Under most conditions, only one small part of the enzyme, called the , complexes with the substrate. it is at this location that the enzyme and substrate fit together, seemingly like a lock and key.

I believe it is the Active site. The active site of an enzyme is part of the enzyme where the substrate binds the enzyme in a rock and key model. It contains two sites namely the binding site and the catalytic site. The binding site is for substrate binding to the enzyme forming and ES complex and the Catalytic site enhances catalysis of the substrate to form a product.