Industrial Biotransformations

5.1 Definitions
operoxidase (CPO) has a tof of 4500 h –1 . However, the molecular masses are 635 and
42 000 g mol –1 , respectively [3, 4].
5.1.1.7 Enzyme Activity
The enzyme activity is defined as the reaction rate per mass of catalyst (protein):
V ˆ @n s
@t m cat
where V = maximum activity of the enzyme under defined conditions (katal kg –1 ,
Umg –1 ); @n s = differential amount of converted substrate (mol, µmol); @t = differential
time for conversion (s, min); and m cat = mass of catalyst (kg, mg).
The SI unit for enzyme activity is the katal (kat = mol s –1 , 1 kat = 6 × 10 7 U), which
results in very low values, so that l-, n- or pkat are often used. A more practical unit of
measurement of enzyme activity is the Unit (1 U = 1 µmol min –1 ). It is very important
that the activity be mentioned along with the substrate used and with all the necessary
reaction conditions, such as temperature, buffer salts, pH value and so on. The concentration
of a protein solution can be determined using indirect photometric tests (e.g.,
Bradford [5]) or analytical methods (e.g., electrophoresis). Only when the enzyme contains
a photometrically active component, e.g., a heme protein, can the concentration be
readily determined by direct photometric absorption. The activity can also be given per
unit of reaction volume (U mL –1 ), if the enzyme concentration cannot be determined
due to a lack of information on its molecular mass or due to the inability to analyze the
mass of the solubilized enzyme.
5.1.1.8 Deactivation Rate
The deactivation rate is defined as the loss of catalyst activity per unit of time:
V 1 ˆ V 0 e k deact …t 1 t 0 †
where k deact = deactivation rate (min –1 ,h –1 ,d –1 ); V 0 = enzyme activity at the start of the
measurement (U mg –1 ); V 1 = enzyme activity at the end of the measurement (U mg –1 );
t 0 = time at the start of the measurement (min, h, d); and t 1 = time at the end of the
measurement (min, h, d).
The deactivation rate expresses the stability of a catalyst.
5.1.1.9 Half-life
The half-life is defined as the time in which the activity is halved:
V 1 ˆ V 0 e k deact …t 1 t 0 †
V 2 ˆ V 0 e k deact …t 2 t 0 †
(9)
(10)
(11)
(12)
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