins differ in the stomach and small intestine?

  Stomach uses pepsin; small intestine uses pancreatic enzymes