Enzyme molecules are, compared to most of their substrate molecules, rather large. Their surface is not evenly structured but displays dent-ins, grooves, pockets, hollows etc. The part that binds to a substrate molecule is termed the active centre or substrate-binding site and is characterized by a shape complementary to that of the substrate molecule. Furthermore are certain amino acid side chains exposed at the active centre. They are engaged in the catalytic turn-over of the substrate.
Numerous enzymes depend on additional factors that are necessary to perform the catalytic reaction.
Often is the enzyme's surface structured in a way that the coenzyme is bound to a specifically shaped pocket. Atoms or ionized groups of the coenzyme take part in the catalytic reaction. These properties explain, why reactions take place at an enzyme's surface that occur in solutions only after a considerable amount of activation energy has been supplied. The reactants are by the binding set into a state of enhanced reactivity recognizable by the close vicinity and the right orientation towards each other. As long as only a few reactants are involved, like in an experiment performed in a chemical laboratory, can such methods produce satisfying results although by-products may lower the yield.