Industrial Biotransformations

5.2 Biocatalyst Kinetics
formation in a substrate, for which it is often specific. The three-dimensional structure
of an enzyme is determined on different levels [13, 14].
. Primary structure: sequence of connected amino acids of a protein chain.
. Secondary structure: hydrogen bonds from the particular type of R–N–H–O=C–R are
responsible for the formation of the secondary structure, the helix or the b-sheet, of
one protein chain.
. Tertiary structure: hydrogen and disulfide bonds, and ionic and hydrophobic forces lead
to the tertiary structure, the folded protein chain.
. Quaternary structure: if several protein chains are combined in the form of subunits,
the quaternary structure is formed. It is not covalent bonds, but molecular interactions
occurring in the secondary, tertiary and quaternary structures that are responsible for
the formation of the well-functioning catalytic system.
5.2.3
Kinetics
In this section the fundamentals of enzyme kinetics will be discussed in brief. For a detailed
description of enzyme kinetics and a discussion of the different kinetic models
please refer to the literature [15–17].
The determination of the kinetic parameters can be carried out in two different ways:
either by measurement of the initial reaction rate under different reaction conditions or
by batch experiments. In both cases a kinetic model has to exist to describe the reaction
rate as a function of the concentrations of the different reaction components. The two
methods differ in the number of variable components. In the case of the initial determination
of the reaction rate, only the concentration of one compound is altered, whereby
all others are constant. On the contrary, in the case of batch reactions, the time course of
all concentrations of all (!) components is measured. Therefore, all mass balances [see
Eqs. (31–35)] are required for the determination of the kinetic parameters that form a
system of coupled differential equations. The values of the kinetic parameters are determined
by fitting the kinetic equations to the measured data by non-linear regression
(Fig. 5.3). In the case of batch experiments, this is supplemented by numerical integration
of the reaction rate equations. An appropriate test of the kinetic model and the
kinetic parameters is the simulation of the time-courses of batch reactor experiments
with different starting concentrations of the substrate. These are then compared with the
actual batch experiments.
The fundamental description of enzyme kinetics dates back to Michaelis and Menten
[18]. In 1913, in their theory on enzyme catalysis they postulated the existence of an enzyme–substrate
(ES) complex that is formed in a reversible reaction from the substrate (S)
and enzyme (E).
E
k1 k2
k-1 + S ES E + P
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