Protein digestion in man:
(a) Protein digestion in the mouth: Proteins do not undergo any change as there is no protease in the mouth.
(b) Protein digestion in the stomach: Gastric juice has two proteases pepsin and rennin.
(i) Pepsin is first secreted as pepsinogen:
It is converted to pepsin by HCI. One HCI has converted some pepsinogen, further conversion is autocatalytic. Pepsin is an endopeptidase. It acts on internally situated peptide bonds of proteins. In the end the proteins are converted to proteoses, peptones and polypeptides.
(ii) Rennin (also called chymosin):
It is a milk curdling enzyme that is mostly present in infants. It is secreted as prorennin. Renin acts on the soluble milk protein, casein, converting it into paracasein. In the presence of Ca2+ of milk it is further converted into calcium para caseinate, which coagulates preventing the rapid passage of milk into the duodenum. It is then hydrolyzed by pepsin to proteoses, peptones & polypeptides
As digestion proceeds in the stomach the food becomes more or less liquified and is referred to as chyme.
(c) Protein digestion in the Intestine:
Here it comes under the action of
(i) Pancreatic proteases &
(ii) Intestinal proteases.
(i) Pancreatic proteases:
These are Trypsin, chymotrypsin and carboxypeptidase. The first two are endopeptidases and the third is and exopeptidase. Trypsin is secreted as trypsinogen & it is activated by enterokinase (enteropeptidase) of the intestinal juice.
chymotrypsinogen which is activated by trypsin. It, too, converts proteins into protein fragments.
Carboxypeptidase an enopeptidase, hydrolyzes terminally situated bonds. The products are tripeptides, Depeptides and amino acids.
(ii) Intestinal proteases:
They may be termed as peptidases because they act on the protein fragments. Proteoses, peptones and polypeptides ammopeptidase tripeptides, dipeptides and
The proteins have been completed digested and the products are free amino acids. They are now ready to be absorbed.
