Editing Enzyme unit (section)

{{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 measurement|unit]] of [[enzyme]]'s [[Enzyme assay|catalytic activity]].<ref>{{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 }}</ref> 

1 U (μmol/min) is defined as the amount of the enzyme that [[catalysis|catalyzes]] the conversion of one [[micro-|micro]][[mole (unit)|mole]] of [[substrate (biochemistry)|substrate]] per minute under the specified conditions of the [[Assay|assay method]].<ref>{{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}}</ref> 

The specified conditions will usually be the [[Enzyme|optimum conditions]], including but not limited to [[temperature]], [[pH]], and [[Substrate (biology)|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.<ref>Principles of Biochemistry, page 94, 4th Edition, Lehninger</ref> 

The enzyme unit was adopted by the [[International Union of Biochemistry and Molecular Biology|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 [[nano-|n]]mol/s;
:16.67 nkat = 16.67 nmol/s;
:Therefore, 1 U = 16.67 n[[katal|kat]]<ref name="isbn_9781461585329">{{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}}</ref>

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<ref name="isbn_9783527606054">{{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.}}</ref> (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, [[Vitamin|vitamins]] and [[Hormone|hormones]]).