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Chapter: Biochemistry: Digestion

Digestion of Proteins

Proteins in diet are from animal sources and vegetable sources. Animal sources like milk, dairy products, meat, fish, liver and eggs are rich sources of proteins.



Proteins in diet are from animal sources and vegetable sources. Animal sources like milk, dairy products, meat, fish, liver and eggs are rich sources of proteins.


Vegetable sources like cereals, pulses, peas, beans and nuts are rich in protein.


1. Digestion in mouth


There are no enzymes in mouth to degrade the protein.


2. Digestion in stomach


HCl : HCl secreted by the gastric mucosa destabilizes the secondary structures of the proteins such that it can be easily acted upon by the enzymes.


The proteolytic enzymes present in the gastric juice are pepsin, rennin, gastricin and gelatinase.


Pepsin :It is a potent proteolytic enzyme and is present in the gastric juices. It issecreted in the inactive zymogen form called as pepsinogen, which has a molecular weight of 42,500 daltons. In the acidic medium, pepsinogen is cleaved to pepsin and the reaction is favoured autocatalytically. Pepsin having a molecular weight of 34,500 daltons is an endopeptidase. An endopeptidase is an enzyme that acts on the peptide linkages in the interior of the protein.


Pepsin acts on protein to convert it to proteoses and peptones, which are low molecular weight peptides.

Proteins------------->  Proteoses + Peptones

It has a broader specificity and acts on peptide linkages constituted by the carboxyl group of an aromatic / hydrophobic amino acid or amino group of a dicarboxylic acid.


It hydrolyzes the soluble casein in milk , which along with calcium forms insoluble paracaesinate.


The optimum pH for pepsin is 1.6 – 2.5



Rennin is present in infants only and it is secreted by the gastric mucosa as pro –rennin. It is converted to active rennin by HCl. It also converts casein in milk to insoluble calcium paracaesinate.


3. Digestion in duodenum

The chief enzymes of the pancreatic juice that acts on proteins are a) trypsin

chymotrypsin c) carboxy peptidase d) elastases and e) collagenases



Trypsin, a proteinase is secreted in the inactive zymogen form called trypsinogen. It is activated by enterokinase and also autocatalytically in the presence of calcium.

It is an endopeptidase that is specific for peptide linkages formed by carboxyl groups of basic amino acids, namely arginine, lysine. The hydrolytic products are polypetides, proteoses, peptones, di and tri peptides. It cannot hydrolyze peptide linkages which involves proline.

It activates proelastase to elastase, chymotrypsinogen to chymotrypsin, fibrinogen to fibrin. The optimum pH for trypsin is 8 – 9.



Itisanendopeptidase,whichissecretedintheinactiveformaschymotrypsinogen. It is activated by trypsin and also autocatalytically. It hydrolyses peptide linkages with carboxyl group of aromatic amino acids like tryptophan, tyrosine and phenyl alanine.

The optimum pH for chymotrypsin is 7 - 8


Carboxy peptidase

Two types of carboxy peptidases, carboxy peptidase A and B are known. Carboxy peptidase A is a metallo enzyme that contains zinc. Both are exopeptidases. Carboxy peptidase A is specific for aromatic amino acids at the C terminal end, while carboxy peptidase B is specific for basic amino acids at the C terminal end.


The optimum pH for both of them lies between 7-8.


4.  Digestion in small intestine


The proteolytic enzymes present in the intestinal juice are enterokinase, amino peptidase, prolidase and di and tri peptidases.


Enterokinase is an enzyme that activates trypsin in the presence of calcium. Aminopeptidases are capable of removing one amino acid from the N terminal end of the peptide. They cannot hydrolyze the linkages of di-peptides or if the N terminal amino acid is proline. Prolidases are enzymes that hydrolyze linkages involving proline.


Thus by the concerted action of all the above enzymes, proteins are broken down to di and tri-peptides. Di and tri peptidases present in the intestinal mucosal cells or inside the absorptive cells cleave them to amino acids.


5.  Absorption of amino acids

Amino acids and small peptides that are absorbed will reach liver through the portal circulation.

Naturally occurring L-amino acids are absorbed actively, while D-amino acids are absorbed passively. Absorption of amino acids are similar to those of carbohydrates and they need a carrier and sodium ions.

Amino acids are absorbed via glutathione cycle. The steps involved in glutathione cycle are


·              Glutathione combines with amino acids to form γ-glutamyl amino acid and cysteinyl glycine.


·              γ -glutamyl amino acid is transported and hydrolyzed to oxo proline and L.amino acid, which is absorbed.


·              Cysteinyl glycine is cleaved to cysteine and glycine.


·              Oxoproline is converted back to glutamate.


·              Glutamate, cysteine and glycine combine together and form glutathione.


6.  Factors affecting absorption


·              Dinitro phenol or cyanide decreases the absorption of amino acids.


·              Amino acids compete with one another for absorption. High concentrations of one amino acid reduces the absorption of the others.


·              Glutathione is needed to absorb the amino acids via glutathione cycle.


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