Principles of Biochemistry

8.2.3.5 Units of measurement

1. Enzyme activity: By international agreement, 1.0 unit of enzyme activity is defined as that amount causing transformation of 1.0 micromole of substrate per minute at 25^oC under optimal conditions of measurement. The specific activity is the number of enzyme units per milligram of protein. Carbohydrates, lipids, and proteins are bulk nutrients or macronutrients.

2. The specific activity :It is a measure of enzyme purity: it increase during purification of an enzyme and becomes maximal and constant when the enzyme is in the pure state.

3. Turnover number: The turnover number of an enzyme is the number of substrate molecules transformed per unit time by a single enzyme molecule when the enzyme concentration alone is the rate limiting factor . The enzyme carbonic anhydrase, an important enzyme found in high concentration in the red blood cell, is among the most active of all enzymes, with a turnover number of 36,000,000 per minute per enzyme molecule. It catalyzes the reversible hydration of dissolved carbon dioxide to form carbonic acid an other wise slow reaction: CO2+ H2O ¬ H2 CO3 ®

Carbonic anhydrase	36,000,000
- Amylase1,	1,100,000
Galactosidase	12,500
Phosphoglucomutase	1,240