GI 17 Flashcards
In addition to dietary intake, where does the intestine also receive large amounts of protein?
In mucus, enzymes and degraded epithelial cells
What do polypeptide chains need to be broken down into?
Individual amino acids
What is protein digestion mainly mediated by?
Proteolytic enzymes (proteases)
Why are many enzymes secreted as inactive precursors?
to protect the cells where they are manufactured because they are very powerful
What are the two categories of Protease enzymes?
- Endopeptidases
* Exopeptidases
Where do Endopeptidases cleave proteins?
At interior peptide bonds
What are examples of endopeptidases?
- Trypsin
- Chymotrypsin
- Elastase
Where do Exopeptidases cleave proteins?
At their n- (aminopeptidases) and c-termini (carboxypeptidases)
Where does Protein digestion begin?
In the stomach
What does the highly acidic environment of the stomach do to proteins?
Denatures them
What cleaves large polypeptides in the stomach?
Pepsin
What is the inactive precursor to Pepsin and how does it get activated?
Pepsinogen and it is activated by gastric acid
Why does pepsin stop working in the duodenum?
Because it requires acidic activity and the duodenum is more alkaline than the stomach
What proteases contribute to intestinal protein digestion?
- Pancreatic enzymes
* Intestinal brush border enzymes
What are the pancreatic enzymes that digest proteins activated by?
Enterokinase and Trypsin
How does Enterokinase and Trypsin activate protein digesting pancreatic enzymes?
Enterokinase on the surface of intestinal epithelial cells cleave trypsinogen into the active trypsin. Trypsin then catalyzes the activation of the other inactive precursors and more trypsin
What are some intestinal brush border enzymes that digest proteins?
- Aminopeptidases
- Carboxypeptidases
- Endopeptidases
What do the activity of the brush border enzymes generate?
- Individual amino acids
- Dipeptides
- Tripeptides
What happens to Dipeptides and Tripeptides?
They can be taken up into intestinal epithelial cells and are further broken down in the cytosol to produce individual amino acids
What are the three locations that protein digestion can occur?
- Intestinal lumen
- Intestinal brush border
- Inside intestinal epithelial cell
What are the products of protein digestion?
- Amino acids
- Dipeptides
- Tripeptides
What does absorption of protein digestion products require?
Uptake and transit through the intestinal epithelium
What are small peptides (di and tripeptides) taken up into epithelial cells by?
A transport protein called PepT1 through secondary active transport
What kind of transport are small peptides (di and tripeptides) taken up into epithelial cells by?
Secondary active transport
What happens once small peptides (di and tripeptides) are taken into epithelia cells?
They are cleaved by intracellular peptidases into amino acids
How do amino acids exit epithelial cells?
By facilitates diffusion through amino acid transporters
What does PepT1 do?
Brings small peptides (di and tripeptides) into the epithelial cell along with H+ using secondary active transport
What is the proton gradient for the secondary active transport of small peptides (di and tripeptides) maintained by?
The sodium hydrogen exchanger (NHE) that brings sodium into the cell and H+ out of the cell
What is the sodium gradient for the secondary active transport of small peptides (di and tripeptides) maintained by?
A basolateral Sodium potassium ATPase
How many transport mechanisms are there that facilitate amino acid uptake into intestinal epithelial cells?
At least seven
What do the transport mechanism that bring amino acids into the epithelial cells rely on?
Sodium ion gradients
How do amino acids enter the cell?
By secondary active transport coupled to a gradient of sodium ions
