3.2.2. Enzyme Activity and Enzyme Concentration
Turnover Number. In addition to KM, and Vmax, the turnover number (molar activity) and the specific activity are important parameters for the characterization of enzyme reactions. Both are determined under substrate saturation. With highly purified enzymes the turnover number reflects the number of substrate molecules converted in unit time by a single enzyme molecule (or a single active center). Catalase, one of the most potent enzymes, has a turnover number of 2 x 105/s.
Specific Activity. The specific activity of enzymes is given in units. One international unit (IU) is the amount of enzyme consuming or forming 1 µmol sub-strate or 1 µmol product per minute under standard conditions. The base unit is 1 katal, corresponding to the amount of enzyme converting 1 mol substrate per second:
Usually U is used instead of IU. For the quantitative determination of enzyme activity, initial rates are measured at different enzyme concentrations and near substrate saturation, in a suitable temperature range (25-37_C) and at optimal pH. In a certain range the enzyme activity is proportional to the enzyme concentration. The enzyme activity of a sample can be estimated from the linear part of the plot.
Fig. 3.8. Lineweaver-Burk plot.
Fig. 3.9. (a) Competitive inhibition; (b) non-competitive inhibition