Enzymes are biological catalysts. They speed up chemical reactions and are not used up in the process.
Enzyme activity is influenced by substrate concentration, temperature and pH.
By adding the enzyme lactase, milk is made lactose-free. The enzyme breaks down lactose (milk sugar) to glucose and galactose.
Immobilized enzymes are often commercially used, because they have a variety of advantages. These enzymes are not soluble, but they are bound to a surface (hence immobilized)