Enzyme Technology
Enzyme units
The amount of enzyme
present or used in a process is difficult to determine in absolute terms (e.g., grams), as its purity is often low and a proportion may be in an inactive, or
partially active, state. More relevant parameters are the activity of the enzyme
preparation and the activities of any contaminating enzymes. These activities
are usually measured in terms of the activity unit (U) which is
defined as the amount which will catalyse the transformation of 1 micromole of
the substrate per minute under standard conditions. Typically, this represents
10−6 - 10−11 Kg for pure enzymes and 10−4 -
10−7 Kg for industrial enzyme preparations. Another unit of enzyme
activity has been recommended. This is the katal
(kat) which is defined as the amount which will catalyse the
transformation of one mole of substance per second (1 kat = 60 000 000 U). It is
an impracticable unit and has not yet received widespread acceptance. Sometimes
non-standard activity units are used, such as Soxhet, Anson and Kilo Novo units,
which are based on physical changes such as lowering viscosity and supposedly
better understood by industry. Rightfully, such units are gradually falling into
disuse. The activity is a measure of enzyme content that is clearly of major
interest when the enzyme is to be used in a process. For this reason, enzymes
are usually marketed in terms of activity rather than weight. The specific
activity (e.g., U Kg−1) is a parameter of interest, some utility as an
index of purity but lesser importance. There is a major problem with these
definitions of activity; the rather vague notion of "standard
conditions". These are meant to refer to optimal conditions, especially
with regard to pH, ionic strength, temperature, substrate concentration and the
presence and concentration of cofactors and coenzymes. However, these so-termed
optimal conditions vary both between laboratories and between suppliers. They
also depend on the particular application in which the enzyme is to be used.
Additionally, preparations of the same notional specific activity may differ
with respect to stability and be capable of very different total catalytic
productivity (this is the total substrate converted to product
during the lifetime of the catalyst, under specified conditions). Conditions for
maximum initial activity are not necessarily those for maximum stability. Great
care has to be taken over the consideration of these factors when the most
efficient catalyst for a particular purpose is to be chosen
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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