Man has used the products of enzyme technology for domestic and commercial purposes for thousands of years. Beer, wine, and bread are probably the earliest products of fermentation. It is not surprising, then, that yeast enzymes, upon which such fermentations depend, were the first to be chemically identified in the early nineteenth century. In cheese making, a technology of almost equal antiquity,
was the first relatively pure enzyme used industrially. The appalling stench of hides being soaked in urine to soften them and remove fats has not been characteristic of the tanning industry for nearly a century;
are now used. Well over 2,000 enzymes have been identified; however, only about 10% have been isolated in pure crystalline form. Consequently, most enzymes used in biotechnology are sold on the basis of units of activity rather than by weight, and many product descriptions include a listing of other enzymes known to be present. Enzymes are sometimes named by their substrate plus -ase, followed by source (e.g., lipase, human pancreas). Some, such as pepsin or trypsin, are not. To avoid confusion, a classification system was adopted by international agreement that organized enzymes into six major classes (see Table 5.1) according to the type of reaction catalyzed.