Amylase is an enzyme that breaks down starch into simple sugars. Explain how the 'lock and key' hypothesis describes the way amylase acts on starch, and explain why amylase cannot break down proteins.

The active site of amylase has a specific shape that fits only a particular substrate. The starch molecule has a complementary shape to this active site, so it fits exactly like a key in a lock. When the starch enters the active site, an enzyme‑substrate complex is formed and the enzyme can cleave the starch into simple sugars. Proteins, however, do not have a shape that matches the amylase active site. Consequently, no enzyme‑substrate complex can form between amylase and a protein, so the protein cannot be broken down.