Answer:
Each enzyme is very specific for a particular substrate and each reaction is catalyzed by a particular enzyme this is because all the enzymes have different 3-D structure of its active site which is responsible for the specific binding of enzymes to its substrate.
Therefore a substrate is the reactant that has to be fit in the active site of the enzyme to be catalyzed so in case of amylase the structure of active site does not allow the binding amylase to protein, therefore, amylase can not break down protein.
Proteases are the enzymes that are secreted by pancreases which breaks down the proteins into peptone and amino acids.