Final answer:
The usual substrate for the pancreatic enzyme peptidase is peptides. Peptidases help in the digestion of proteins, breaking them into peptides and amino acids in the small intestine.
Step-by-step explanation:
The usual substrate for the pancreatic enzyme peptidase is d. peptides. Peptidases are a group of enzymes that assist in breaking down proteins into peptides and amino acids, which are then further digested or absorbed. This enzyme is a part of the protein digestion process, which is completed in the small intestine. The pancreatic juice contains other enzymes for protein digestion like trypsin and chymotrypsin, which also break proteins down into peptides. The digestive process for carbohydrates and fats involve different enzymes such as pancreatic amylase for starch and lipase for fats.