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Enzymes are catalysts that occur in all living organisms. They control a wide variety of chemical processes necessary for the maintenance, growth, and reproduction of cells. It has long been known that the role of enzymes in life's processes is to modify naturally occurring compounds. More recently it has become apparent that enzymes will also carry out highly controlled reactions on nonnatural compounds. Moreover, the natural catalysts often are able to promote conversions that are impossible to emulate using other techniques of chemistry. Enzyme-catalyzed conversions (biotransformations) have been used to make highly pure intermediates to new pharmaceuticals - for example, antibiotics, anticancer compounds, and vitamins. In a curious twist, enzymes isolated from bacteria have been used to prepare intermediates for novel antibacterial substances! Enzymes may be purified before use in biotransformations or they may be used in their native environment (e.g., in whole bacterial cells). Sometimes the enzyme complement of the whole cells is altered by genetic engineering to provide better access to a desired chemical feedstock.