Enzymes are remarkably specific both for the substrate and the reaction that is catalyzed. In general, enzymes recognize either a unique substrate or a class of substrates that share common chemical structures. In addition, enzymes catalyze only one reaction or a very limited number of reactions.
For example, the enzyme succinate dehydrogenase acts only on succinate, and the enzyme β-(beta-)galactosidase acts only on a specific class of molecules. The enzyme β-galactosidase catalyzes the cleavage of the glycosidic bond that links galactose to glucose in the disaccharide lactose, as well as any glycosidic bond that links galactose to one of several types of molecule. In this case, the enzyme does not recognize the whole substrate, but a particular structural motif within it, and very small differences in the structure of this motif affect the activity of the enzyme. For example, β-galactosidase cleaves β-galactoside but does not cleave α-(alpha-)galactoside, which differs from β-galactoside only in the orientation of the glycosidic bond. Hence, the enzyme is able to discriminate between two identical bonds with different orientations within a molecule. The specificity of enzymes can be attributed to the structure of their active sites. The enzyme active site interacts only with substrates having a precise three-dimensional structure.