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The enzyme unit, or international unit for enzyme (symbol U, sometimes also IU) is a unit of enzyme's
catalytic activity Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
. 1 U (μmol/min) is defined as the amount of the enzyme that
catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
the conversion of one
micro Micro may refer to: Measurement * micro- (μ), a metric prefix denoting a factor of 10−6 Places * Micro, North Carolina, town in U.S. People * DJ Micro, (born Michael Marsicano) an American trance DJ and producer * Chii Tomiya (都宮 ち ...
mole Mole (or Molé) may refer to: Animals * Mole (animal) or "true mole", mammals in the family Talpidae, found in Eurasia and North America * Golden moles, southern African mammals in the family Chrysochloridae, similar to but unrelated to Talpida ...
of substrate per minute under the specified conditions of the assay method. The specified conditions will usually be the optimum conditions, which including but not limited to temperature, pH, and substrate 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 is not an SI base unit of time, the enzyme unit is discouraged in favor of the
katal The katal (symbol: kat) is the unit of catalytic activity in the International System of Units (SI) used for quantifying the catalytic activity of enzymes (that is, measuring the enzymatic activity level in enzyme catalysis) and other catalyst ...
, the unit recommended by the General Conference on Weights and Measures in 1978 and officially adopted in 1999. One katal is the enzyme activity that converts one mole of substrate per second 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 The concept of enzyme unit should not be confused with the one of
international unit In pharmacology, the international unit (IU) is a unit of measurement for the effect, not mass of a substance; the variance is based on the biological activity or effect, for the purpose of easier comparison across similar ''forms'' of substanc ...
(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 A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutri ...
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 by complex biological processes to regulate physiology and behavior. Hormones are required fo ...
).


See also

*
Turnover number Turnover number has two different meanings: In enzymology, turnover number (also termed ''k''cat) is defined as the maximum number of chemical conversions of substrate molecules per second that a single active site will execute for a given enzyme ...
* Enzyme assay * Enzyme catalysis


References

Units of catalytic activity {{Enzyme-stub