Digestion and Absorption of Proteins

Summary

This video explains the digestion and absorption of proteins, detailing the enzymatic breakdown starting in the stomach with pepsin, activated from pepsinogen by hydrochloric acid. As the partially digested proteins move to the small intestine, the pancreas secretes zymogens like trypsinogen, which are activated by enterokinase. Active enzymes, including trypsin and chymotrypsin, further digest polypeptides into smaller peptides and amino acids. Finally, enzymes on the intestinal lining, such as aminopeptidases and dipeptidases, convert these peptides into free amino acids, which are absorbed into the bloodstream for various bodily functions.

Takeaways

• 😀 Protein digestion begins with their breakdown into proteases, peptones, and large polypeptides.
• 😀 The stomach plays a crucial role by secreting pepsinogen, which is activated by hydrochloric acid to form pepsin.
• 😀 Pepsin is an endopeptidase that breaks down proteins into smaller peptides.
• 😀 In infants, casein protein in milk requires the enzyme renin for digestion before pepsin can act on it.
• 😀 The pancreas releases zymogens (trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase) into the small intestine.
• 😀 Enterokinase, secreted by the duodenal mucosa, converts trypsinogen into trypsin, which then activates other zymogens.
• 😀 Trypsin hydrolyzes peptide bonds next to lysine and arginine, while chymotrypsin acts near aromatic amino acids.
• 😀 Carboxypeptidases A and B remove terminal amino acids from the carboxyl end of peptides.
• 😀 Brush border enzymes like aminopeptidases, dipeptidases, and tripeptidases further digest peptides into free amino acids.
• 😀 Free amino acids enter circulation through specific transporters, often coupled with sodium ions.

Q & A

What is the initial step in protein digestion?

Proteins are first broken down into proteoses, peptones, and large polypeptides.

What role do gastric G cells play in protein digestion?

Gastric G cells secrete pepsinogen, the inactive form of the enzyme pepsin, which is activated in the presence of hydrochloric acid (HCl).

How is pepsinogen converted into pepsin?

Pepsinogen is converted into pepsin by the action of hydrochloric acid secreted by parietal cells in the stomach.

What enzymes are secreted by the pancreas for protein digestion?

The pancreas secretes zymogens including trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase into the small intestine.

What is the function of enterokinase in protein digestion?

Enterokinase activates trypsinogen to trypsin, which then activates other zymogens into their active enzyme forms.

What are the main functions of trypsin, chymotrypsin, and elastase?

Trypsin acts on peptide bonds next to lysine and arginine; chymotrypsin acts on bonds formed by aromatic amino acids; elastase acts on bonds formed by glycine, alanine, and serine.

How do carboxypeptidase A and B differ in function?

Carboxypeptidase A acts on peptide bonds connected to tyrosine, phenylalanine, or tryptophan, while carboxypeptidase B acts on bonds connected to arginine and lysine.

What are aminopeptidases, dipeptidases, and tripeptidases, and where are they located?

These are enzymes present on the brush border of enterocytes in the small intestine that act on peptides to release free amino acids.

What triggers the release of secretin and cholecystokinin (CCK) in the small intestine?

The low pH of chyme entering the duodenum triggers the release of secretin, which stimulates bicarbonate secretion, while CCK stimulates the release of pancreatic juice.

How do amino acids enter the bloodstream after digestion?

Free amino acids enter the circulation via amino acid carriers in the enterocytes, along with sodium ions, which are then transported by the sodium-potassium pump.