Proteolysis is the breakdown of protein into smaller peptides or amino acids by enzymes called proteases. It takes place naturally in bacteria, plants, animals and humans and plays important roles in physiological and cellular processes. It can also be used as an analytical procedure in the lab for studying proteins, and in industry, for instance, in food processing. In the human body, proteases like pepsin in the stomach and trypsin, chymotrypsin and elastase in the small intestine, contribute to the digestion of proteins from the diet.

Only the smaller building blocks of proteins, the amino acids, and small peptides, consisting of only few amino acids, can be absorbed through the gut wall and become available for building new proteins with various functions in the body. Proteins that are not fully digested and absorbed by proteolysis in the small intestine will end up in the colon, and can often be further degraded by the gut bacteria, that have their own, additional proteolytic enzymes.

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