Photoenzymatic Catalysis Enables Radical-Mediated Ketone Reduction in Ene-Reductases

Article abstract of DOI:10.1002/anie.201902005

Flavin-dependent ene-reductases (EREDs) are known to stereoselectively reduce activated alkenes, but are inactive toward carbonyls. Demonstrated here is that in the presence of photoredox catalysts, these enzymes will reduce aromatic ketones. Mechanistic experiments suggest this reaction proceeds through ketyl radical formation, a reaction pathway that is distinct from the native hydride-transfer mechanism. Furthermore, this reactivity is accessible without modification of either the enzyme or cofactors, allowing both native and non-natural mechanisms to occur simultaneously. Based on control experiments, we hypothesize that binding to the enzyme active site attenuates the reduction potential of the substrate, enabling single-electron reduction. This reactivity highlights opportunities to access new catalytic manifolds by merging photoredox catalysis with biocatalysis.

Full text of DOI:10.1002/anie.201902005

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Photoenzymatic Catalysis Enables Radical- Mediated Ketone
Reduction in ‘Ene’-Reductases
Authors: Todd Hyster, Braddock Sandoval, Sarah Kurtoic, Megan
Chung, and Kyle Biegasiewicz
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902005
Angew. Chem. 10.1002/ange.201902005
Link to VoR: http://dx.doi.org/10.1002/anie.201902005
http://dx.doi.org/10.1002/ange.201902005

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
Photoenzymatic Catalysis Enables Radical-Mediated Ketone
Reduction in ‘Ene’-Reductases
Braddock A. Sandoval, ‡[a] Sarah I. Kurtoic, ‡[a] Megan M. Chung, [a] Kyle F. Biegasiewicz, [a] Todd K.
Hyster*[a]
Abstract: Flavin-dependent ‘ene’-reductases (EREDs) are known to
stereoselectively reduce activated alkenes, but are inactive toward
carbonyls. Herein we demonstrate that in the presence of photoredox
catalysts, these enzymes will reduce aromatic ketones. Mechanistic
elusive. 6 This is due, in part, to the challenge of achieving
productive interactions between enzymatic and small molecule
catalytic cycles.
Enzymes use a variety of strategies to enable kinetically
experiments suggest this reaction proceeds via ketyl radical formation, challenging reactions. Oxidoreductases, for instance, use a series
a reaction pathway that is distinct from the native hydride transfer
mechanism. Furthermore, this reactivity is accessible with no
modification of the enzyme or cofactors, allowing both native and non-
natural mechanisms to occur simultaneously. Based on control
experiments, we hypothesize that binding to the enzyme active site
attenuates the reduction potential of the substrate, enabling single
electron reduction. This reactivity highlights opportunities to access
new catalytic manifolds by merging photoredox catalysis with
biocatalysis.
of hydrogen bonding interactions to render substrates more
electrophilic, priming them for reduction by various cofactors. We
recognized that if this mechanism of substrate activation could be
merged with single electron reductants, it would be possible to
use oxidoreductases that typically use ionic mechanisms to
catalyze reactions that proceed via radical intermediates.7,8 To
this end, we recently demonstrated that nicotinamide-dependent
double bond reductases (DBRs) can catalyze a ‘new to nature’
enantioselective radical deacetoxylation of α-acetoxyketones
when exposed to visible light and a xanthene dye photoredox
catalyst.9 In this study, binding to the DBR activates the ketone
for single electron reduction by the photocatalyst and orients the
resulting a-acyl radical for selective hydrogen atom transfer from
NADPH.
The discovery of new catalytic reactions is a fundamental
goal in organic synthesis. Small molecule catalysts, such as
transition metal complexes or small organic molecules, have
proven to be fertile scaffolds on which new chemical reactivity is
developed.1 Enzymes are rarely used as general scaffolds for
discovering new chemical reactions because of their perceived
reaction specificity. The realization that substrate permissive
enzymes can also exhibit catalytic promiscuity has opened the
door to enzymes being used for reactions beyond their native
function. 2 From this perspective, devising new strategies for
revealing non-natural reaction pathways in enzymes can
significantly expand the potential applications of biocatalysis.
Synergistic catalysis is an attractive strategy for expanding
the synthetic capabilities of a particular catalyst.3 By weaving
together two catalytic cycles, it becomes possible to achieve
chemical reactivity that would otherwise be inaccessible to either
catalyst individually.4 For instance, the merger of enamine and
transition metal catalysis enabled cross-coupling reactions that
were previously unknown.5 Despite the growing trend of using
biocatalytic approaches to organic synthesis, examples of
synergy between small molecule catalysts and enzymes remain
O
O
OH
Ph
Ph
Ph
ERED
ERED
Me
Me
Non-Natural Reactivity
Natural Reactivity
[carbonyl reduction]
[alkene reduction]
Can ‘ene’-reductases be used to catalyze carbonyl reductions?
■ This work
N
Ru
N
N
N
N
N
O
OH
Me
Me
‘ene’-reductase + photocatalyst
aryl ketones
chiral alcohols
H
H
O
Ph
Me
Attenuated
Reduction Potential
[a]
B. A. Sandoval, S. I. Kurtoic, M. M. Chung, Dr. K. F. Biegasiewicz,
Prof. Dr. T. K. Hyster
Department of Chemistry
Princeton University
Figure 1. Merging photoredox catalysis with biocatalysis
Frick Chemical Laboratory, Princeton, NJ 08544
E-mail: thyster@princeton.edu
Building on this mechanistic understanding, we questioned
whether a similar mode of activation could be achieved within the
active site of flavin-dependent ‘ene’-reductases (EREDs). These
substrate-promiscuous enzymes are widely used for the
asymmetric reduction of activated olefins, which occurs via
Author Contributions:
‡These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
stereoselective delivery of hydride from flavin hydroquinone
of Ru(bpy)3Cl2 and irradiation with blue light enabled all nine
EREDs to reduce acetophenone to the corresponding alcohol (SI
Table 2). The most active enzyme was morphinone reductase
from P. putida (MorB), which provided product in quantitative yield
as a scalemic mixture favoring the (R)-enantiomer (80:20 e.r.)
(Table 1, entry 1). Comparable yields can be achieved with 0.5
mol % enzyme, although extended reaction times are required to
reach full conversion. Interestingly, pH had a negligible effect on
reaction efficiency (SI Table 6). Preferential formation of the
opposite enantiomer of the alcohol is achieved by 12-
oxophytodienoate reductase (OPR-1) (82:18 e.r.) and YersER
(86:14 e.r.), albeit with diminished yield by comparison to MorB
(Table 1, entries 2 and 3). A brief survey of photocatalysts
revealed Ru(bpy)3Cl2 and Ru(bpz)3Cl2 to be most effective (SI
Table 4).
10
(FMNhq).
Two
hydrogen
bond
donors
(typically
Histidine/Histidine, or Histidine/Asparagine) are used to activate
and orient the substrate for reduction. We envisioned that these
residues could similarly activate ketones for single electron
reduction. Furthermore, recent studies have shown that EREDs
are highly compatible with photoredox catalysis, although no
examples currently exist of using photocatalysis to achieve new
reactivity with these enzymes.11 ,17 As a model system, we
targeted the reduction of ketones to alcohols, a reaction pathway
that is not observed in EREDs, or any other flavoenzyme known
in nature. We imagined that this reaction would be attractive for
streamlining the biocatalytic reduction of enones to saturated
alcohols, a process that currently requires two enzymes using
natural reaction mechanisms. 12 Furthermore, mechanistically
novel biocatalytic carbonyl reductions may offer advantages, such
as altered selectivity profiles and irreversibility.
A series of control experiments were conducted with MorB
to confirm the importance of each component of the system
(Table 1, entries 4-7). Both NAD+ and GDH-105 were required for
reactivity, suggesting that reduced flavin (FMNhq) is required for
ketone reduction (Table 1, entry 7 and SI Table 1, entry 2).
Importantly, these studies suggest the reaction is not occurring
via hydride transfer from NADPH to acetophenone (Table 1, entry
From a design perspective, we imagined that this non-
natural carbonyl reaction could proceed via the mechanism
indicated in Figure 2. Initial reduction of the ERED with NADH
affords reduced flavin (FMNhq). The enzyme binds an equivalent
of the ketone, activating it for single electron reduction (E1/2 = -2.1
V in MeCN). 13 Concurrently, visible light photoexcitation of a
Ru(II) photocatalyst produces the excited state Ru(II)* that is
known to be readily quenched by NADH analogues to afford a
more strongly reducing Ru(I) species. 14 Subsequently, Ru(I)
5). Surprisingly, replacing NAD(P)+ and GDH-105 with
a
photochemical reducing system involving TEOA and Ru(bpy)3Cl2
provided only low conversion, presumably because cofactor
turnover is faster with the GDH/glucose system.17 (SI Table 5).
II/I
(E1/2 = -1.33 V) reduces the ERED-bound ketone to the
Table 1. Reaction Optimization
corresponding ketyl radical. 15 We hypothesize that the ketyl
radical is quenched via hydrogen atom transfer from flavin and
MorB (1 mol %)
Ru(bpy)3Cl2 (1 mol %)
O
OH
Me
electron transfer from FMNsq to Ru(II)* regenerates FMNox .
16 As
Me
NAD(P)+ (2 mol %)
GDH-105, Glucose
460 nm LEDs, 24 h
active sites of EREDs are partially exposed to the bulky solution,
we anticipate that electron transfer to substrates bound within the
active site can occur without requiring a bioconjugation event.
1
2
‘Ene’-Reductase
entry[a]
Change from standard conditions
no change
Yield (%)
e.r.
OH
1
2
3
4
5
6
7
99
38
80:20
Ph
NAD
Me
FMNsq
*RuII
FMNox
RuI
H
H
O
O
OPR1 instead of MorB
YersER instead of MorB
no MorB
18:82
Ph
Me
Ph
Me
20
14:86
Photoredox
Catalytic
Cycle
FMNhq
NADH
N.R.
N.R.
N.R.
N.R.
-
-
-
-
RuII
SET
d1-glucose and no Ru(bpy)3Cl2
no irradiation
H
H
O
Ph
Me
H
‘Ene’-Reductase
Catalytic Cycle
FMNox
no GDH-105 and Glucose
H
H
RuI
O
SET
*RuII
Ph
Me
[a]
Standard Reaction Conditions: 10 µmol substrate, 1 mol% MorB, 1 mol%
FMNhq
+
Ru(bpy) Cl , 2 mol% NAD(P) , 9 mg glucose, 0.25 mg GDH-105, 250 µL
reaction volume.
3
2
H
H
O
HAT
Ph
Me
H
FMNsq
To probe the formation of a ketyl radical, an aromatic ketone
containing a pendant cyclopropane was prepared and subjected
to the reaction conditions. Two products were identified as the
ring-opened ketone and the corresponding ring-opened alcohol,
with the remainder of the mass balance being unreacted starting
material (Figure 3A and SI Figure 1). Importantly, in the absence
of the ERED only starting material was observed, supporting the
Figure 2. Proposed Reaction Mechanism
We initiated our investigation by examining the reduction of
acetophenone with eight structurally diverse EREDs. While no
reaction was observed in the absence of photocatalyst, addition
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
hypothesis that enzyme-mediated substrate activation is required
to achieve single electron reduction and ketyl radical formation (SI
Figure 1).
role (Figure 3c).19 To probe this possibility, isotope incorporation
experiments were performed with acetophenone and wild type
MorB. When the reaction was run using KPi buffered D2O as the
solvent, no deuterium was incorporated into the alcohol product,
as evidenced by 13C NMR. This strongly suggests that tyrosines
in the active site are not serving as H-atom donors. Interestingly,
when the standard reaction conditions were repeated using d1-
glucose as an in situ deuterium source for labeling FMN, no
deuterium incorporation was observed in the product. In both
cases, the reaction goes to full conversion within 24 hours with no
change in enantioselectivity from the standard reaction conditions.
Surprised by these unusual isotope incorporation results,
we hypothesized that a kinetic isotope effect could prevent
deuterated flavin from quenching the ketyl radical. Instead, the
deuterium at the flavin N-5 position could exchange with solvent,
followed by hydrogen atom transfer to the ketyl radical. (SI Figure
3). Indeed, when a rate study was performed, the reaction
became significantly slower in the presence of d1-glucose. (SI
Figure 4). This effect was further magnified when both glucose
and the solvent were deuterated. Due to the pKa of the anionic
FMNhq (pKa > 20) and lack of solvent exchange observed under
standard olefin reduction experiments, we postulate that this
exchange is mediated by oxidation of FMNhq by photoexcited
Ru(II)* to generate the neutral flavin semiquinone.20 Due to its
increased acidity (pKa = 8.3), flavin semiquinone could exchange
rapidly with solvent under the reaction conditions (SI Figure 3).
We next focused on determining which residues are
responsible for substrate binding and activation (Figure 3B). Like
other EREDs, MorB is understood to bind its natural substrate via
hydrogen bonding from a His/Asn pair, making these side chains
the most likely candidates. To probe this hypothesis, we used site-
directed mutagenesis to mutate each residue to alanine and
tested the reduction of acetophenone. When asparagine was
mutated to alanine (N189A), the product was observed in
quantitative yield, albeit with significantly diminished e.r. (57:43).
Scrutton previously observed that this mutation results in
cyclohexanone binding in two different conformations, potentially
accounting for the poor selectivity observed with this variant.18
Mutation of the histidine to alanine (H186A), however, only formed
the product in 15% yield. This mutation is known to inhibit the
A
O
MorB (1 mol %)
Ph
O
Ph
Ru(bpy)3Cl2 (1 mol %)
4a
Ph
Ph
OH
NAD+ (2 mol %)
GDH-105, Glucose
460 nm LEDs, 24 h
Ph
3
Ph
4b
34% yield
Supports the intermediacy of ketyl radical
2.1:1 (4a:4b)
B
Figure 4. Substrate Scope
Mutated Binding
Residues
MorB (1 mol %)
O
OH
Ru(bpy)3Cl2 (1 mol %)
H186
N189A
R
R
NAD(P)+ (2 mol %)
GDH-105, Glucose
460 nm LEDs, 24 h
X
X
>99% yield
N189
43:57 e.r.
Morphinone
Reductase (MorB)
5-13
14-22
H186A
15% yield
67:33 e.r.
OH
Me
OH
Me
X
X
C
MorB (1 mol %)
X = OMe (14) 50% yield, 88:12 era
X = CF3 (15) >99% yield, 76:24 er
X = Me (17) 82% yield, 75:25 er
O
HO H/D
Me
Ru(bpy)3Cl2 (1 mol %)
X = Cl
(18) 98% yield, 81:19 er
Me
NAD+ (2 mol %)
X = F
(16) 64% yield, 86:14 er
GDH-105, glucose
460 nm LEDs, 24 h
1
2
Me OH
OH
OH
OH
C5H11
Deuterium Source
Yield (%)
> 99
e.r.
Deuterium incorporation
S
Me
Me
80:20
80:20
0%
0%
deuterated buffer
d1-glucose
> 99
19
20
21
22
15% yield
67:33 e.r.
90% yielda
65% yield
67:33 e.r.
62% yielda
oxidative half-reaction with cyclohexanone, presumably due to
diminished substrate binding. Collectively, these results suggest
that the canonical binding residues are responsible for orienting
the substrate and activating it for single electron reduction.
88:12 e.r.
52:48 e.r.
a 48 h reaction time
To better understand the scope and limitations of this
reaction, a panel of aromatic ketone substrates were tested for
reduction with MorB and Ru(bpy)3Cl2. Para- and meta-
substituents are tolerated, forming the product in good yields and
with promising levels of enantioselectivity (Figure 4, 14-18).
Interestingly, electron withdrawing substituents proved to be more
reactivity, presumably due to their increased reduction potentials.
An ortho-substituent is also tolerated and provides product in low
yield with poor enantioselectivity (Table 4, 19). A more sterically
napthyl ketone is also reactive, although extended reaction times
Figure 3. Mechanistic studies to determine the presence of
intermediate and identification of the H-atom source.
a radical
To gain insight into the radical termination event, we
investigated the source of the hydrogen atom. While our previous
studies indicated that reduced FMNhq could function as a
hydrogen atom source, we recognized that a cysteine or tyrosine
located within the enzyme active site could also serve a similar
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
are required to reach high conversions (Table 4, 20). Increasing
the steric bulk of the other substituent on the ketone provided
products with poor levels of enantioselectivity (Table 4, 21,22).
This limitation can be overcome by using FlOYE as a catalyst
(98:2 er), although this reaction occurs with diminished yields
(Supplemental Table 2). Alternatively, we imagine protein
engineering could be applied to increase the enantioselectivity of
a particular transformation. Finally, non-acetophenone derivatives
were unreactive, presumably due to their low reduction potentials.
Finally, we envisioned that a single enzyme with ene-
reductase and ketoreductase catalytic activity could streamline
biocatalytic synthesis. When ketone 23 was subjected to the
reaction conditions, we observed global reduction of the substrate,
presumably with the ERED conducting its native reactivity,
followed by a non-natural ketone reduction to provide the desired
product in 89% yield and 83:17 er (Figure 5, 24). In contrast, in
the absence of light only alkene reduction is observed in 98%
yield (Figure 5, 25).
range of chemical transformations. Herein, we demonstrate that
a new mechanistic pathway can be accessed to generate chiral
products that are unexpected for this enzyme class without
biocatalyst modification. Furthermore, as significant efforts are
often made to engineer enzymes to tolerate conditions typically
used in organic synthesis, finding more examples of catalytic
promiscuity expands their synthetic utility and maximizes the
return on investment for such engineering projects.
Experimental Section
General Reaction Procedure. In an anaerobic chamber (Coy,
Grass Lake, MI), a stock solution of turnover mix containing GDH-
105 (5 mg) and glucose (180 mg) in 1 mL of degassed KPi (100
mM, pH 8) was prepared. A catalyst mix solution containing NAD+
(14.3 mg) and Ru(bpy)3Cl2 (8 mg) in 1 mL of degassed KPi (100
mM, pH 8) was prepared. To a dram vial containing 150 µL of
degassed KPi (100 mM, pH 8) was added 50 µL of turnover mix,
followed by 10 µL of catalyst mix and enzyme (volume of enzyme
varies with concentration). Finally, 10 µL of ketone (1.0 M in
DMSO) was added. The vial was sealed with a cap and removed
from the anaerobic chamber. The reaction was placed on a
shaker and illuminated with 460 nm light for 24 hours.
MorB
Ru(bpy)3Cl2
MorB
Ru(bpy)3Cl2
OH
O
O
Me
Me
Me
25
Ph
Ph
Ph
No Light
460 nm LED
Me
24
Me
23
98% yield
89% yield
83:17 er
Acknowledgements
T.K.H thanks the NIH-NIGMS (R01 GM127703), Searle Scholars
Award (SSP-2017-1741), Sloan Research Fellowship, the
Princeton Catalysis Initiative, and Princeton University for support.
B.A.S. and S.I.K. thank the Edward C. Taylor Fellowship for
support.
Figure 5. Change in Product Selectivity
One of the current limitations of biocatalysis in organic
synthesis is the perception that enzymes are limited to their
natural catalytic activity, suggesting that novel reactivity in
biocatalysis can only be obtained by screening nature for new
transformations, or with significant engineering of the biocatalyst.
This perception is in stark contrast to small molecule chiral
catalysts, which are often used to impart selectivity in a wide
Keywords: biocatalysis • photoredox • reduction • hydrogen
atom transfer • ‘ene’-reductase
[1]T.P. Yoon, E.N. Jacobsen, Science. 2003, 299, 1691-1693.
97-105.
[2] a) U.T. Bornscheur; R.J. Kazlauskas, Angew. Chem. Int. Ed. 2004, 43,
6032-6040. b) H. Renata, Z.J. Wang, F.H. Arnold, Angew. Chem. Int. Ed.
2015, 54, 3351-3367. c) F. H. Arnold, Angew. Chem. Int. Ed. 2017, 57, 4143-
4148.
[11] a) S.H. Lee, D.S. Choi, M. Pesic, Y.W. Lee, C.E. Paul, F. Hollmann, C. B.
Park, Angew. Chem. Int. Ed. 2017, 56, 8681-8685. b) Z.C. Litman, Y. Wang,
H. Zhao, J.F. Hartwig, Nature, 2018, 560, 355-359.
[12] a) E. Brenna, F.G. Gatti, D. Monti, F. Parmeggiani, A. Sacchetti.
ChemCatChem 2012, 4, 653-659. b) H.S. Toogood, N.S. Scrutton, ACS Catal.
2018, 8, 3532-3549.
[3] A.E. Allen, D.W.C. MacMillan, Chem. Sci. 2012, 3, 633-658.
[4] a) D. A. Nicewicz, D.W.C. MacMillan, Science, 2008, 322, 5898, 77-80. b)
J. Du, K. Skubi, D. M. Schultz, T. P. Yoon 2014, 344, 392. c) A. Gil, F.
Albericio, M. Alvarez Chem. Rev. 2017, 117, 8420.
[13] S. Fukuzumi, K. Ishikawa, K. Hironaka, T. Tanaka, J. Chem. Soc. Perkin
Trans. II 1987, 751-760.
[5] a) S. Krautwald, D. Sarlah, M. A. Schafroth, E. M. Carreira, Science 2013,
340, 1065. (b) I. Ibrahem, A. Córdova, Angew. Chem. Int. Ed. 2006, 45, 1952-
1956.
[14] a) R. Maidan, I. Willner, J. Am. Chem. Soc. 1986, 108, 1080-1082. b) C.
Pac, M. Ihama, M. Yasuda, Y. Miyauchi, H. Sakurai, J. Am. Chem. Soc. 1981,
103, 6496-1497.
[6] M. Hönig, P. Sondermann, N.J. Turner, E.M. Carreira, Angew. Chem. Int.
Ed. 2017, 56, 8942-8973.
[15] C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113,
5322-5363.
[7] H. Zipse. Org. Biomol. Chem. 2003, 1, 692-699.
[16] B. A. Sandoval, A. J. Meichan, T. K. Hyster, J. Am. Chem. Soc. 2017,
139, 11313-11316.
[8] a) A.K. Turek, D.J. Hardee, A. M. Ullman, D.G. Nocera, E.N. Jacobsen,
Angew. Chem. Int. Ed. 2016, 55, 539-544. b) H. Zipse. Org. Biomol. Chem.
2003, 1, 692-699.
[17] M.K. Peers, H.S. Toogood, D.J. Heyes, D. Mansell, B.J Coe, N. S.
Scrutton, Catal. Sci. Technol., 2016, 6, 169-177.
[9] K.F. Biegasiewicz, S.J. Cooper, M.A. Emmanuel, D.C. Miller, T.K. Hyster,
Nat. Chem. 2018, 10, 770-775.
[18] H.L. Messiha, A.W. Munro, N.C. Bruce, I. Barsukov, N.S. Scrutton, J. Biol.
Chem. 2005, 280, 10695-10709.
[10] a) R.M. Kohli, V. Massey, J. Biol. Chem. 1998, 273(98), 32763-32770. b)
C.K. Winkler, G. Tasnádi, D. Clay, M. Hall, K Faber, J. Biotechnol. 2012, 162,
381-389. c) C.K. Winkler, K. Faber, M. Hall, Curr. Opin. Chem. Biol. 2018, 43,
[19] a) J. Stubbe, D.G. Nocera, C.S. Yee, M.C.Y. Chang, Chem. Rev. 2003,
103(6), 2167-2202. b) B.I. Morinaka, A.L. Vagstad, M.J. Helf, M. Gugger, C.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
[20] P. Macheroux, S. Ghisla, C. Sanner, H. Rüterjans, F. Müller. BMC
Biochemistry, 2005, 6, 1-11.
Kegler, M.F. Freeman, H.B. Bode, J. Piel, Angew. Chem. Int. Ed. 2014, 53,
8503-8507.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/anie.201902005
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Braddock A. Sandoval, Sarah I. Kurtoic,
Megan M. Chung, Kyle F. Biegasiewicz
Todd K. Hyster*
Page No. – Page No.
Photoenzymatic Catalysis Enables
Radical-Mediated Ketone Reduction
in ‘Ene’-Reductases
O
OH
Me
H
H
O
2+
Me
Ru(bpy)3
Ph
Me
Attenuated
photocatalyst
Reduction Potential
‘ene’-reductase
Flavin Misbehavin’: ‘Merging photocatalysis with biocatalysis leads to new chemical reactivity for flavin-dependent ‘ene’-reductases.
Mechanistic studies support a mechanism in which enantioenriched alcohols are produced via hydrogen atom transfer from flavin to
the ketyl radical.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.