In the picture below, the active site of the enzyme and the substrate have complementary shapes. This is so illustrated to indicate that the enzyme can recognize the substrate based, at least in part, on its shape.
Secondly, it is meant to illustrate that
the enzyme and substrate form a very close interaction.
Thirdly, it shows that the substrate will preferentially bind to the active site and not to other sites on the enzyme.
In the induced fit model of enzyme-substrate
binding, the shape of the active site of the unbound enzyme is not the exact
complement of the shape of the substrate. However, the enzyme does bind to the
substrate. After binding of the enzyme to the substrate is initiated, a conformational
change in the shape of the active site which results in a new shape of the active
site that is complementary to the shape of the substrate.