Enzymes have a specific 3D shape (tertiary structure) as a result of their amino-acid sequence.A region of the enzyme, known as the active site, creates a small depression in the enzyme that is responsible for the enzyme's catalytic activity.The substrate or substrates fit into the active site to form an enzyme-substrate complex because the shape of the active site is complementary to the shape of the substrate.Temporary bonds form between the enzyme and substrate which hold the substrate in place and weaken the bonds within the substrate molecules.Active sites are clefts or grooves on the surface of an enzyme, usually composed of amino acids from different parts of the polypeptide chain that are brought together in close proximity in the tertiary structure of the folded protein.Substrates initially bind to the active site by non-covalent interactions, including hydrogen bonds, ionic bonds, and hydrophobic interactions.