Peroxygenases are heme‐dependent enzymes that use peroxide‐borne oxygen to catalyze a wide range of oxyfunctionalization reactions. Herein, we report the engineering of an unusual cofactor‐independent peroxygenase based on a promiscuous tautomerase that accepts different hydroperoxides (t‐BuOOH and H2O2) to accomplish enantiocomplementary epoxidations of various α,β‐unsaturated aldehydes (citral and substituted cinnamaldehydes), providing access to both enantiomers of the corresponding α,β‐epoxy‐aldehydes. High conversions (up to 98 %), high enantioselectivity (up to 98 % ee), and good product yields (50–80 %) were achieved. The reactions likely proceed via a reactive enzyme‐bound iminium ion intermediate, allowing tweaking of the enzyme's activity and selectivity by protein engineering. Our results underscore the potential of catalytic promiscuity for the engineering of new cofactor‐independent oxidative enzymes.
Class I aldolases
catalyze asymmetric aldol addition reactions
and have found extensive application in the biocatalytic synthesis
of chiral β-hydroxy-carbonyl compounds. However, the usefulness
of these powerful enzymes for application in other C–C bond-forming
reactions remains thus far unexplored. The redesign of class I aldolases
to expand their catalytic repertoire to include non-native carboligation
reactions therefore continues to be a major challenge. Here, we report
the successful redesign of 2-deoxy-
d
-ribose-5-phosphate aldolase
(DERA) from
Escherichia coli
, an archetypical
class I aldolase, to proficiently catalyze enantioselective Michael
additions of nitromethane to α,β-unsaturated aldehydes
to yield various pharmaceutically relevant chiral synthons. After
11 rounds of directed evolution, the redesigned DERA enzyme (DERA-MA)
carried 12 amino-acid substitutions and had an impressive 190-fold
enhancement in catalytic activity compared to the wildtype enzyme.
The high catalytic efficiency of DERA-MA for this abiological reaction
makes it a proficient “Michaelase” with potential for
biocatalytic application. Crystallographic analysis provides a structural
context for the evolved activity. Whereas an aldolase acts naturally
by activating the enzyme-bound substrate as a nucleophile (enamine-based
mechanism), DERA-MA instead acts by activating the enzyme-bound substrate
as an electrophile (iminium-based mechanism). This work demonstrates
the power of directed evolution to expand the reaction scope of natural
aldolases to include asymmetric Michael addition reactions and presents
opportunities to explore iminium catalysis with DERA-derived catalysts
inspired by developments in the organocatalysis field.
An interesting rearrangement of arylvinylidenecyclopropanes having three substituents at the 1- and 2-positions of the corresponding cyclopropane catalyzed by Lewis acids to give 6aH-benzo[c]fluorine derivatives via a double intramolecular Friedel-Crafts reaction or to give an indene derivative via an intramolecular Friedel-Crafts reaction is described.
The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new‐to‐nature enzymes. Organocatalysis was originally bio‐inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new‐to‐nature enzymes.
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