Question

What fits into the active site of an enzyme based on its shape

Answer 1

A substrate- it is specific to the enzyme

Answer 2

substrate
Induced fit
The matching between an enzyme's active site and the substrate isn't just like two puzzle pieces fitting together (though scientists once thought it was, in an old model called the “lock-and-key” model). Instead, an enzyme changes shape slightly when it binds its substrate, resulting in an even tighter fit.
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