Discuss the main steps in the digestion of proteins as the food passes through different parts of the alimentary canal.

Proteins digestion begins in the stomach and it gets completed in the small intestine. The enzyme proteases act on protein digestion. 

Digestion proteins in the stomach:
The digestive juice secreted in the gastric glands situated on the walls of the stomach is called gastric juice. HCl, pepsinogen, and rennin are the main constituents of gastric juice. The food that enters the stomach becomes acidic when mixed with this gastric juice.
The inactive pepsinogen is converted into active pepsin in the acidic medium. This active pepsin then converts proteins into proteases and peptides.

The enzyme rennin plays a very important role in the coagulation of milk.

Digestion of protein in the small intestine:

As soon as the food enters the small intestine from the stomach it is acted upon by three enzymes pancreatic juice, intestinal juice (known as succus entericus), and bile juice that are present in the small intestine.

The action of pancreatic juice:

Pancreatic juice contains various inactive enzymes such as trypsinogen, chymotrypsinogen, and procarboxypeptidase. These enzymes are present in an inactivated state. They are activated by the enzyme enterokinase which is secreted from the intestinal mucosa and converts trypsinogen into trypsin.

Now the other enzymes of pancreatic juice are activated by active trypsin. Chymotrypsinogen is a proteolytic enzyme that is responsible for the breakdown of proteins into peptides.

On the carboxyl end of the peptide chain, the carboxypeptidases act and help in releasing the smallest unit of proteins i.e. amino acids.

The action of bile juice:
Bile juice contains bile salts such as bilirubin and biliverdin which is responsible for the breakdown of large, fat globules into smaller globules so that pancreatic enzymes can easily act on them. This process of breakdown is known as the emulsification of fats. Bile juice is responsible for making the medium alkaline and it also activates lipase. The active lipase then breaks down the fats into diglycerides and monoglycerides.
 

The action of intestinal juice:
Intestinal juice contains a variety of enzymes. Pancreatic amylase digests polysaccharides into disaccharides. Then further digestion of disaccharides is done by the action of enzyme disaccharidases such as maltase, lactase, sucrase, etc. The enzyme proteases hydrolyze or converts peptides into dipeptides and finally into amino acids.

By the action of the enzyme pancreatic lipase, the fats get converted into diglycerides and monoglycerides. Similarly, the enzyme nucleases convert the nucleic acids into nucleotides and nucleosides.