L21: Digestion and Absorption of Proteins

Question 1

LO1: Define proenzymes, endopeptidases, and exopeptidases

Answer 1

PROENZYMES: inactive precursor enzymes that are activated after secretion into lumen of GI tract (activation requires limited proteolysis)

ENDOPEPTIDASES: cleave proteins by hydrolyzing peptide bonds within the polypeptide chain

EXOPEPTIDASES: cleave AAs from either the N or C terminal ends of peptides and proteins

Question 2

LO2: Name a protease (proenzymes) that is produced by the stomach, pancreas, and small intestine

Answer 2

stomach: pepsinogen (inactive precursor of pepsin)
pancreas: trypsin (catalyzes proteolysis of pancreatic zymogens, including chymotrypsin)

small intestine: peptidases (cleave amino acids from carboxyl end)

Question 3

LO3: Name two non-enzymatic components of gastric juice and describe how each contributes to protein digestion

Answer 3

NON-ENZYMATIC COMPONENTS OF GASTRIC JUICE
-gastrin, a peptide hormone secreted by the mucosal cells of the stomach that stimulates HCL secretion by parietal cells and pepsinogen secretion of chief cells

-HCL, secreted by parietal cells with a few electrolytes; decreases the pH of the contents entering the stomach, denaturing dietary protein, providing the proper pH environment for pepsin, and initiating limited proteolysis of pepsinogen to produce pepsin

Question 4

LO4: Describe the cascade mechanism by which the pancreatic proteases are released and activated in the GI tract

Answer 4

1. Chyme moves through duodenum, signalling secretin and CCK to be released. Secretin stimulates release of pancreatic juice, while CCK binds to exocrine cells of pancreas and stimulates release of pancreatic zymogens (inactive precursor forms) into lumen of small intestine via pancreatic duct.
2. Enteropeptidases, found in brush border of intestinal cells, catalyze the conversion of trypsinogen to trypsin
3. Trypsin catalyzes the limited proteolysis of all the remaining zymogens to produce active forms of chymotrypsin, elastase, CPA, and CPB

Question 5

LO5: List the products of protein digestion within the lumen of the small intestine

Answer 5

Final products of hydrolysis by brush border peptidases: mixture of free amino acids, dipeptides, and tripeptides

Question 6

LO6: Describe the mechanisms by which amino acids are transported from the lumen of the small intestine to the portal blood

Answer 6

• transport proteins in brush border membrane (co-transport sodium)
  - neutral AA transporter
  - aromatic/hydrophobic AA transporter
  - Imino acid transporter
  - acidic AA transporter
  - basic AA transporter

-transport proteins in the basolateral membrane (sodium independent)

Question 7

LO7: Explain why inherited defects in intestinal absorption of specific AAs correlate with increased renal excretion of those AAs

Answer 7

• many brush border transporters (sodium dependent) are also located in the renal tubules (kidneys)
• therefore if intestinal absorption is deficient due to an inherited transporter defect, this transporter will be defective in the kidneys as well and will not take up the corresponding AA
• corresponding AA will instead be excreted in the urine

Question 8

LO8: Name two disorders that can result from long-term use of PPIs

Answer 8

Long-term use of PPIs can result in B12 deficiency, as they reduce gastrin (needed for absorption of B12)

• anemia
• peripheral sensory and motor neural deficiencies