Although on a superficial glimpse, peroxidases appear to be more easy to use than
monooxygenases, several points limit their practical application considerably: Given
the tendency of H 2 O 2 to deactivate proteins in general, the operational stability of
peroxidases in the presence of substantial concentrations of the oxidant is very limited
and in order to achieve reasonable turnover numbers (usually only a few hundred) it has
to be continuously added via autotitration using a H 2 O 2 -sensitive electrode (‘peroxystat’ [1495]) by maintaining a constant low level (~10 mM). Although this also
minimizes spontaneous (non-selective) oxidation, reaction rates are modest too. Consequently, at this point, peroxygenases are no serious competitors for monooxygenases.
2.4 Formation of Carbon–Carbon Bonds
The majority of enzymatic reactions exploited for biotransformations involve
functional group manipulations via bond-breaking reactions. The following enzymatic systems, which are capable of forming carbon–carbon bonds in a highly
stereoselective manner, belong to the class of lyases and are gaining increasing
attention in view of their potential in synthesis. Since these enzymes are involved in
the biosynthesis and biodegradation of sugars, lyase-catalyzed reactions are generally equilibrium-controlled. The following strategies can be applied to drive C–C
bond forming reactions towards completion:
• The primary hydroxycarbonyl products often spontaneously cyclize to yield a
stable hemiacetal.
• C–C bond forming steps are often embedded in a reaction cascade, which pulls
out the formed product from the equilibrium (Sect 3.2, cascade reactions).
• Some C-donor molecules, such as pyruvate, undergo decarboxylation, which
provides a strong driving force.
For the sake of clarity, the donor representing the umpolung reagent is drawn
with bold C–C bonds (blue) throughout this chapter.
• Aldol reactions catalyzed by aldolases are useful for the elongation of aldehydes
by a two- or three-carbon unit yielding β-hydroxy compounds.
• Aldehydes of various size can be coupled in a head-to-head fashion to furnish
α-hydroxycarbonyl compounds (acyloins, benzoins).
• A hydroxyacetyl C 2 -fragment (equivalent to hydroxyacetaldehyde) is transferred via transketolase reactions.
• For the addition of the C 1 -synthon cyanide to aldehydes by hydroxynitrile lyases
see Sect. 2.5.3.
2.4.1 Aldol Reactions
Asymmetric C–C bond formation based on catalytic aldol addition reactions
remains a challenging subject in synthetic organic chemistry. Although many
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2 Biocatalytic Applications
monooxygenases, several points limit their practical application considerably: Given
the tendency of H 2 O 2 to deactivate proteins in general, the operational stability of
peroxidases in the presence of substantial concentrations of the oxidant is very limited
and in order to achieve reasonable turnover numbers (usually only a few hundred) it has
to be continuously added via autotitration using a H 2 O 2 -sensitive electrode (‘peroxystat’ [1495]) by maintaining a constant low level (~10 mM). Although this also
minimizes spontaneous (non-selective) oxidation, reaction rates are modest too. Consequently, at this point, peroxygenases are no serious competitors for monooxygenases.
2.4 Formation of Carbon–Carbon Bonds
The majority of enzymatic reactions exploited for biotransformations involve
functional group manipulations via bond-breaking reactions. The following enzymatic systems, which are capable of forming carbon–carbon bonds in a highly
stereoselective manner, belong to the class of lyases and are gaining increasing
attention in view of their potential in synthesis. Since these enzymes are involved in
the biosynthesis and biodegradation of sugars, lyase-catalyzed reactions are generally equilibrium-controlled. The following strategies can be applied to drive C–C
bond forming reactions towards completion:
• The primary hydroxycarbonyl products often spontaneously cyclize to yield a
stable hemiacetal.
• C–C bond forming steps are often embedded in a reaction cascade, which pulls
out the formed product from the equilibrium (Sect 3.2, cascade reactions).
• Some C-donor molecules, such as pyruvate, undergo decarboxylation, which
provides a strong driving force.
For the sake of clarity, the donor representing the umpolung reagent is drawn
with bold C–C bonds (blue) throughout this chapter.
• Aldol reactions catalyzed by aldolases are useful for the elongation of aldehydes
by a two- or three-carbon unit yielding β-hydroxy compounds.
• Aldehydes of various size can be coupled in a head-to-head fashion to furnish
α-hydroxycarbonyl compounds (acyloins, benzoins).
• A hydroxyacetyl C 2 -fragment (equivalent to hydroxyacetaldehyde) is transferred via transketolase reactions.
• For the addition of the C 1 -synthon cyanide to aldehydes by hydroxynitrile lyases
see Sect. 2.5.3.
2.4.1 Aldol Reactions
Asymmetric C–C bond formation based on catalytic aldol addition reactions
remains a challenging subject in synthetic organic chemistry. Although many
204
2 Biocatalytic Applications