Digestion and Absorption of Proteins and Carbohydrates

Question 1

What is digestion?

Answer 1

• The breakdown of complex dietary molecules to small molecules via digestive enzymes.

Question 2

What are the principle sites of protein digestion?

Answer 2

• The stomach.
• The small intestines.

Question 3

What are proteolytic enzymes?

Answer 3

• Proteases released to breakdown proteins.
• Secreted into the stomach
• Released from the pancreas into the small
  intestine.

Question 4

Discuss the process of protein digestion in the stomach.

Answer 4

• pH- 2
• Proteases released by chief cells in the
  form of pepsinogen.
• Pepsinogen converted to Pepsin by HCI.
• Pepsin: an endopeptidase that cleaves at amino side of hydrophobic aa.

Question 5

Discuss the process of protein digestion in the small intestines.

Answer 5

*Enzymes released by pancreas as zymogens

Question 6

List the pancreatic enzyme type and their catalytic mechanism.

Answer 6

1. Trypsin chymotrypsin, elastase:
   - Type: Endopeptidase
   - Mechanism: Serine protease
2. Carboxypeptidase A, Carboxypeptidase B:
   - Type: Carboxypeptidase
   - Mechanism: Metalloprotease (Zn2+)

Question 7

Discuss the cleavage specificity of different enzymes.

Answer 7

Cleavage specificity of dietary proteins by proteases from the pancreas.
1. Trypsin
- Peptide bonds adjacent to basic amino acids
2. Chymotrypsin
- Peptide bonds adjacent to hydrophobic amino acids
3. Elastase
- Peptide bonds adjacent to small amino acids
4. Carboxypeptidase A
- Hydrophobic amino acids at C-terminus
5. Carboxypeptidase B
- Basic amino acids at C-terminus

Question 8

What are products of protein digestion.

Answer 8

• Cleaved by peptidases:
  1. Tetra-peptides
  2. Tri-peptides
• Amino acids.

Question 9

What are the sites of carbohydrate digestion?

Answer 9

1. The mouth: this is where carbohydrate digestion starts.
2. The small intestine

*No carbohydrate digestion takes place in the stomach

Question 10

What are sources of carbohydrates in our diet?

Answer 10

• Starch, lactose, sucrose, trehalose, cellulose.

Question 11

Discuss carbohydrate digestion in the mouth.

Answer 11

1. Mechanical digestion: chewing
2. Chemical digestion:
   - Salivary glands in the oral cavity secrete saliva
   - Saliva contains the enzyme salivary amylase
   - Salivary amylase hydrolyses only starch into maltose and dextrin
   - pH in mouth is ~ 6.8 – 7.0
   - Only about 5% of starch is broken down in the mouth

Question 12

What are the enzymes involved in carbohydrate digestion in the small intestine?

Answer 12

• Pancreatic enzyme: a-amylase
• Brush border enzymes: disaccharidases and oligsaccharidases.
  *Most of the digestion is due to pancreatic ⍺-amylase because food does not stay in the mouth for a long time.

Question 13

What is the action of pancreatic alpha amylase?

Answer 13

• It’s an endoglycosidase: does not act on terminal glycosidic bonds.
• It is specific for a1,4 linkages and does not act on a1,6 linkages
• The hydrolytic products are maltose, maltotriose and a-limit dextrins

Question 14

What are the actions of brush border enzymes?

Answer 14

Question 15

What is absorption?

Answer 15

• Transportation of the products of digestion and other small dissolved molecules, with ions and water across the epithelial cell membranes.
• Occurs mainly in the small intestine.

Question 16

What are the units of absorption?

Answer 16

1. The villus
   - Length varies between 0.5 and 1.5mm
   - Covered by simple columnar epithelium
   - These have cytoplasmic extensions at the lumen: brush border

Question 17

List the enzymes that are anchored to the microvilli.

Answer 17

• The brush border contains catalytic domains of enzymes that protrude into the intestine:
  1. Aminopeptidase
  2. Endopeptidase
  3. Carboxypeptidase
  4. Di-peptidase
  5. Disaccharidases
  6. Oligsaccharidases

Question 18

What are enterocytes?

Answer 18

• These are simple columnar epithelium found on the lining of the stomach, intestines, gallbladder, uterine tubes & collecting ducts of the kidneys.
• Function: protection, secretion and absorption.

Question 19

What are the mechanisms of absorption?

Answer 19

1. Passive diffusion:
   - Down a concentration gradient.
2. Facilitated diffusion:
   - Uses membrane proteins in the enterocyte.
3. Active transport:
   - Energy dependent membrane proteins in the enterocyte.

Question 20

Discuss entry into the enterocyte by peptide absorption.

Answer 20

• Form in which the majority of proteins are absorbed.
• More rapid than absorption of free amino acids.
• Coupled to H+ gradient
• Na+/H+ exchange maintains gradient.
• Metabolised into free amino acids in enterocyte.
• Only free amino acids absorbed into blood.

Question 21

Discuss transport across the baso-lateral membrane.

Answer 21

• Released into interstitial fluid by facilitated
  diffusion and co-transport.
  -Transported to the liver via the hepatic portal vein.
• Glutamate and aspartate are utilised as energy and not transported out of the enterocyte.

Question 22

Discuss the entry of monosaccharides into the enterocyte.

Answer 22

• By passive diffusion (very slow).
• By membrane-associated transporters:
  1. Na+/glucose co-transporter (SGLT1)
  2. Na+ independent transporter (GLUT5)