Topic 58 - Catabolism and absorption of the proteins

Question 1

Words to include in catabolism of proteins

Answer 1

• Luminal digestion
• Brush border digestion
  
  • Amino peptidases
  • Dipeptidases
  • Dipeptidil-aminopeptidases
• Proteolytic enzymes
• Endopeptidases (inner region of amino acids)
• Exopeptidases (terminal region of amino acids)
• Peptidases
• Enterocytes
• Peptides
• Dipeptides
• Tripeptides
• Amino acids
• Luminal membrane
• Secondary active transport
• Na⁺-Aa co-transporter system
• Stomach
  
  • Peptide digestion
  • Chief cells
  • Pepsinogen
  • HCl
  • Pepsin
  • Autocatalytic
  • Hydrolyze
  • Phe, Tyr, His (aromatic amino acids)
• Small intestines
  
  • Peptide digestion
  • Small intestine, proximal region
  • Luminal membrane
  • Enterocytes
  • Peptidases
  • Polypeptides
  • Amino acids
  • Oligopeptides
  • Carboxylpeptidases
    
    • Pancreatic juice
    • Trypsin
    • Enteropeptidases
  • Trypsin
    
    • Pancreatic juice
    • Carbonyl bonds
      
      • Arginine
      • Lysine
  • Chymotrypsin
    
    • Pancreatic juice
    • Carbonyl bonds
    • Tyrosine
    • Phenylalanine
• Large intestines
  
  • Bacteria
  • Colon
  • Urea
  • Nitrogen-metabolism
  • Pseudo-cecotrophy (rabbits)
  • Microbes

Question 2

Words to include in protein absorption

Answer 2

• Amino acids
• Peptides
• Enterocyte
• Facilitated diffusion
• Active transport (symport)
• Luminal side
  
  • Na⁺ symport system
    
    • Neutral amino acids
    • Phenylalanine, methionine
    • Proline, hydroxyproline
  • Facilitated transport
    
    • Hydrophobic, neutral
    • Alkaline
• Basolateral side
  
  • Na-symport
    
    • Small hydrophilic amino acids
    • Structure dependent uptake of neutral amino acids
  • Facilitated transport
    
    • Hydrophobic, neutral
    • Alkaline

Question 3

Topics to include in the essay

Answer 3

1. Catabolism of proteins
   
   • Luminal and brush border digestion
   • Stomach
   • Small intestines
   • Large intestines
2. Absorption of proteins
   
   1. Luminal side
   2. Basolateral side

Question 4

Catabolism of proteins

Which type of digestion is performed?

Answer 4

1. Luminal digestion
   
   • With proteolytic enzymes produced by the stomach and pancreas
   • Small peptides get through the luminal membrane
     
     • By secondary active transport
2. Brush border digestion
   
   • ​There are Na⁺-Aa co-transporter systems in the brush border membrane
   • During brush border digestion enzymes which are embedded in the luminal membrane finish the digestion of proteins:
     
     • Amino peptidases
       
       • Cleave an amino-acid from the N-terminal of the peptide
     • Dipeptidases
       
       • Cleave dipeptides into amino acids
     • Dipeptidil-aminopeptidases
       
       • Cleave dipeptides from the N-terminal of the peptides
   • Peptidases of enterocytes catabolize peptides to:
     
     • Dipeptides
     • Tripeptides
     • Amino acids
   • ​These substances are released and they can enter the cell by:
     
     • Facilitated diffusion
     • Na⁺ co-transport

Question 5

Cleaving of proteins

Answer 5

• Cleaving of proteins may happen:
  
  • At the inner regions amino acid (endopeptidases)
  • At the terminal amino acid (exopeptidases)

Question 6

Catabolism of proteins

Stomach

Answer 6

• Peptide digestion starts in the stomach
• Chief cells produce pepsinogen
  
  • Pepsinogen is cleaved by HCl → active pepsin
• Pepsin will activate other pepsinogens in an autocatalytic way
• Pepsin can hydrolyze bonds at pH 1.8-3.8 consisting of:
  
  • Phe, Tyr and His (aromatic amino acids)

Question 7

Catabolism of proteins

Small intestines

Answer 7

• Peptide digestion mainly occurs in the proximal region of small intestines
• Peptidases catabolize polypeptides into:
  
  • Amino acid
  • Oligopeptides
• Carboxylpeptidases
  
  • Origin: pancreatic juice
  • Becomes activated by the effect of trypsin and enteropeptidases in the intestines
  • Cleaves the free terminal amino acids from the C-terminal of the polypeptide
• Trypsin
  
  • Origin: pancreatic juice
  • Becomes active when entering the intestines
  • Hydrolyzes carbonyl bonds with arginine and lysine
• Chymotrypsin
  
  • Origin: pancreatic juice
  • Becomes active when entering the intestines
  • Cleaves carbonyl bonds with tyrosine and phenylalanine

Question 8

Catabolism of proteins

Large intestines

Answer 8

• The bacteria in the colon receive urea from blood to balance their nitrogen-metabolism (like in forestomach digestion of ruminants)
• Microbes digest proteins
• Bacteria synthesize proteins
• Rabbits: pseudo-cecotrophy

Question 9

Protein absorption

Answer 9

• Amino acids and small peptides can get into the enterocyte by:
  
  • Facilitated diffusion
  • Active transport (symport)
• Luminal side:
  
  • Na⁺ symport systems are responsible for the transport of:
    
    1. Neutral amino acids
    2. Phenylalanine, methionine
    3. Proline, hydroxyproline
  • Amino acid groups which can get into the cell by facilitated transport:
    
    • Hydrophobic, neutral
    • Alkaline
• Basolateral side:
  
  • Na-symport, two are important:
    
    1. Small hydrophilic amino acids
    2. Structure dependent uptake of neutral amino acids
  • Amino acid groups which can get into the cell by facilitated transport:
    
    • Hydrophobic, neutral
    • Alkalin