The enzyme unit, or international unit for enzyme (symbol U, sometimes also IU) is a unit of

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

's catalytic activity. 1 U (μmol/min) is defined as the amount of the enzyme that catalyzes the conversion of one micro mole of substrate per minute under the specified conditions of the assay method. The specified conditions will usually be the optimum conditions, including but not limited to

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

, pH, and substrate

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'', ...

, that yield the maximal substrate conversion rate for that particular enzyme. In some assay method, one usually takes a temperature of 25°C.Principles of Biochemistry, page 94, 4th Edition, Lehninger The enzyme unit was adopted by the International Union of Biochemistry in 1964. Since the

minute A minute is a unit of time defined as equal to 60 seconds. It is not a unit in the International System of Units (SI), but is accepted for use with SI. The SI symbol for minutes is min (without a dot). The prime symbol is also sometimes used i ...

is not an SI base unit of time, the enzyme unit is discouraged in favor of the katal, the unit recommended by the

General Conference on Weights and Measures The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...

in 1978 and officially adopted in 1999. One katal is the enzyme activity that converts one mole of substrate per

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

under specified assay conditions, so :1 U = 1 μmol/min = 1/60 μmol/s ≈ 16.67 nmol/s; :16.67 nkat = 16.67 nmol/s; :Therefore, 1 U = 16.67 n kat While the katal may be recommended, almost all scientific research today still uses the system based on the minute, for the simple reason that enzyme assays are measured in minutes, not seconds. The concept of enzyme unit should not be confused with the one of international unit (IU). Although it is true that 1 U = 1 IU (because, for many enzymes, the existing U was adopted as the later IU), international units can be defined for the biologic activity of many other kinds of substance besides enzymes (for example,

vitamins Vitamins are organic molecules (or a set of closely related molecules called vitamers) that are essential to an organism in small quantities for proper metabolic function. Essential nutrients cannot be synthesized in the organism in suff ...

and

hormones A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physiology and behavior. Hormones a ...

References

Units of catalytic activity {{Enzyme-stub

See also

References