Which enzyme breaks down protien?

in general, enzymes as we know are classified by which particular substrate they break down and are named accordingly. Protease enzymes are responsible for the break down of Proteins into simpler forms such as amino acids.

In terms of digestion, large protein chains are broken down by gastric protease (aka pepsin (which first comes in the form of pepsinogen which is activated to pepsin by the HCL acid in the stomach)) to form large polypeptides.

they are then broken down further by pancreatic protease (aka trypsin) into smaller peptides. They are then further broken down by peptidase into the simpest form - amino acids.