For example, there are peptidases and enzymes that break glycosidic bonds, like lactase or sucrase . Lyases - the catalyze the addition reactions of H 2 O, NH 3 or CO 2 to double bonds. Ligase - these catalyze the condensation or joining of two small molecules to form one larger molecule. In enzyme catalyzed reactions, the enzyme and substrate must first bind to form a complex. The portion of the enzyme in contact with the substrate is called the active site .
Active sites are usually pockets or clefts in the surface of the enzyme.
The amino acid R-groups involved in the binding and in the catalysis are called catalytic groups . Only a small proportion of the R-groups in the enzyme are actively involved in this way. This theory suggests that enzymes and substrates combine because they have complementary molecular geometries. The enzyme is pictured as being conformationally rigid, with a crevice in the protein chain suitable for binding a substrate. Binding only occurs if the two have complementary shapes, like a lock and key.