Answer
The shape of a molecule gives it its function, and enzymes are large molecules with very specific shapes. Their active sites can change based on what else is in their environment. Thus, their specific shape leads to the very specific nature of how they can behave in different substrates.
Work Step by Step
The key concept here is that molecular shape = molecular function. Review the nature of enzymes and see how complex they are and how specific their shape is to their function.