Industrial Biotransformations

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5 Basics of Bioreaction Engineering
5.1.1.5 Turnover Number
The turnover number (tn) is the number of synthesized molecules per number of catalyst
molecules used:
tn ˆ n p
n cat
1
m p
where tn = turnover number (–); n p = amount of product p at the end of the reaction
(mol); m p = stoichiometric factor for the product p (–); and n cat = amount of catalyst (mol).
The turnover number is a measure of the efficiency of a catalyst. Kinases, dehydrogenases
and aminotransferases have turnover numbers of the order of 10 3 . Enzymes
such as carbonic anhydrase and superoxide dismutase have turnover numbers of the
order of 10 6 [2]. Clearly the turnover numbers have to be high for commercial applications
of biotransformations. Particularly, while using expensive catalysts, the tn should
be as high as possible to reduce the final production cost.
It is very important to name the defined reaction parameters in combination with the
tn to make the values comparable. Instead of the tn, the deactivation rate or half-life may
also be given.
The turnover number can also be given for cofactors/coenzymes.
The turnover number is more frequently used, in addition to the turnover frequency
and the selectivity, in order to evaluate a catalyst in homogeneous (chemical) catalysis. In
biocatalysis, possibly because the molar mass has to be taken into account to obtain a
dimensionless number, this parameter is used less frequently.
5.1.1.6 Turnover Frequency
The turnover frequency is a mass-independent quantity for describing the activity of the
biocatalyst and is defined as:
tof ˆ n s
t n cat
where tof = turnover frequency (s –1 ); n s = moles of converted starting material s (mol,
µmol); t = time for conversion (s, min); and n cat = moles of active sites (mol, µmol).
The turnover frequency allows an evaluation of the performance between different catalyst
systems, irrespective of whether they are biological or non-biological. Its threshold
is at the value of one event per second per active site.
According to this definition, the tof can be determined only if the number of active
sites is known. For an enzymatic reaction obeying Michaelis–Menten kinetics (vide infra),
the turnover frequency is given by:
tof ˆ 1
(8)
kcat Biological catalysts have high turnover frequencies compared with chemical catalysts.
However, when substrate/product solubility, stability and molecular mass of the biocatalyst
are taken into consideration, this advantage is lost. As an example, the epoxidation
and sulfidation catalyst Mn-Salen has a tof of 3 h –1 , while its enzymatic counterpart chlor-
(6)
(7)