Digestion and Absorption of Protein | Biochemistry

In this article we will discuss about the Digestion and Absorption of Protein from Gastrointestinal Tract.

Digestion of Protein:

The proteolytic enzymes secreted in gastric juice, pancreatic juice and also present in the intestinal mucosa cause the hydrolysis of pro­tein in the gastrointestinal tract.

Stomach:

Pepsin, the endopeptidase, is present in gastric juice and hydrolyzes the peptide bonds in the interior of the protein molecule.

Pepsin hydrolyzes the dietary protein into a mixture of polypeptides:

Renin has a strong clotting action on milk. This is very important in the digestion of milk pro­teins in infants. The pH of the gastric juice becomes low in achlorhydria, achylia gastrica (both pepsin and HCl absent) and in pernicious anemia. Then dietary protein will not be digested in the stomach.

Small Intestine:

The polypeptides formed in the stomach are digested in the intestine by trypsin, chymotrypsin and carboxy-peptidases secreted in pancreatic juice and amino-peptidases present in the intestinal mucosa.

Trypsin hydrolyzes peptide linkages containing arginine or lysine and chymo­trypsin hydrolyzes peptide linkages containing tyrosine or phenylalanine:

Carboxypeptidase A hydrolyzes the end group of peptides containing aromatic or aliphatic amino acid and releases free amino acids. Carboxypeptidase B hydrolyzes peptides containing arginine and lysine residues.

The intestinal mucosa also con­tains tripeptidase, di-peptidase etc., which hydrolyze tri- and dipeptides:

The final products of digestion of proteins are amino acids which are absorbed.

Absorption of Protein:

1. Three different active processes are in­volved in the transport of amino acids. One process involves cystine and the ba­sic amino acids, another the amino acids proline and hydroxyproline and the third the neutral (L-) amino acids.

2. D-amino acids are absorbed by simple dif­fusion. But the neutral (L-) amino acids require a carrier system in the absorption. Na⁺ is also required. This is similar to that of active transport of glucose. Vitamin B₆ (pyridoxal phosphate) is also involved in the process. The amino acid associates with the carrier and Na⁺ in the microvilli and the complex travels to the inner side of the membrane where it dissociates, re­leasing the amino acid Na into the cytosol. The carrier returns back and func­tions repeatedly. Na⁺ is then actively trans­ported out of the cell.

3. If one amino acid is fed in excess, it re­tards the absorption of another. This is similar to those made with respect to reabsorption of amino acids by the renal tu­bules.

4. Sometimes the whole protein is absorbed into the blood. A protein is antigenic and accounts for food allergies. In the young mammal, the permeability of the mucosa, in this respect, is greater than that in the adult.

5. Food proteins are generally readily di­gested (90 to 97 per cent) under normal conditions, very little escapes in the fae­ces. The insoluble fibrous protein, kera­tin, is not hydrolyzed by enzymes of the human digestive tract.

These are altered by heating to coagulation and hydrolyzed by superheated steam. The biological val­ues of these proteins are not affected by such procedures. Cooked egg albumin is digested more readily than raw. The nutri­tional value of cereal proteins is lowered by overheating or toasting.