Asymmetric visible-light photobiocatalytic reduction of β-keto esters utilizing the cofactor recycling system in Synechocystis sp. PCC 6803

Article abstract of DOI:10.1016/j.tetlet.2020.151973

The asymmetric reduction of β-keto esters employing a wild-type strain of cyanobacterium Synechocystis sp. PCC 6803 under illumination of red LED light at 25 °C for 24 h was evaluated. As a result, the corresponding (R)-β-hydroxy esters were obtained as major products. The R-selectivity was shown to increase for bulkier substrates. Moreover, it was also found that the R-selectivity increased with decreasing substrate concentrations. This can be explained by the assumption that the K_m value of the R-selective reductase is smaller than that of the S-selective enzyme involved in the reaction. Additionally, it was demonstrated that the R-selective reductase required the light-dependent production of reduced nicotinamide adenine dinucleotide phosphate (NADPH) for effective reaction; however, the S-selective variant did not. Overall, cyanobacterium was employed as a sustainable photobiocatalyst proliferating under illumination of light, while utilizing inorganic salts and atmospheric carbon dioxide (CO₂). Employing the whole-cell system allowed for the preparation of industrially-important chiral compounds, such as optically active β-hydroxy esters.

Full text of DOI:10.1016/j.tetlet.2020.151973

Journal Pre-proofs
Asymmetric visible-light photobiocatalytic reduction of β-keto esters utilizing
the cofactor recycling system in Synechocystis sp. PCC 6803
Shusei Tanaka, Hideo Kojima, Satomi Takeda, Rio Yamanaka, Tetsuo
Takemura
PII:
S0040-4039(20)30416-0
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.151973
TETL 151973
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
28 February 2020
21 April 2020
23 April 2020
Please cite this article as: Tanaka, S., Kojima, H., Takeda, S., Yamanaka, R., Takemura, T., Asymmetric visible-
light photobiocatalytic reduction of β-keto esters utilizing the cofactor recycling system in Synechocystis sp. PCC
6803, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.151973
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Asymmetric visible-light photobiocatalytic
reduction of -keto esters utilizing the
cofactor recycling system in Synechocystis sp.
PCC 6803
Leave this area blank for abstract info.
Shusei Tanaka, Hideo Kojima, Satomi Takeda, Rio Yamanaka, Tetsuo Takemura
Synechosystis sp. PCC 6803
OH
OH
O
O
O
O
+
OR
OR
OR
H2O, 25°C, 24 h

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Asymmetric visible-light photobiocatalytic reduction of -keto esters utilizing the
cofactor recycling system in Synechocystis sp. PCC 6803
Shusei Tanakaa , Hideo Kojimaa,  , Satomi Takedab , Rio Yamanakac , and Tetsuo Takemurad
aDepartment of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
bDepartment of Biological Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
cFaculty of Pharmaceutical Sciences, Himeji Dokkyo University, 7-2-1 Kamiono, Himeji, Hyogo 670-8524, Japan
dFaculty of Science, Josai University, 1-1 Keyakidai, Sakadoi, Saitama 350-0295, Japan
———
ARTICLE INFO
ABSTRACT
 Corresponding author. Department of Chemistry, Graduate School of Science, Osaka Prefecture University; e-mail: kojima@c.s.osakafu-
u.ac.jp
Article history:
The asymmetric reduction of -keto esters employing a wild-type strain of cyanobacterium
Received
Received in revised form
Accepted
Synechocystis sp. PCC 6803 under illumination of red LED light at 25 °C for 24 h was
evaluated. As a result, the corresponding (R)--hydroxy esters were obtained as major products.
The R-selectivity was shown to increase for bulkier substrates. Moreover, it was also found that
the R-selectivity increased with decreasing substrate concentrations. This can be explained by
the assumption that the Km value of the R-selective reductase is smaller than that of the S-
selective enzyme involved in the reaction. Additionally, it was demonstrated that the R-selective
reductase required the light-dependent production of reduced nicotinamide adenine dinucleotide
phosphate (NADPH) for effective reaction; however, the S-selective variant did not. Overall,
cyanobacterium was employed as a sustainable photobiocatalyst proliferating under illumination
of light, while utilizing inorganic salts and atmospheric carbon dioxide (CO2). Employing the
whole-cell system allowed for the preparation of industrially-important chiral compounds, such
as optically active -hydroxy esters.
Available online
Keywords:
Optically active -hydroxy esters
Cyanobacterium
Synechocystis sp. PCC 6803
Photobiocatalyst
2019 Elsevier Ltd. All rights reserved.
Due to excellent enantioselectivities of enzymes inside the
cells, whole-cell asymmetric reduction of ketones is a particularly
powerful tool for the production of chiral alcohols.1 Although
reductive biocatalysis is a promising method for optically active
alcohols, the method has a fundamental problem; in this method,
the redox coenzyme is used for reduction and is oxidized
according to the reduction of a ketone. Therefore, it is necessary
to regenerate the oxidized coenzyme to the reduced form.
Recently, asymmetric visible-light photocatalytic methods have
attracted increased attention due to the necessity to develop eco-
utilizing inorganic salts and atmospheric carbon dioxide (CO2).7a
Thus, using techniques employing cyanobacteria as
photobiocatalysts is expected to contribute to energy-saving and
CO2 reduction. A number of studies considering the reduction of
C=C bonds8 and C=O bonds9 utilizing cyanobacteria have been
reported. Nonetheless, only a few reports on the synthesis of -
hydroxy esters using cyanobacteria as photobiocatalysts have
been described. Takemura et al. investigated the synthesis of tert-
butyl (S)-3-hydroxybutanoate from tert-butyl 3-oxobutanoate
utilizing knockout mutants of cyanobacterium Synechocystis sp.
PCC 6803, which is a genus of fresh water cyanobacteria.10 In the
present study, we evaluated the factors controlling the
stereoselectivity of the asymmetric reduction of -keto esters
using a wild-type strain of Synechocystis sp. PCC 6803 as a
photobiocatalyst.
friendly
techniques
for
biotransformation.2
In
the
photobiocatalytic method, the redox cofactor nicotinamide
adenine dinucleotide phosphate (NADPH) acting as a reducing
agent is regenerated by
a photosynthetic system under
illumination of visible-light.3 We have previously reported that
whole-cells of plants, such as Nicotiana tabacum4 and
Arabidopsis thaliana,5 worked as effective photobiocatalysts for
the preparation of -hydroxy esters, versatile chiral building
blocks in the pharmaceutical, agrochemical, and food industries,6
by the asymmetric reduction of -keto esters.
Ito et al. reported that employing a red light-emitting diode
(LED) lamp as the light source resulted in a more effective
asymmetric reduction of 2’,3’,4’,5’,6’-pentafluoroacetophenone
using Synechocystis sp. PCC 6803 than utilizing blue or green
LED light.9b Consequently, we employed a red LED lamp as a
light source during the reactions. In the first instance, we
investigated the effect of steric hindrance of the substrate on the
reaction. The reactions of -keto esters 1a-f (10 g/mL)
containing ester moieties of various bulkiness, were carried out in
the presence of Synechocystis sp. PCC 6803 under illumination
of red LED light (660 nm, 10 mol photons m-2 s-1) at 25°C for
Similarly to plants, cyanobacteria are photosynthetic
organisms;
however,
their
potential
as
whole-cell
photobiocatalysts for the production of various chemicals is
considerably higher than that of plants.7 Cyanobacteria are
excellent sustainable catalysts, as they proliferate more
efficiently under illumination of solar light than plants, while

2
Tetrahedron
24 h (Scheme 1). The yields and enantiomeric excess (ee)
shown in Figure 1. It is noteworthy that the yield of (R)-2f
increased with the increase in the light intensity (photon flux
density: PFD) up to 2.5 mol photons m-2 s-1 and stayed high
thereafter. On the other hand, the yield of (S)-2f decreased a little
up to 2.5 mol photons m-2 s-1 and stayed low thereafter. These
results implied that the R-selective reductase required the light-
dependent production of NADPH, whereas the S-selective
reductase did not. It was previously reported that the reduction of
exogenous ketones, such as acetophenone derivatives, using
cyanobacterium Synechococcus sp. PCC 7942, depended on
reduced NADPH. The cofactor typically acts as a reducing agent
during anabolic reactions in the cells and is recycled during the
photosynthesis in photosynthetic organisms.9d Similarly, the
asymmetric reduction of -ketoesters 1 by Synechocystis sp. PCC
6803 may be controlled by the amount of NADPH in the cells. At
2.5 mol photons m-2 s-1 or higher, the yield of (R)-2f remained
nearly constant, possibly because a sufficient quantity of
NADPH was produced by the photosynthetic system at those
light intensities. Thus, the described reaction is eco-friendly, as
the use of low intensity and low energy red-light was sufficient to
achieve effective transformations.
values were determined by gas chromatography (GC). All values
are the mean of three experimental results (Table 1).
Synechocystis sp. PCC 6803
OH
OH
O
O
O
O
+
OR
OR
2a-f
OR
2a-f
H2O, 25°C, 24 h
1a-f
(R)-
(S)-
a: R = CH2CH3
a: R = CH2CH3
b: R = (CH2)2CH3
c: R = (CH2)3CH3
d: R = CH(CH3)2
e: R = CH(CH2CH3)2
f: R = C(CH3)3
b: R = (CH2)2CH3
c: R = (CH2)3CH3
d: R = CH(CH3)2
e: R = CH(CH2CH3)2
f: R = C(CH3)3
Scheme 1.
In all cases, the corresponding (R)--hydroxy esters 2a-f were
obtained in moderate yields, as shown in Table 1. Notably, in the
case of -hydroxy esters 2a-c possessing straight-chain ester
moieties, the yields of (R)-2 increased considerably with the
increase of the number of carbon atoms in the chains (Table 1,
entries 1-3). On the other hand, for the -hydroxy esters 2d-f
containing branched-chain ester functionalities, the highest ee
value was achieved in the case of 1f, probably due to the large
steric hindrance of the bulky tert-butyl group (Table 1, entries 4-
6). Hence, overall, the R-selectivity tended to increase for bulkier
substrates.
We subsequently changed the substrate concentration of 1f
from 10 g/mL to 50 g/mL and 250 g/mL, which resulted in a
decrease in the ee values from 75% to 57% and 39%,
respectively. Thus, the R-selectivity was higher at lower substrate
concentrations. Takemura et al. have previously reported that 3-
oxyacyl-[acyl-carrier-protein]reductase (the Slr0315 protein) was
the major R-selective reductase in the asymmetric reduction of
tert-butyl 3-oxobutanoate using Synechocystis sp. PCC 6803.10
Moreover, it was determined that the stereoselectivity could be
regulated by changing the substrate concentration in the
microbial reduction of ketones due to the difference in the
Michaelis constants (Km) of the R-selective enzyme and the S-
selective enzyme in the Michaelis-Menten kinetic model. At low
substrate concentrations, the enzyme with the smallest Km value
catalyzed the reaction preferentially over other enzymes with
larger Km values.11 Based on these findings, it is considered that
the Km value of the R-selective 3-oxyacyl-[acyl-carrier-
protein]reductase was also smaller than that of the S-selective
reductase involved in the reaction.
Figure 1. The effect of light during the reaction of 1f. Reaction conditions: 1f
(10 g/mL); cyanobacteria (Abs680-750 0.33); 25 °C, 24 h. All values are the
mean of three experimental results (n = 3).
It was previously described that the ratio of NADPH to
reduced nicotinamide adenine dinucleotide (NADH) under light
conditions was 1.7-fold higher than that under dark conditions in
cyanobacterial cells.12 To further investigate the effect of the
quantity of NADPH on the yield of (R)-2f, we examined the
influence of light during the precultivation stage, prior to the
reaction. Thus, the precultivation was carried out for 2 days in the
dark or under illumination of fluorescence light (20 mol
photons m-2 s-1) before conducting the reaction. The asymmetric
reduction of 1f was subsequently performed in the dark or under
illumination of red LED light (660 nm, 10 mol photons m-2 s-1)
at 25 °C for 3 h. For the dark reactions, the yield of (R)-2f in the
light precultivation were slightly higher than those in the dark
precultivation (DD vs. LD in Figure 2). On the other hand, for the
light reactions, the light conditions at the precultivation stage did
not have a significant effect on the yield of (R)-2f (DL vs. LL in
Figure 2). Thus, the yield of (R)-2f increased greatly under
illumination of light during the reaction (DD vs. DL and LD vs.
LL in Figure 2). These results suggest that (R)-2f was produced
by NADPH, which was regenerated under illumination of light
during the precultivation stage or during the reaction.
Table 1. Asymmetric reduction of -keto esters 1a-f using a wild-type strain
of cyanobacterium Synechocystis sp. PCC 6803.a
Entry
Substrate
1a
Product, %b
2a, 16
2b, 76
2c, 96
(R)-2, %
10
(S)-2, %
ee % (R)b
27
1
2
3
4
5
6
6
1b
63
13
17
17
13
5
66
1c
79
64
1d
2d, 53
2e, 95
36
34
1e
82
73
1f
2f, 89
84
75
a Reaction conditions: 1 (10 g/mL); cyanobacteria (Abs680-750 0.33); red LED
light (660 nm, 10 mol photons m-2 s-1); 25°C, 24 h.
b Determined by GC. All values are the mean of three experimental results (n
= 3).
Subsequently, we investigated the effects of light on the
progress of the reaction. The reaction of 1f was carried out under
illumination of light with intensity of 0.5-20 mol photons m-2 s-1.
The yields of (R)-2f and (S)-2f obtained in each reaction are

3
In conclusion, in the current work, we have investigated the
asymmetric reduction of -keto esters 1 using a wild-type strain
of cyanobacterium Synechocystis sp. PCC 6803 under
illumination of red LED light (660 nm) at 25°C for 24 h. As a
result, the corresponding (R)--hydroxy esters
2
were
successfully obtained. We determined that the R-selectivity
tended to increase for bulkier substrates. Moreover, the R-
selectivity increased with decreasing substrate concentrations.
This can be explained by assuming that the Km value of the R-
selective reductase is smaller than that of the S-selective enzyme
involved in the reaction. Moreover, we have demonstrated that
the R-selective reductase required the light-dependent production
of NADPH, while the S-selective reductase did not. Notably, the
described photobiocatalytic system is sustainable, as the
cyanobacteria proliferate under illumination of light, while
utilizing inorganic salts and atmospheric CO2. Thus, the use of
cyanobacteria as whole-cell photobiocatalysts allowed for
efficient preparation of industrially-relevant chiral compounds,
such as optically active -hydroxy esters.
Figure 2. The effect of light at the precultivation stage or during the
reaction. Precultivation conditions: 25 °C; 2 days; fluorescence light (20
mol photons m-2 s-1) or dark. Reaction conditions: 1f (10 g/mL);
cyanobacteria (Abs680-750 0.33); 25 °C, 3 h; red LED light (660 nm, 10 mol
photons m-2 s-1) or dark. DD, dark in the precultivation stage-dark during the
reaction; LD, light-dark; DL, dark-light; LL, light-light. All values are the
mean of three experiments (n = 3).
Furthermore, we also investigated the effect of the quantity of
cyanobacterial cells on the progress of the reactions (Table 2).
The quantity of cells was adjusted on the basis of the absorbance
at 680 nm in the ultraviolet (UV)-visible absorption spectrum of
cyanobacteria dispersed in aqueous solution, setting the
absorbance at 750 nm to zero (Abs680-750). The yields and ee
values of 2f increased with the increasing quantity of cells for
Abs680-750 of 0.99 or bellow (Table 2, entries 1-5). Moreover, for
Abs680-750 of 0.99, the yield and ee values increased to 96% and
89%, respectively (Table 2, entry 5). In the presence of a low
quantity of cells, the yield and ee values were also low, possibly
because of cell damage caused by the excess amounts of
substrates (Table 2, entries 1 and 2).
Acknowledgments
The authors would like to express our deep appreciation to Dr.
Kaoru Nakamura for his sincere and helpful comments, which
improved the quality of the manuscript. The authors would also
like to thank Mr. Kazuma Ujigawa, Mr. Yuta Otani, Mr. Fumito
Okubo, Mr. Takuya Katsuhara, and Ms. Yuki Nakamura in
Osaka Prefecture University for their helpful contributions.
Supplementary Material
Supplementary data to this article can be found online at
https://.
Furthermore, for Abs680-750 equal to 2.97, the ee value
decreased to 57% (Table 2, entry 7). This may be a consequence
of unequal exposure of the cells to light. The phenomenon of
self-shadowing has previously been reported for high-cell-density
cultures. It results in a lower amount of light reaching the
microorganisms, and thus a lower metabolic rate.13 Moreover, in
the present study, it was found that the use of LED light of 60
mol photons m-2 s-1 in the case of Abs680-750 of 1.32 led to the
increase in the ee value from 67% to 77% (Table 2, entries 6 and
8). This outcome also suggested that NADPH generated by light
was an essential cofactor for the production of (R)-2f.
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Highlights
- Asymmetric reduction of -keto esters using a
cyanobacterium
- (R)--Hydroxy esters obtained as major products
- R-selective reductase with the light-dependent
production of NADPH
- Sustainable photobiocatalyst utilizing the cofactor
recycling system