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.
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 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 nkat
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 and hormones).
- Nomenclature Committee of the International Union of Biochemistry (NC-IUB) (1979). "Units of Enzyme Activity". Eur. J. Biochem. 97 (2): 319–20. doi:10.1111/j.1432-1033.1979.tb13116.x.
- "Terminology of bioanalytical methods (IUPAC Recommendations 2018)". Chemistry International. 40 (3): 34. 2018-07-01. doi:10.1515/ci-2018-0319. ISSN 1365-2192.
- Principles of Biochemistry, page 94, 4th Edition, Lehninger
- Wharton, Christopher W.; Eisenthal, Robert (2013), Molecular Enzymology, Tertiary Level Biology, Springer Science and Business Media, p. 82, ISBN 9781461585329.
- Bommarius, Andreas S.; Riebel-Bommarius, Bettina R. (2007), Biocatalysis: Fundamentals and Applications, John Wiley and Sons, p. 30, ISBN 9783527606054.
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