• Enzymes act under mild conditions.
Enzymes act within a range of about pH 5–8 (typically around pH 7) and in a
temperature range of 20–40
C (preferably at around 30
C). This minimizes
problems of undesired side-reactions such as decomposition, isomerization,
racemization, and rearrangement, which often plague traditional methodology.
• Enzymes are compatible with each other.
6
Since enzymes generally function under the same or similar conditions,
several biocatalytic reactions can be performed in a cascade-like fashion in a
single flask. Such systems are particularly advantageous if unstable intermediates are involved and furthermore, an unfavorable equilibrium can be shifted
towards the desired product by linking consecutive enzymatic steps. Multienzyme cascades are often denoted as ‘artificial metabolism’ (Sect. 3.2) [22].
• Enzymes are not restricted to their natural role.
Many proteins exhibit a high substrate tolerance by accepting a large variety
of man-made nonnatural substances. If advantageous for a process, the aqueous
medium can often be replaced by an organic solvent (Sect. 3.1).
• Enzymes can catalyze a broad spectrum of reactions.
Like catalysts in general, enzymes can only accelerate reactions but have no
impact on the position of the thermodynamic equilibrium of the reaction. Thus, in
principle, enzyme-catalyzed reactions can be run in both directions. The catalytic
flexibility of enzymes is generally denoted as ‘catalytic promiscuity’ [23–29],
which is divided into ‘substrate promiscuity’ (conversion of a nonnatural substrate), ‘catalytic promiscuity’ (a nonnatural reaction is catalyzed), and ‘condition
promiscuity’ (catalysis occurring in a nonnatural environment).
There is an enzyme-catalyzed process equivalent to almost every type of organic
reaction [30], for example:
• Hydrolysis-synthesis of esters [31], amides [32], lactones [33], lactams [34],
ethers [35], acid anhydrides [36], epoxides [37], and nitriles [38].
• Oxidation of alkanes [39], alcohols [40], aldehydes, sulfides, sulfoxides [41],
epoxidation of alkenes [42], hydroxylation and dihydroxylation aromatics [43],
and the Baeyer-Villiger oxidation of ketones [44, 45].
• Reduction of aldehydes/ketones, alkenes, and reductive amination [46].
• Addition-elimination of water [47], ammonia [48], hydrogen cyanide [49].
• Halogenation and dehalogenation [50], electrophilic Friedel-Crafts-type alkylation [51] and acylation [52], nucleophilic aromatic substitution [53], O- and Ndealkylation [54], Kolbe-Schmitt carboxylation [55], and decarboxylation [56],
isomerization [57], acyloin [58] and aldol reactions [59]. Even Michael additions [60], Stetter reactions [61], Nef reactions [62], Wittig-olefination [63],
Mitsunobu-type inversions [64], the Cannizzaro-reaction [65], Prinsrearrangement [66], and Diels-Alder reactions [67–70] have been reported.
6 Only proteases are exceptions to this rule for obvious reasons.
4
1 Introduction and Background Information
Enzymes act within a range of about pH 5–8 (typically around pH 7) and in a
temperature range of 20–40
C (preferably at around 30
C). This minimizes
problems of undesired side-reactions such as decomposition, isomerization,
racemization, and rearrangement, which often plague traditional methodology.
• Enzymes are compatible with each other.
6
Since enzymes generally function under the same or similar conditions,
several biocatalytic reactions can be performed in a cascade-like fashion in a
single flask. Such systems are particularly advantageous if unstable intermediates are involved and furthermore, an unfavorable equilibrium can be shifted
towards the desired product by linking consecutive enzymatic steps. Multienzyme cascades are often denoted as ‘artificial metabolism’ (Sect. 3.2) [22].
• Enzymes are not restricted to their natural role.
Many proteins exhibit a high substrate tolerance by accepting a large variety
of man-made nonnatural substances. If advantageous for a process, the aqueous
medium can often be replaced by an organic solvent (Sect. 3.1).
• Enzymes can catalyze a broad spectrum of reactions.
Like catalysts in general, enzymes can only accelerate reactions but have no
impact on the position of the thermodynamic equilibrium of the reaction. Thus, in
principle, enzyme-catalyzed reactions can be run in both directions. The catalytic
flexibility of enzymes is generally denoted as ‘catalytic promiscuity’ [23–29],
which is divided into ‘substrate promiscuity’ (conversion of a nonnatural substrate), ‘catalytic promiscuity’ (a nonnatural reaction is catalyzed), and ‘condition
promiscuity’ (catalysis occurring in a nonnatural environment).
There is an enzyme-catalyzed process equivalent to almost every type of organic
reaction [30], for example:
• Hydrolysis-synthesis of esters [31], amides [32], lactones [33], lactams [34],
ethers [35], acid anhydrides [36], epoxides [37], and nitriles [38].
• Oxidation of alkanes [39], alcohols [40], aldehydes, sulfides, sulfoxides [41],
epoxidation of alkenes [42], hydroxylation and dihydroxylation aromatics [43],
and the Baeyer-Villiger oxidation of ketones [44, 45].
• Reduction of aldehydes/ketones, alkenes, and reductive amination [46].
• Addition-elimination of water [47], ammonia [48], hydrogen cyanide [49].
• Halogenation and dehalogenation [50], electrophilic Friedel-Crafts-type alkylation [51] and acylation [52], nucleophilic aromatic substitution [53], O- and Ndealkylation [54], Kolbe-Schmitt carboxylation [55], and decarboxylation [56],
isomerization [57], acyloin [58] and aldol reactions [59]. Even Michael additions [60], Stetter reactions [61], Nef reactions [62], Wittig-olefination [63],
Mitsunobu-type inversions [64], the Cannizzaro-reaction [65], Prinsrearrangement [66], and Diels-Alder reactions [67–70] have been reported.
6 Only proteases are exceptions to this rule for obvious reasons.
4
1 Introduction and Background Information