Where is the active site of an enzyme?

The active site of an enzyme is a region of the enzyme where the substrate molecules bind and undergo a chemical reaction. It is a structural element of protein formed by the folding pattern of the protein. The active site appears as a groove or pocket and is made up of amino acid residues that establish temporary bonds with the substrate. 

The substrate can bind to the enzyme only at the active site and not at any other region. This is because the groove at the active site is shaped to accommodate the structure of the enzyme that works on it.