- Proteins of the ingested food are broken down into amino acids by proteases (peptidases).
- The proteases are secreted in inactive pro-enzymes to protect the mucous membranes of the gut from hydrolysis.
- Protein digestion starts in the stomach and is completed in the small intestine because saliva contains no protease.
I. Stomach:
- Food undergoes both mechanical and chemical changes in the stomach.
a. Mechanical change:
- Wall of the stomach undergoes periodic muscular contractions.
- This churns and breaks the food mechanically and mixes thoroughly with the gastric juice for proper action of the enzymes.
- The cardiac and pyloric sphincters remain closed during the churning of food.
b. Chemical change:
- The gastric juice contains mucus, HCl and two inactive proteases: pepsinogen (propepsin) and prorennin.
1. Hydrochloric acid: It serves many functions:
- It disinfects the food by killing bacteria.
- It stops the action of salivary enzymes in the due course of time.
- It activates pepsinogen to pepsin and prorennin to rennin.
- It produces an appropriate pH (acidic) for protein digestion.
- It softens the food and breaks down the cementing material between the cells of the food.
- It denatures many food proteins, increasing exposure of their peptide bonds to pepsin which facilitates the action of pepsin on them.
2. Pepsinogen:
- It is first changed by HCl into an active enzyme pepsin which then itself continues to activate additional pepsinogen, the process called autocatalytic activation.
- Pepsin hydrolyses the proteins partially to proteoses and peptones in an acidic medium (pH 2).
- It can digest collagen of the white fibers of connective tissues but no keratin of hair, horns and nails.
Pepsinogen + HCl ———-> Pepsin
Proteins ———-> Proteoses and Peptones or peptides (by pepsin)
3. Prorennin:
- It is first hydrolyzed by HCl to an active enzyme rennin.
- Rennin hydrolyses the soluble milk protein casein into paracasein and whey protein.
- Paracasein is spontaneously precipitated in the presence of calcium and insoluble calcium paracaseinate, forming solid curd, or coagulated milk.
- The coagulated milk stays in the stomach for a longer period for proper action of pepsin.
- Pepsin converts this calcium paracaseinate also into peptones.
- Rennet tablets containing rennin obtained from calf are commercially used for curdling milk.
Prorennin + HCl ———-> Rennin
Casein ———-> Paracasein + Whey protein (by rennin)
Paracasein + Calcium ———-> Calcium Paracaseinate
Calcium paracaseinate ———-> peptones (by pepsin)
- As a person matures, rennin production decreases greatly and stops altogether.
II. Small Intestine:
- In the small intestine, food receives three alkaline secretions: bile (pH 8) from the liver, pancreatic juice (pH 8.8) and intestinal juice (pH 8.3) from the intestinal glands.
- Their alkalinity stops the action of pepsin.
a. Bile: It contains no enzymes and hence has no action on proteins.
b. Pancreatic juice:
- It contains 3 inactive proteases: trypsinogen, chymotrypsinogen and Procarboxypeptidases.
- Being inactive, the proteases don’t digest the pancreas itself during their secretion and storage.
i. Trypsinogen:
- It is activated to an enzyme trypsin by a non-digestive enzyme enterokinase or enteropeptidase present in the intestinal juice.
- Once formed, trypsin activates additional trypsinogen by autocatalysis.
- Trypsin breaks more proteins, especially basic proteins, into proteoses and peptones (peptides).
- It has a limited action on collagen and no action on casein and keratin.
- It coagulates the blood by hydrolyzing its fibrinogen into fibrin in predatory animals.
Trypsinogen ———-> Trypsin (by enterokinase)
Proteins ———-> Peptides (by trypsiin)
ii. Chymotrypsinogen:
- It is activated to an active enzyme chymotrypsin by tryspsin.
- It also splits the proteins into peptides.
- It also coagulates milk in an alkaline medium by hydrolyzing milk protein casein to paracasein and whey proteins.
- Paracasein coagulates in the presence of calcium, forming calcium paracaseinate.
Chymotrypsinogen ———-> Chymotrypsin (activation by trypsin)
Proteins ———-> Peptides (by chymotrypsin)
iii. Procarboxypeptidases:
- These are activated to enzymes carboxypeptidases by trypsin.
- These enzymes separate individual amino acids adjacent to the free terminal carboxyl group of the peptides, reducing them into dipeptides. i. e. they produce amino acids and dipeptides.
Procarboxypeptidases ———-> Carboxypeptidases (by trypsin)
Peptides ———-> Dipeptides + Amino acids (by carboxypeptidase)
III. Intestinal juice:
- It contains two digestive proteases: aminopeptidases and dipeptidases, and a non-digestive protease enterokinase (enteropeptidase).
a. Enteropeptidase: It activates the pancreatic trypsinogen into trypsin as above mentioned.
b. Aimnopeptidase:
- These separate individual amino acids adjacent to the free terminal amino group of the peptides, reducing them into dipeptides.
- In other words, they produce amino acids and dipeptides like in carboxipeptidases of pancreatic juice.
Peptides ———-> Dipeptides + Amino acids (by aminopeptidase)
- Dipeptidases: These hydrolyze the dipeptides into amino acids.
Dipeptides ———-> Amino acids (by dipeptidase)