α-Haloacid Dehalogenases are specific for short-medium chain
2-halocarboxylic acids [2022, 2023]. In contratst to haloalkane dehalogenases,
they can also convert α-fluoroalkanoates,
59 but they are inactive on (nonactivated)
haloalkanes. Interestingly, the reactivity of halides with α-haloacid dehalogenases
depends on the source of enzyme. In some cases it increases from iodine to fluorine
derivatives, which is in sharp contrast to the corresponding chemical reactivity with
nucleophiles such as hydroxyl ion [2024]. The most intriguing aspect of α-haloacid
dehalogenases is their enantiospecificity [2025]. Depending on the growth conditions, the microbial production of stereo-complementary (R)- or (S)-specific
enzymes may be induced [2026–2028].
(S)-2-Chloropropionic acid is a key chiral synthon required for the synthesis of a
range of important α-aryl- and α-aryloxypropionic acids used as anti-inflammatory
agents and herbicides, respectively (Scheme 2.35). Several strategies to resolve
racemic 2-chloropropionic acid via enzymatic ester hydrolysis using ‘classic’
hydrolases proceed with varying degrees of selectivity [2029]. An elegant approach
makes use of an (R)-specific α-haloacid dehalogenase from Pseudomonas putida
NCIMB 12018 (Scheme 2.232) [2030–2032]. Thus, from a racemic mixture of
α-haloacid, the (R)-enantiomer is converted into the (S)-hydroxyacid product via
inversion of configuration leaving the (S)-α-haloacid behind. Some minor structural
variations of the substrate are tolerated. This process has been scaled-up to industrial production at a capacity of 2000 t/year [2033].
R 1
R 2
Br
rac
R
1
R
2
OH
R 1
R 2
Br
+
S
S
Haloalkane
Dehalogenase
Buffer
R 1
R 2
Enzyme
E
Me CO-NHCH 2 Ph DbjA >200
Et CO-NHCH 2 Ph LinB
>200
Me CO 2 Me
DbjA >200
Et CO 2 Me
DbjA >200
R 1
R 2
Enzyme E
Me Et
DhaA
2
Me n-Pr
DbjA 145
Me n-Bu
68
Me n-Pent
28
DbjA
DbjA
Scheme 2.231 Kinetic resolution of 2-bromoalkanes, α-bromoesters and amides using haloalkane
dehalogenases
Hal
CO 2 H
R
Hal
CO 2 H
R
OH
CO 2 H
R
Hal
CO 2 H
R
R = Me, Et, Br-CH 2
+
+
rac
dehalogenase
buffer
e.e. >98%
α-haloacid
S N 2 inversion
Hal = Cl, Br
(S)
(S)
(R)
(S)
[OH ]
Scheme 2.232 Resolution of 2-chloropropanoic acid derivatives by α-haloacid dehalogenase
59 Fluoroacetate is extremely toxic.
258
2 Biocatalytic Applications
2-halocarboxylic acids [2022, 2023]. In contratst to haloalkane dehalogenases,
they can also convert α-fluoroalkanoates,
59 but they are inactive on (nonactivated)
haloalkanes. Interestingly, the reactivity of halides with α-haloacid dehalogenases
depends on the source of enzyme. In some cases it increases from iodine to fluorine
derivatives, which is in sharp contrast to the corresponding chemical reactivity with
nucleophiles such as hydroxyl ion [2024]. The most intriguing aspect of α-haloacid
dehalogenases is their enantiospecificity [2025]. Depending on the growth conditions, the microbial production of stereo-complementary (R)- or (S)-specific
enzymes may be induced [2026–2028].
(S)-2-Chloropropionic acid is a key chiral synthon required for the synthesis of a
range of important α-aryl- and α-aryloxypropionic acids used as anti-inflammatory
agents and herbicides, respectively (Scheme 2.35). Several strategies to resolve
racemic 2-chloropropionic acid via enzymatic ester hydrolysis using ‘classic’
hydrolases proceed with varying degrees of selectivity [2029]. An elegant approach
makes use of an (R)-specific α-haloacid dehalogenase from Pseudomonas putida
NCIMB 12018 (Scheme 2.232) [2030–2032]. Thus, from a racemic mixture of
α-haloacid, the (R)-enantiomer is converted into the (S)-hydroxyacid product via
inversion of configuration leaving the (S)-α-haloacid behind. Some minor structural
variations of the substrate are tolerated. This process has been scaled-up to industrial production at a capacity of 2000 t/year [2033].
R 1
R 2
Br
rac
R
1
R
2
OH
R 1
R 2
Br
+
S
S
Haloalkane
Dehalogenase
Buffer
R 1
R 2
Enzyme
E
Me CO-NHCH 2 Ph DbjA >200
Et CO-NHCH 2 Ph LinB
>200
Me CO 2 Me
DbjA >200
Et CO 2 Me
DbjA >200
R 1
R 2
Enzyme E
Me Et
DhaA
2
Me n-Pr
DbjA 145
Me n-Bu
68
Me n-Pent
28
DbjA
DbjA
Scheme 2.231 Kinetic resolution of 2-bromoalkanes, α-bromoesters and amides using haloalkane
dehalogenases
Hal
CO 2 H
R
Hal
CO 2 H
R
OH
CO 2 H
R
Hal
CO 2 H
R
R = Me, Et, Br-CH 2
+
+
rac
dehalogenase
buffer
e.e. >98%
α-haloacid
S N 2 inversion
Hal = Cl, Br
(S)
(S)
(R)
(S)
[OH ]
Scheme 2.232 Resolution of 2-chloropropanoic acid derivatives by α-haloacid dehalogenase
59 Fluoroacetate is extremely toxic.
258
2 Biocatalytic Applications