Another important class of compounds produced by biotechnology is enzymes. These protein catalysts are used widely in both medical and industrial research. Proteases, enzymes that break down proteins, are particularly important in detergents, in tanning hides, in food processing, and in the chemical industry. One of the most significant commercial enzymes of this type is subtilisin, which is produced by a bacterium. Because many stains contain proteins, the manufacturers of laundry detergents include subtilisin in their product. Subtilisin is 274 amino acids long, and one of these, the methionine at position 222, lies right beside the active site of the enzyme. This is the site on the enzyme's surface where the substrate is bound, and where the reaction that is catalyzed by the enzyme takes place. In this instance the substrate is a protein in a stain, and the reaction results in the breaking of a peptide bond in the backbone of the protein. Unfortunately, methionine is an amino acid that is very easily oxidized, and laundry detergents are often used in conjunction with bleach, which is a strong oxidizing agent. When used with bleach, the methionine in subtilisin is oxidized and the enzyme is inactivated, preventing the subtilisin from doing its work of breaking down the proteins present in food stains, blood stains, and the like.
To overcome this problem, genetic engineering techniques were used to isolate the gene for subtilisin, and the small part of the gene that codes for methionine 222 was replaced by chemically synthesized DNA fragments that coded for other amino acids. The experiment was done in such a way that nineteen new subtilisin genes were produced, and every possible amino acid was tried at position 222. Some of the altered genes gave rise to inactive versions of the enzyme, but others resulted in fully functional subtilisin. When these subtilisins were tested for their resistance to oxidation, most were found to be very good (except when cysteine replaced methionine: It too is easily oxidized). So now it is possible to use laundry detergent and bleach at the same time and still remove protein-based stains. This type of gene manipulation, which has been called "protein engineering," has already been used for making beneficial changes in other industrial enzymes, and in proteins used for medical purposes.