majority of organic compounds are only poorly soluble in aqueous media. Thus,
shifting enzymatic reactions from an aqueous to an organic medium would be
highly desired, but the unavoidable price one has to pay is usually some loss of
catalytic activity, which is often in the order of one magnitude [99].
• Enzymes are bound to their natural cofactors.
It is a striking paradox, that although enzymes are extremely flexible
for accepting nonnatural substrates, they are almost exclusively bound to
their natural cofactors which serve as storage for chemical energy (ATP),
as molecular shuttles of redox equivalents (heme, flavin, nicotinamide),
or as carriers for toxic amines (pyridoxal phosphate) and highly reactive
carbanion intermediates (thiamine diphosphate). These ‘biological reagents’
are prohibitively expensive to be used in stoichiometric amounts and some
are relatively unstable molecules. With very few exceptions, they cannot
be replaced by more economical man-made substitutes [100]. Although
many cofactors can be efficiently recycled, others cannot (Sects. 2.1.4
and 2.2.1).
• Enzymes are prone to inhibition phenomena.
The tight binding of small molecules may lead to substrate and/or product
inhibition, which causes a drop in reaction rate at elevated substrate and/or
product concentrations, a factor which limits the efficiency of the process.
Whereas substrate inhibition can be circumvented comparatively easily by
keeping the substrate concentration at a low level through continuous addition,
product inhibition is a more complicated problem. The gradual removal of
product by physical means is usually difficult. Alternatively, a consecutive
step may be linked to the reaction to effect in-situ chemical removal of the
product.
• Enzymes may cause allergies.
Enzymes may cause allergic reactions. However, this may be minimized if
enzymes are regarded as chemicals and handled with the same care.
1.3.3 Isolated Enzymes Versus Whole Cell Systems
The physical state of biocatalysts which are used for biotransformations
can be very diverse. The final decision as to whether one should use isolated,
more-or-less purified enzymes or whole microorganisms – either in a free
or immobilized form – depends on many factors, such as (i) the type of
reaction, (ii) whether there are cofactors to be recycled, and (iii) the scale in
which the biotransformation has to be performed [101, 102],. The general pros
and cons of using isolated enzymes versus whole (microbial) cells are outlined in
Table 1.2.
8
1 Introduction and Background Information
shifting enzymatic reactions from an aqueous to an organic medium would be
highly desired, but the unavoidable price one has to pay is usually some loss of
catalytic activity, which is often in the order of one magnitude [99].
• Enzymes are bound to their natural cofactors.
It is a striking paradox, that although enzymes are extremely flexible
for accepting nonnatural substrates, they are almost exclusively bound to
their natural cofactors which serve as storage for chemical energy (ATP),
as molecular shuttles of redox equivalents (heme, flavin, nicotinamide),
or as carriers for toxic amines (pyridoxal phosphate) and highly reactive
carbanion intermediates (thiamine diphosphate). These ‘biological reagents’
are prohibitively expensive to be used in stoichiometric amounts and some
are relatively unstable molecules. With very few exceptions, they cannot
be replaced by more economical man-made substitutes [100]. Although
many cofactors can be efficiently recycled, others cannot (Sects. 2.1.4
and 2.2.1).
• Enzymes are prone to inhibition phenomena.
The tight binding of small molecules may lead to substrate and/or product
inhibition, which causes a drop in reaction rate at elevated substrate and/or
product concentrations, a factor which limits the efficiency of the process.
Whereas substrate inhibition can be circumvented comparatively easily by
keeping the substrate concentration at a low level through continuous addition,
product inhibition is a more complicated problem. The gradual removal of
product by physical means is usually difficult. Alternatively, a consecutive
step may be linked to the reaction to effect in-situ chemical removal of the
product.
• Enzymes may cause allergies.
Enzymes may cause allergic reactions. However, this may be minimized if
enzymes are regarded as chemicals and handled with the same care.
1.3.3 Isolated Enzymes Versus Whole Cell Systems
The physical state of biocatalysts which are used for biotransformations
can be very diverse. The final decision as to whether one should use isolated,
more-or-less purified enzymes or whole microorganisms – either in a free
or immobilized form – depends on many factors, such as (i) the type of
reaction, (ii) whether there are cofactors to be recycled, and (iii) the scale in
which the biotransformation has to be performed [101, 102],. The general pros
and cons of using isolated enzymes versus whole (microbial) cells are outlined in
Table 1.2.
8
1 Introduction and Background Information