The main enzyme that breaks down proteins into smaller peptides and amino acids is called protease or peptidase. There are many different types of proteases, each with its own specificity for cleaving particular peptide bonds. Some well-known proteases include:

1. Pepsin: This protease is produced in the stomach and is active in acidic conditions. It breaks down proteins into smaller peptides.

2. Trypsin and Chymotrypsin: These proteases are produced in the pancreas and work in the alkaline environment of the small intestine. They further break down peptides into smaller fragments and amino acids.

3. Elastase: Another protease produced in the pancreas, elastase is responsible for digesting elastin, a protein found in connective tissue.

4. Carboxypeptidases and Aminopeptidases: These proteases remove amino acids one at a time from the ends of peptides. Carboxypeptidases cleave amino acids from the C-terminus, while aminopeptidases cleave from the N-terminus.

5. Cathepsins: These proteases are found in lysosomes, cellular organelles responsible for breaking down various macromolecules. Cathepsins are involved in the intracellular digestion of proteins.

These are just a few examples of proteases, and there are many more involved in protein digestion, metabolism, and various cellular processes. Proteases play a crucial role in breaking down dietary proteins into forms that can be absorbed and used by the body, as well as in regulating protein turnover and function within cells.