The best answer is (2).
This can be described by two theories. The first theory is the key and lock model which proposes that a specific enzyme and its substrate have complementary shapes such that they fit into each other like a lock and its key.
The second theory is the induced fit model which proposes that the enzyme's active site and its substrate don't fit perfectly but instead they alter their shape to be able to bind together thus facilitating a reaction.
Either way, the shape of the enzyme is critical to its function. When denatured, an enzyme loses shape and cannot function.
