Proteases and lipases are both enzymes involved in digestion. Explain why a protease enzyme cannot break down a lipid molecule, and describe what happens when a protease is used to digest a protein at its optimum temperature.

Proteases have an active site that is shaped to fit only peptide bonds in proteins – the lock and key hypothesis shows that the enzyme’s active site is complementary to the substrate.

A lipid molecule is a long hydrocarbon chain with a polar head; it does not fit into the protease active site, so the enzyme cannot recognise or cleave it.

When a protease acts on a protein at its optimum temperature, the protein enters the active site forming an enzyme‑substrate complex. The enzyme catalyses the hydrolysis of peptide bonds, breaking the protein into amino acids. The products are released and the protease is unchanged and can bind another protein molecule.