Page 224 - Cascade biocatalysis

200 9 Stereoselective Hydrolase-Catalyzed Processes in Continuous-Flow Mode

Table 9.1 Comparison of homogeneous and heterogeneous continuous-flow systems.

Homogeneous Heterogeneous
Reactants (and catalysts) are mixed and Reactions are performed with a
remain a homogeneous single phase heterogenized/immobilized catalyst

Advantages Flexibility (one device for many Catalyst retention/recovery is
reactions) integrated
Disadvantages No catalyst recovery (only possible Less flexibility (only the reaction
in a separate device) mediated by the embedded
catalyst is possible)
HTS possibilities a Substrates, reactants, catalysts, and Substrates, reactants, and
conditions; analytical devices conditions; synthetic methods
and analytical devices

a High-throughput screening.

biotransformations can be achieved basically in the following ways [18–22]: (i)
heterogenization of the soluble enzyme by coupling to an insoluble support by
adsorption or covalent binding, (ii) by cross-linking of the enzyme or entrapment
in a lattice or in microcapsules, (iii) by fixation of the enzyme on ultrafiltration
membranes [23–26], or (iv) applying whole cells using their enzyme apparatus
[27–30]. Biosensors are a very special form of carrier-fixed biocatalysts [31, 32].
Any of the above listed heterogenization modes can be applied in continuous-flow
systems although the various modes of enzyme retention may require different
types of reactors [33].
There are two ideal types of continuously operated reactors [34]: continuous
ideally stirred tank reactor (CISTR) and plug-flow reactor (PFR). In CISTR, complete
mixing renders the degree of conversion independent of the position in the reactor
and therefore the conditions within the CISTR are the same as those in the outlet
stream (usually low substrate and high product concentrations). In PFR, conversion
depends on the length of the reaction vessel. Thus the conditions within the reactor
are uneven, often with temperature and concentration gradients normal to flow
direction. When choosing between real reactor types (Figure 9.1), one should
consider kinetic and operational features such as the kinetic parameters of the
enzyme reaction, solubility of substrate and product, and enzyme stability.
Thus, for Michaelis–Menten kinetics, a PFR type reactor, predominantly a
packed-bed reactor (PBR, Figure 9.1b) is preferred to the continuous stirred-tank
reactor (CSTR, Figure 9.1a), since it requires less biocatalyst to reach the same
level of conversion. In this case, ideal reactors are those with high space time/yield
to increase the efficiency of the transformation. PBRs with immobilized catalyst
have a clear advantage in that voidage is low: 34% compared to over 80–90% for
CSTR [35]. However, if pH control is required, the use of a PFR is not advised.
In case of substrate inhibition, a CSTR (Figure 9.1a) operated at high conversion
is to be preferred. On the other hand, when product inhibition is pronounced, a