58. Catabolism and absorption of the proteins

Question 1

Catabolism of proteins

Answer 1

Catabolism of proteins begins through the luminal digestion with proteolytic enzymes produced by the stomach and the pancreas. Proenzymes are cleaved to form activated enzymes.

Question 2

Cleavage of proteins

Answer 2

Cleavage of proteins may happen at inner regions (endopeptidases), or at the terminal amino acid (exopeptidases). The result is peptides with 2-6 amino acids, appearing in the small intestines. Some free amino acids are also present here.
Further catabolism proceeds by brush border digestion, when peptidases of enterocytes catabolize peptides to tri- and dipeptides, as well as free amino acids.

Most of the small peptides get through the luminal membrane by secondary active transport. There is 8-10 Na+-Aa co-transporter systems with different specificity in the brush border membrane. Besides the co-transporter system, several facilitated transporter systems are present in the membrane.

Question 3

Stomach

Answer 3

• Peptide digestion starts in the stomach.
• Chief cells produce pepsinogen, which is cleaved by HCl into active pepsin.
• Pepsin will activate other pepsinogens in an autocatalytic way.
• Pepsin can hydrolyse bonds consisting of Phe, Tyr and His (aromatic amino acids), at pH 1,8-3,8.

Question 4

Small intestines

Answer 4

• Peptide digestion primarily occurs in proximal region of the small intestines.
• Luminal membrane of duodenal and jejunal enterocytes contain large amount of peptidases. These enzymes catabolize polypeptides into amino acids and oligopeptides.
• Carboxypeptidases:
• Originates from the 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:
• Hydrolyses carbonyl bonds with Arg and Lys.
• Chymotrypsin:
• Cleaves carbonyl bonds with Tyr or Phe.
• Trypsin and chymotrypsin originates from pancreas and become active when they enter the intestine.

Question 5

Luminal and brush border digestion

Answer 5

• Brush border enzymes accomplish the catabolism of peptides. They are located in enterocytes membrane.
• During brush border digestion enzymes 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

-At last, di- and tripeptides and amino acids are released, which can enter the cell by facilitated diffusion and by Na+ co-transport.

Question 6

Protein absorption

Answer 6

• Amino acids and small peptides can get into the enterocyte by facilitated diffusion or by active transport (symport). The more hydrophobic and concentrated an amino acid it, the more significant diffusion is on its absorption.
• Luminal side:
• Na+ symport systems is a active transport mechanism
• Are responsible for the transport of neutral amino acids, phenylalanine, methionine, proline and hydroxyproline.
• By facilitated transport:
• Hydrophobic neutral amino acids and alkaline amino acids.
• Basolateral side:
• Transport processes at the basolateral side are different from those which can be found at the luminal membrane.
• From among the transport mechanisms operating with Na-symport, two are important:
• Small hydrophilic amino acids
• Structure dependent uptake of neutral amino acids
• Facilitated transport plays a role only in the transport of hydrophobic neutral amino acids.

Question 7

Large intestines

Answer 7

• Protein metabolism here shows many common features to the forestomach digestion in ruminants. The bacteria in the colon receive urea from blood to balance their N-metabolism.
• Rabbits are special because they show pseudo-cecotrophy.
• Microbes digest and bacteria synthetize proteins, but these are not absorbed.