Enzyme-specific index reaction

Enzymes are excellent catalysts for two reasons great specificity and high turnover rates. With but few exceptions, all reac tions in biological systems are catalyzed by enzymes, and each enzyme usually catalyzes only one reaction. For most of the important enzymes and other proteins, the amino-acid sequences and three-dimensional structures have been determined. When the molecular struc ture of an enzyme is known, a precise molecular weight could be used to state concentration in molar units. However, the amount is usually expressed in terms of catalytic activity because some of the enzyme may be denatured or otherwise inactive. An international unit (lU) of an enzyme is defined as the amount capable of producing one micromole of its reaction product in one minute under its optimal (or some defined) reaction conditions. Specific activity, the activity per unit mass, is an index of enzyme purity. 

In addition, at each step the four-figure EC number (see p. 88) for the enzyme responsible for a reaction is given in italics. The enzyme name and its systematic classification in the system used by the Enzyme Catalogue are available in the following annotated enzyme list (pp. 420-430), in which all of the enzymes mentioned in this book are listed according to their EC number. The book s index is helpful when looking for a specific enzyme in the text. 

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