Amolecule that closely resembles the shape of a substrate and therefore also fits into the active site of an enzyme might serve as an

inhibitor

Explanation:

Enzymes have a shape that closely resemble that of the substrate hence they fit into the substrate by lock and key mechanism. The activity of the enzyme hinges on its similarity with the substrate in shape. Any other molecule similar to the shape of the substrate can fit into the substrate thereby preventing the enzyme from locking with the substrate. Such substances are called inhibitors. They decrease the activity of an enzyme.

true

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