enzyme unit

{{Short description|Unit of catalytic activity}}

{{distinguish|international unit{{!}}the international unit (IU) used in biological activity}}

The enzyme unit, or international unit for enzyme (symbol U, sometimes also IU) is a unit of enzyme's catalytic activity.{{cite journal |author=Nomenclature Committee of the International Union of Biochemistry (NC-IUB) |title=Units of Enzyme Activity |journal=Eur. J. Biochem. |volume=97 |pages=319–20 |year=1979 |doi=10.1111/j.1432-1033.1979.tb13116.x |issue=2|doi-access=free }}

1 U (μmol/min) is defined as the amount of the enzyme that catalyzes the conversion of one micromole of substrate per minute under the specified conditions of the assay method.{{Cite journal|date=2018-07-01|title=Terminology of bioanalytical methods (IUPAC Recommendations 2018)|journal=Chemistry International|volume=40|issue=3|pages=34|doi=10.1515/ci-2018-0319|issn=1365-2192|doi-access=free|hdl=10138/237212|hdl-access=free}}

The specified conditions will usually be the optimum conditions, 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 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{{Citation|last1=Wharton|first1=Christopher W.|title=Molecular Enzymology|url=https://books.google.com/books?id=Y_oGCAAAQBAJ&dq=1+U+%3D+16.67+nkat&pg=PA82|pages=82|year=2013|postscript=.|series=Tertiary Level Biology|publisher=Springer Science and Business Media|isbn=9781461585329|last2=Eisenthal|first2=Robert}}

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{{Citation|last1=Bommarius|first1=Andreas S.|title=Biocatalysis: Fundamentals and Applications|url=https://books.google.com/books?id=JRyt1AyNQ8gC&pg=PA30|pages=30|year=2007|postscript=.|publisher=John Wiley and Sons|isbn=9783527606054|last2=Riebel-Bommarius|first2=Bettina R.}} (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).