Protein Digestion And Absorption: From Stomach To Small Intestine

Protein Digestion And Absorption: From Stomach To Small Intestine

How are proteins digested? Indicate the specific site at which Proteolytic enzymes act. Add a note on the amino acid pool.
Answer:

Digestion of Proteins:

• Proteins are digested in the stomach, pancreas, and small intestine.
• It doesn’t occur in the mouth due to the absence of proteases in saliva.

1. Digestion by gastric secretion in the stomach.

• Gastric juice contains hydrochloric acid and pepsinogen.
• Renin also helps in digestion, which is found in infants.

1. Hydrochloric acid.
   • It is secreted by the parietal cells of the gastric gland.
   • It has two important functions.
     • Denaturation of proteins.
     • Killing of micro-organisms.
2. Pepsin.
   • Produced by serous cells of the stomach as pepsinogen.
   • It cleaves peptide bonds formed by amino groups of phenylalanine or tyrosine.
   • It results in the formation of peptides and a few amino acids which act as stimulants for the release of the hormone cholecystokinin.
3. Renin.
   1. Involved in curdling of milk in infants.
   2. Converts milk protein casein to calcium paracaseinate which can be digested by pepsin.

2. Digestion by proteases present in the pancreas.

• Proteases are secreted by zymogens.
• They are initiated by cholecystokinin and secret in hormones.
• Zymogens produce active proteases namely chymotrypsin, elastase, and carboxypeptidase.
• Trypsin cleaves the peptide bond.
• Carboxypeptidase acts on peptidase bonds of COOH – terminal amino acid.
• The combined effect of all pancreatic proteases causes the formation of free amino acids and small peptides.

3. Digestion in the small intestine.

• It occurs by aminopeptidases and dipeptidases are present.

1. Aminopeptidase.
   • It cleaves N-terminal amino acids one by one to produce free amino acids and smaller peptides.
2. Dipeptidases.

• Acts on different dipeptides to liberate amino acids.

Amino acid pool:

1. An adult has about 100 g of free amino acids which represent tire amino acid pool of the body.
2. Glutamate and glutamine constitute about 50% and essential amino acids about 10% of the body pool.
3. This amino acid pool is maintained by the balance between the sources and utilization of amino acids.