Digestion and Absorption of Proteins and Carbohydrates Flashcards
What is digestion?
- The breakdown of complex dietary molecules to small molecules via digestive enzymes.
What are the principle sites of protein digestion?
- The stomach.
- The small intestines.
What are proteolytic enzymes?
- Proteases released to breakdown proteins.
- Secreted into the stomach
- Released from the pancreas into the small
intestine.
Discuss the process of protein digestion in the stomach.
- 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.
Discuss the process of protein digestion in the small intestines.
*Enzymes released by pancreas as zymogens
List the pancreatic enzyme type and their catalytic mechanism.
- Trypsin chymotrypsin, elastase:
- Type: Endopeptidase
- Mechanism: Serine protease - Carboxypeptidase A, Carboxypeptidase B:
- Type: Carboxypeptidase
- Mechanism: Metalloprotease (Zn2+)
Discuss the cleavage specificity of different enzymes.
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
What are products of protein digestion.
- Cleaved by peptidases:
1. Tetra-peptides
2. Tri-peptides - Amino acids.
What are the sites of carbohydrate digestion?
- The mouth: this is where carbohydrate digestion starts.
- The small intestine
*No carbohydrate digestion takes place in the stomach
What are sources of carbohydrates in our diet?
- Starch, lactose, sucrose, trehalose, cellulose.
Discuss carbohydrate digestion in the mouth.
- Mechanical digestion: chewing
- 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
What are the enzymes involved in carbohydrate digestion in the small intestine?
- 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.
What is the action of pancreatic alpha amylase?
- 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
What are the actions of brush border enzymes?
What is absorption?
- 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.
What are the units of absorption?
- The villus
- Length varies between 0.5 and 1.5mm
- Covered by simple columnar epithelium
- These have cytoplasmic extensions at the lumen: brush border
List the enzymes that are anchored to the microvilli.
- 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
What are enterocytes?
- 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.
What are the mechanisms of absorption?
- Passive diffusion:
- Down a concentration gradient. - Facilitated diffusion:
- Uses membrane proteins in the enterocyte. - Active transport:
- Energy dependent membrane proteins in the enterocyte.
Discuss entry into the enterocyte by peptide absorption.
- 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.
Discuss transport across the baso-lateral membrane.
- 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.
Discuss the entry of monosaccharides into the enterocyte.
- By passive diffusion (very slow).
- By membrane-associated transporters:
1. Na+/glucose co-transporter (SGLT1)
2. Na+ independent transporter (GLUT5)
